Evaluation of Growth and Production Parameters of Raspberries and Blackberries Cultivated in Romania

Abstract

There are a multitude of raspberry and blackberry varieties, and each of them develops differently depending on environmental factors and cultivation technology, so much research is needed to see which variety has the best yield in a desired area. This paper studied the growth under natural soil and specific climate conditions in the Bucharest–Ilfov region of Romania of a raspberry plantation and a blackberry plantation, both in their first year of vegetation. The studied interval, the period of June to October 2024, was established from the beginning of the ripening of the first fruits to the late ripening of the fruits. The study analyzed the correlations between the vegetative and productive parameters of the raspberry variety “Delniwa” and the blackberry variety “Thornfree” on productivity per plant. During the study period, good shoot formation was observed, with an average height of 1400 mm for raspberries and 3474 mm for blackberries (r = 0.99 to raspberries and r = 0.98 to blackberries); a good development of the average stem diameter of 8.54 mm for raspberries and 12.78 mm for blackberries (r = 0.96 la zmeur si r = 0.89 la mur), of the number of ripe fruits harvested (r = 0.68 to raspberries and r = 0.58 to blackberries), all of which are correlated with increased productivity of 820 g/plant for raspberries and 2050 g/plant for blackberries. The experimental data were statistically analyzed using linearized, polynomial and hyperbolic models to identify the relationships between the studied variables and to highlight growth variations and fruit production in raspberries and blackberries throughout the season. In the first year of vegetation, both crops recorded constant growth but with different rhythms: raspberries showed constant fruit production, with a peak in June–July and a slight resumption in September, while blackberries had high production at the beginning of summer, followed by a significant decrease in August. The results obtained support farmers who grow raspberries and blackberries under similar natural pedo-climatic conditions, contributing to crop planning and production optimization.

1. Introduction

Growing demand for fresh berries has led to the rapid development of advanced production methods for raspberries and blackberries, ensuring healthy products, rich in antioxidants, vitamins and minerals [1] and phenolic compounds [2], throughout the year. In addition, raspberry and blackberry plants can offer a wide range of bioactive substances that can be obtained from different vegetative organs, such as young shoots, leaf buds or leaves, which are available throughout the growing season [3,4].

Raspberries, Rubus idaeus, and blackberries, Rubus fruticosus, both species described by Linnaeus, are plants of the genus Rubus which have a perennial root system that produces biennial stems, called “canes”, which usually grow vegetative in the first year, and are called “primocanes”; in the second year, after dormancy, they are called “floricanes”, and produce flowers and fruit [5].

Mexico is the main producer of fresh blackberries, supplying the United States and Europe, while Russia is the largest producer of raspberries, with over 100,000 tonnes annually, followed by Serbia and Poland, each with over 80,000 tonnes [5]. In Romania, in 2023, the area under raspberries was 190 hectares, with a production of 430 tonnes, and for blackberries, 1200 hectares, with a production of 3840 tonnes [6].

In countries such as Norway, production is limited by low temperatures but in recent decades, Poland, the Netherlands, the United Kingdom and Belgium have invested in growing raspberries and blackberries in tunnels and plastic greenhouses to extend the season and meet market demand [7,8]. At the same time, research indicates the possibility of cultivating blackberries in shaded systems, integrated into productive forestry, to obtain commercially viable crops [9].

The expansion of berry cultivation areas around the world has led farmers to overuse nitrogen fertilizers in a bid to increase yields. However, proper fertilizer use is essential for healthy crops [10,11] and an appropriate dosage depends on the level of organic matter in the soil and the plant’s nutrient reserves [12].

The external natural factors that influence the processes of initiation, growth and development of shoots of raspberry varieties and floral buds of blackberry varieties are temperature and photoperiod [1,13,14], light, humidity and carbon dioxide [5].

Climate change has brought and will bring warmer weather, more variable climate and different precipitation, thus, significant challenges for the cultivation of berries, especially in temperate regions. Plants will have to adapt to much more variable weather conditions, and changes in temperature and precipitation will influence both plant health and crop quality [15].

There are many studies [16,17,18,19,20,21,22] regarding the vegetative and productive parameters of berries under certain growing conditions, in different climate and soil areas, to obtain optimal yields to satisfy the demands of consumers around the world.

Recent studies have analyzed various aspects of raspberry and blackberry growth and productivity under different climatic and planting conditions. In Moldova, the study [17] evaluated the growth phenophases and drought resistance of 29 raspberry varieties. In Bulgaria, the studies [16] and [18] examined the vegetative and productive characteristics of the raspberry varieties “Magdalena” and “Shopska Alena”, highlighting the influence of planting distance (0.3 m and 0.5 m) on the number (35.5), height (1.83 m, 1.38 m, respectively) and thickness of shoots (6.31–7.52 mm, respectively, 8.41 mm), as well as yield (1.69 kg/1 m², for the variety “Shopska Alena”). In the “Magdalena” variety, the highest number of shoots was recorded for a planting distance of 0.3 m and the highest shoot height was for a planting distance of 0.5 m. Also, a correlation was observed between the height, thickness of the shoots and yield in the second and third years after planting.

In Kosovo, the study [19] evaluated the fruit quality of five raspberry cultivars, identifying the cultivar “Delniwa” as having the largest fruit diameter (24.79 mm, and the cultivar “Mapema” with the highest values of fruit length (29.84 mm) and fruit weight (6.10 g). In another study [22], the impact of raspberry cultivars on growth parameters and biomass characteristics was analyzed, with the cultivar “Polana” having the highest number of lateral shoots and “Polonez” having the highest shoot length and thickness.

For blackberries, the study [20] identified the genotype “Chester Thornless” as the most suitable for commercial cultivation in the Mediterranean region of Turkey, due to its superior fruit yield and quality. In Serbia, the study [21] demonstrated that seven blackberry cultivars (“Dirksen Thornless, Thornfree, Čačanska Bestrna, Black Satin, Loch Ness, Chester Thornless, Navaho”) have adapted well to acidic soil and difficult climatic conditions, and are recommended for commercial cultivation.

Previous studies have focused on analyzing the vegetative and productive parameters of raspberries and blackberries starting from the second or third year of production without correlating the influence of environmental factors on the development of plants in the first year and without applying mathematical modeling for yield forecasting. The current research is innovative, analyzing for the first time the development of raspberries “Delniwa” and blackberries “Thornfree” from the first year of vegetation, under the climatic and pedological conditions of Bucharest. It also uses advanced statistical models for the forecast of staggered production, thus contributing to the optimization of production and the forecast of yield per plant depending on the vegetative parameters.

In this context, the present work shows the potential of Romania for the development of raspberry and blackberry crops under natural climate and soil conditions. The experiment was carried out using raspberry and blackberry seedlings, respectively, in the first year of vegetation, which developed well and produced fruits from the first year of planting. In this study, an in-depth analysis of the vegetative and productive parameters (shoot heights, stem diameters, number of fruits and their sizes) was carried out in raspberry and blackberry plants in the first year of vegetation. The aim of our study was to investigate how the stem, shoots and fruits of raspberry and blackberry plants develop under natural environmental conditions (air temperature, precipitation, atmospheric humidity, solar radiation, evapotranspiration, soil temperature and humidity, volume content of ions). Another objective was to evaluate the growth and development indices of raspberry and blackberry plants under the influence of climate on the yield and quality of fruits. Also, statistical modeling was performed for the staggered production of a raspberry and blackberry plant, depending on the vegetative and productive indices, which resulted in a polynomial and a hyperbolic formula, suitable for both scientific research and agricultural management. Therefore, this study provides some new clues for studying the correlation of raspberry and blackberry growth indices with plant productivity in the future. The main objectives of the study were:

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determining the impact of climatic conditions on the development and productivity of “Delniwa” raspberry and “Thornfree” blackberry plants in the first year of vegetation;

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analysis of the correlation between vegetative parameters and fruit yield;

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development of statistical formulas: linear, polynomial and hyperbolic, to predict staggered production per plant depending on vegetative parameters and environmental conditions and to compare the performance of the two crops;

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establishing recommendations for optimizing yield from the first year of vegetation in the climatic conditions of Romania (Bucharest–Ilfov area).

The research hypothesis for the study of raspberry and blackberry crops under natural climatic and soil conditions in the Bucharest–Ilfov area consists in the adaptability of the plants to the specific conditions of the capital, given the warmer and more urbanized climate, the yield and quality of production obtained on a sandy–loamy soil. This hypothesis could contribute to a deeper understanding of how urban, climatic and pedological conditions in the studied area influence the development and productivity of these crops and could provide recommendations for local producers or for the development of sustainable urban agriculture strategies.

2. Materials and Methods

2.1. Research Area and Growing Conditions

The research was carried out in open field conditions (agro-climatic), in the Bucharest–Ilfov Region, location Figure 1, southern Romania (latitude 44°28′56″ N; longitude 26°04′41″ E) and north-eastern Europe.

The experimental plantation has a humid continental climate with very hot summers, with temperature variations between a minimum of −30 °C (in January and February 1942) and a maximum of +42 °C (in July and August 2000), with an average annual temperature in the period from 2019 to 2022 of +13 °C and total precipitation of 623.8 mm. The experimental area is a lowland with terraces, characterized by the presence of numerous terraces along the rivers that drain it, an area made up of exclusively quaternary deposits represented by loess and loessoid deposits, with a pH of 6.5–7.2 and an average humus content of 3.7–4.3% [24].

2.2. The Varieties Used in the Experiments

The plantations were established in the spring of 2024, the raspberry plantation on an area of 1000 m² and the blackberry plantation on an area of 500 m², the distance between the rows being 3.3 m and the distance between the plants being 0.75 m. The study included two varieties, one of raspberry “Delniwa” and one of blackberry “Thornfree”. After planting the seedlings, the trellis support system, essential for ensuring real growing conditions, and drip irrigation system were established for both plantations. The irrigation system was prepared as a precaution in case of extreme drought but no irrigation was done during the experiment.

The cuttings of the raspberry variety “Delniwa”, used to establish the raspberry crop, are part of the primocane variety, in which the appearance of the stems, flowering and fruiting took place in the same vegetation season. The cycle of shoot growth and fruiting takes place in a single year, on the first-year wood, fruiting from June 15 to October 15. It is a hybrid created by crossing the “Polka, Polana” and “Himbotop” varieties, a premium variety, with fruits resistant to transport and handling; they do not darken after harvesting but retain their natural color 2 to 3 days after harvesting, pink [22]. Usually, the fruits are large, with large drupes with small seeds inside each one welded together, red in color, rounded conical in shape, intensely aromatic, sweet–sour in taste, weighing between 4.3 and 6.1 g [25].

The cuttings of the blackberry variety “Thornfree”, used to establish the blackberry crop, are part of a vigorous and productive variety, which develops very well even in arid areas with harsh winters, and usually do not need abundant irrigation. This blackberry variety, originating in Europe, is perennial, multiplies itself, giving new shoots from the root every year, which produce fruit, has no thorns, bears fruit from July 15 to August 25 and develops a root up to 60 to 70 cm deep; the shoots can reach a height of up to 3 to 4 m, needing a support system. The fruits are medium to large in size (12 g), have an elongated conical shape, black epidermis with a very shiny purple tint, slightly acidic taste, are very aromatic and juicy [25,26].

In the images in Figure 2 and Figure 3, the two crops and their fruits can be observed during their monitoring.

The requirements of raspberries and blackberries for ecological factors are average for light; the average optimum atmospheric temperature is 16 to 17 °C, the soil temperature is approximately 16 °C; the optimum atmospheric humidity is 70 to 80% during flowering and 65 to 70% during the rest of the vegetation period. They grow well on sandy–loamy, fertile, drained, aerated soil, with pH 5.6 to 6.5, rich in nitrogen and proteins [26].

The experiment monitored the growth and fruiting of the raspberry variety “Delniwa” and the blackberry variety “Thornfree” under the influence of climatic factors in the experimental site of the crops developed within the National Institute of Agricultural Machinery in the Bucharest–Ilfov area, Romania. Thus, monthly measurements were made with a tape measure on the height of the shoots, with a caliper on the diameter of the stem at 10 cm above the ground, and the fruits harvested throughout the ripening period were weighed with a Kern PCB 6000-0 balance (precision 1 g) (Kern&Sohn GmbH, Ziegelei, Balingen, Germany). To establish the average values with the heights, thicknesses and weights shown in the Section 3, measurements were made on five plants for each parameter.

The raspberry varieties “Delniwa” and “Thornfree” were selected for this study based on the following criteria: their adaptability to local conditions (Poland is the country of origin for the “Delniwa” raspberry variety, and the United States for the “Thornfree” blackberry variety), both having good tolerance to heat and cold, resistance to common forest berry diseases, high yield and large, sweet and aromatic, juicy fruits, as well as low maintenance requirements. Also, both varieties adapt well to different soil types, including slightly acidic or neutral ones, which are common in the Bucharest region, well-drained and fertile, making them suitable for this area. Both varieties have flowering and harvesting periods that are well suited to the climatic conditions of Bucharest–Ilfov. “Delniwa” produces early fruits, while “Thornfree”, also known for its lack of thorns, produces fruits later, being able to ensure a prolonged harvest throughout the season. In addition, raspberries and blackberries are highly valued on the local and international market; therefore, selecting these varieties can ensure a stable income for growers, due to high demand and competitive prices. In conclusion, these varieties offer an optimal combination of fruit quality and economic efficiency, which makes them ideal for cultivation in this region.

2.3. Soil Analysis

Five soil samples were randomly taken from the field at a depth of 30 cm before the establishment of the two crops. After sampling, the soil was ground, air-dried and filtered through a 2.2 mm sieve before analysis. The analysis methods and the values obtained for general soil parameters are presented in

Table 1. General soil parameters.

Parameter	Analysis Method	Value Obtained	References
pH	Potentiometric method in aqueous suspension	6.6	[27,28]
Bulk density	Gravimetric method	1.31 g/cm3	[29]
Porosity	Bulk density and soil solid particle density method	53%	[28,30]
Organic matter	Walkley-Black method	3.08%	[31,32]
Soil texture	Sedimentation method	Sandy loamy	[33]

.

These values suggest that the analyzed soil presents suitable conditions for the cultivation of raspberries and blackberries, with a good balance between moderate acidity, water and nutrient retention capacity and a physical structure favorable to root development. A sandy–loamy texture ensures good drainage and proper aeration of the roots and allows efficient absorption of water and nutrients.

The analysis of heavy metals and minerals in the soil was carried out using standardized procedures in the field of soil analysis, using an atomic absorption spectrometer. The results are shown in

Table 2. Soil analysis before the establishment of the two crops.

Heavy Metals and Minerals	Content, mg/kg	Macro and Microelements	Content, mg/kg
Lead	17.00	Aqueous extract chlorides	88.6
Nickel	18.20	Sulfates (S-SO42−)	72.0
Copper	18.62	Soluble calcium	269.0
Cadmium	0.33	Soluble magnesium	31.0
Manganese	493.3	Soluble sodium	11.3
Zinc	79.8	Exchangeable calcium	4310.0
Nitrogen and Nitrogen Compounds	Content, mg/kg	Exchangeable magnesium	400.6
Ammonium (N-NH4+)	5.77	Exchangeable sodium	161.7
Nitrite (N-NO2−)	0.73	Exchangeable potassium	331.6
Nitrate (N-NO3−)	3.42

.

Analysis of the values obtained for heavy metals and minerals in the soil indicates that the soil has a balanced profile in terms of heavy metals and micronutrients. The low levels of lead and cadmium ensure a safe environment, and the values for nickel, copper, manganese and zinc are within normal limits for soils intended for raspberry and blackberry crops.

The analysis of macro and micronutrients shows that the soil has a good content of calcium and potassium, which is beneficial for raspberries and blackberries. Magnesium could be slightly limiting compared to calcium and the low level of sodium is positive; and sulfates are within a safe range, nitrogen is low, so a supplement with compost, manure or organic fertilizers could be beneficial. In conclusion, the soil is suitable for the development of these crops, providing both food security and the nutrients necessary for optimal growth.

Meteorological parameters, humidity, temperature and volumetric ion content in the soil were measured by the meteorological station located in the field, next to the two crops.

2.4. Monitoring Growing Conditions

During the growing period of the two crops, meteorological parameters were recorded by a weather station located 50 m from the two research crops and are presented in

Table 3. Meteorological parameters in the experimental area, period June–October 2024.

Months	Average Air Temperature, °C	Total Precipitation, mm	Relative Air Humidity, %	Solar Radiation, W/m2	Evapotranspiration, mm
June	25.24	87.6	55.95	265	121.1
July	26.86	61.6	53.26	262	151.3
August	26.16	18.2	49.07	216	123.4
September	19.87	68.4	63.4	135	70.3
October	12.43	7.6	76.33	101	36.1

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The month of October was marked by the lowest average temperatures during the growing season of the two crops, which only reached the value of 12.43 °C, compared to the highest values of over 26 °C, in the months of July and August. Especially in summer, air temperature ranged between +25.24 °C and +26.86 °C. The phases of phenological development of raspberry plants depend on the variety and other factors, but especially on the climatic conditions of the year [17,34].

According to Table 1, the analysis of meteorological data shows that the minimum monthly precipitation accumulated in October was 7.6 mm, and the largest amount was collected in June in the amount of 87.6 mm, although precipitation during the vegetation period was evenly distributed except for August in the amount of 18.2 mm, this month being a critical period for the growth and development of raspberry and blackberry plants.

Air humidity in August was 49%, which is at the lower limit for raspberry and blackberry plants. Lower relative humidity can favor dehydration of the plants and growth can be slower, especially in warmer periods. In contrast, the maximum value of 76% is an optimal humidity, especially during fruiting periods. This helps maintain plant health and prevent premature drying of the fruits [26].

Raspberries and blackberries need good sun exposure to produce quality fruit. Plants should be exposed to sunlight for at least 6 h per day to ensure adequate photosynthesis. Red raspberry fruit is prone to a disorder that has been identified as a form of sun injury. The condition, most commonly seen in areas with high temperatures and solar radiation, is characterized by drupes that enlarge but fail to turn red [35].

Solar radiation of 265 W/m² in June is sufficient to support plant growth, but not ideal for maximum production. Plants could benefit from more sun exposure or a sunnier location to reach their maximum growth potential [9].

Also, a balance between temperature, humidity and light is essential to ensure a good harvest [36,37]. In the biennial raspberry, flower initiation occurs when temperatures are lower and/or photoperiods are shorter than in the case of primocane fruiting varieties, which are closely related to solar radiation, day length and temperature [14].

Raspberries and blackberries require a significant amount of water to support their growth and produce quality fruit. Their evapotranspiration will depend on climatic conditions and water management. During periods of active growth (spring and summer), when solar radiation is high, in the study it was over 200 W/m² and temperatures are higher; in the study it was over 25 °C, and evapotranspiration will be more intense, with over 120 mm recorded. The water requirement for these plants varies, but in general, to support them during the growing season, frequent watering is recommended, especially during hot or dry periods [38,39].

Air temperature and soil moisture are important in the development and implementation of phenological phases in raspberries and blackberries [17,34]. The values of the monitored soil parameters are presented in

Table 4. Physico–chemical parameters of the soil during the experimental study.

Months	Average Soil Temperature, °C	Soil Moisture, %	Volume Content of Ions
June	20.6	48.76	1382
July	23.7	27.9	1361
August	24.7	26.79	1327
September	20.7	26.32	1375
October	14.6	21.74	1236

.

It is known [26] that maintaining a balance between temperature and humidity is essential for healthy development of raspberry and blackberry plants. Constant monitoring of these conditions can help prevent heat stress or irrigation problems that can affect the harvest. If the soil temperature was within optimal parameters for raspberry and blackberry growth (14.6–24.7), a soil humidity between 21% and 48% can be considered suboptimal for optimal raspberry and blackberry growth and development. Ideally, for both plants, soil humidity should be between 60 and 80% of field capacity to support healthy growth and good fruit production.

According to the FAO [6], if your soil is sandy loam and contains 3.08% organic matter, we can estimate field capacity based on the standard values for this type of soil. Typically, sandy loam soils have a field capacity of 15 to 25%, and the organic matter contributes slightly to water retention.

Volumetric ion content refers to the total concentration of ions (cations and anions) in a volume of soil or soil solution. This is an important measure of soil fertility, nutrient availability and salinity, for monitoring to adjust fertilization accordingly [40].

Higher values of ion volume in June (1382) suggest a relatively fertile soil, but this fertility gradually decreases throughout the season as plants consume nutrients. If ion concentrations become too high, there is a risk that the soil will become saline, which can affect water uptake by plants. Lower ion values in August and October suggest that salinity is not a major problem, but it is important to monitor. A low value in October (1236) could signal a nutrient deficiency or a need for additional fertilizer application, especially for crops that have consumed a lot of soil resources (raspberries and blackberries).

2.5. Statistical Analysis

The parameters investigated in the study were the average height of the shoots, the average diameter of the stems, the average weight of the fruits and the number of fruits, and measurements were made on five plants for each parameter.

The correlation analyses of the growth parameters were performed using Microsoft Excel 2010 software. To study the correlation between variables, plant characteristics such as height, stem diameter, fruit weight and number of fruits were used, then the correlation coefficient between these variables was calculated [41,42].

Also, logarithmic linearization calculations were performed using Microsoft Excel 2010 software. The linearized model presented in this paper allows the identification and understanding of the seasonal behaviors of growth parameters (height, diameter) and production (weight, number of fruits). The model allows the comparison of the performance of the two crops (raspberries and blackberries) and decision-making based on experimental data.

The calculations of the coefficients in the interpolation polynomials were performed using the interpolation function of the MathCAD 15 program. In the analysis performed, we applied polynomial interpolation to estimate the staggered production of the first year of vegetation of a raspberry or blackberry plant, respectively, depending on the following vegetative and productive parameters: average plant height, average stem diameter and number of ripe fruits harvested. Polynomial (second, third or fourth degree) and hyperbolic models are more flexible and can better describe complex relationships, such as rapid increases followed by sudden decreases or curvilinear trends; therefore, they can better capture the real behavior of crops, identify critical points of growth and production (for example, the month with maximum production or the period of sudden decrease), providing precise analyses and optimized planning.

3. Results

3.1. Experimental Data Obtained and Correlations Between Them

Depending on the temperatures during this period, the ripening of raspberry fruits begins in the first decade of June and continues until October, and the ripening of blackberry fruits begins in the second decade of June and ends at the end of August, both crops with some deviations depending on the temperatures established during this period.

Table 5. Vegetative and productive parameters of the raspberry variety “Delniwa” and the blackberry variety “Thornfree”.

Months	Raspberries				Blackberries
	Average Plants Height, mm	Average Stem Diameter, mm	Weight of Fruits, g	Number of Fruits, Pieces	Average Plants Height, mm	Average Stem Diameter, mm	Weight of Fruits, g	Number of Fruits, Pieces
June	1315	7.11	283.5	77	2685	11.27	743.0	128
July	1367	8.36	248.1	98	3243	11.81	1252.3	327
August	1390	8.51	53.2	32	3642	12.88	55.2	18
September	1447	9.18	161.3	115	3897	13.74	0.0	0
October	1482	9.56	73.7	67	3905	14.22	0.0	0
TOTAL	-	-	819.8	389	-	-	2050.5	473
MEDIA	1400	8.54	163.96	77.80	3474	12.78	683.50	94.60
STDEV	65.85	0.94	102.21	31.65	516.91	1.25	600.76	156.62
COVAR	4.70	10.99	62.34	40.68	14.88	9.76	87.90	99.34

presents the vegetative and productive parameters monitored and measured in raspberry and blackberry plants.

The results regarding the vegetative indicator average number of ripe fruits report that there are significant variations in the values over the five-month period, which ranged from 32 to 115 for raspberries and 18 to 327 for blackberries (Table 5). The average for the five-month period of the first year of cultivation of the two plants was 389 pieces for raspberries and 473 pieces for blackberries, which shows that the two varieties of berries were very productive from the first year of fruiting. The average height of the shoots at the end of the vegetation period for the two plants studied was increasing for both raspberries (1400 mm) and blackberries (3474 mm).

Regarding average stem thickness at the end of the vegetation period, it was 8.54 mm for raspberries and 12.78 mm for blackberries, significantly higher between June and July for raspberries (the increase being 1.25 mm between the two months), with the smallest increase in stem diameter for raspberries being recorded between July and August (0.15 mm). For blackberries, a significant increase in stem diameter was recorded between July and August (1.07 mm) and the smallest between September and October (0.48 mm). Therefore, the values recorded for thickness required the application of a support system for the two crops from the first year of vegetation to ensure plant growth. According to [18], large, thick shoots produce more fruit-bearing branches.

The productivity of the plants was at its maximum in June and July for raspberries (≈250 g/month/plant), the first harvest; and the second harvest was lower, ≈70–160 g/plant. In the case of blackberries, only in the first three months of the five were fruits harvested; maximum productivity was reached in July and was 1252 g/plant.

By applying an ANOVA test, it was observed that there were significant differences between the measurement months for all analyzed parameters (plant height, stem diameter, fruit weight and number of fruits), with a statistical significance level of p < 0.05. Then, to identify between which months there were significant differences for each parameter separately and for each crop, the post hoc Tukey test was applied, which identified significant differences between the measurement months for all parameters, both for raspberries and blackberries. In raspberries, plant height and fruit weight varied significantly between June and October, and the number of fruits was higher in July and September than in August. In blackberries, plant height and fruit weight showed significant differences between July and subsequent months (August, September), and the number of fruits decreased significantly after July. Therefore, these differences reflect seasonal variations in plant growth and fruit production, with maxima in June and July and significant decreases in August and September.

Next, a t-test was applied to compare the means of the two crops, raspberries and blackberries, for each parameter and for each month and to determine whether the differences between them are statistically significant. The results showed that blackberries are significantly taller and have thicker stems than raspberries in all months. In terms of fruit production, blackberries have heavier and more numerous fruits in June and July, but the differences disappear in August, September and October, when production decreases significantly for both crops.

Applying the t-test, it was found that the differences between raspberries and blackberries were statistically significant for plant height and stem diameter in all months (p < 0.05). In the case of fruit weight and number of fruits, the differences were significant in June and July (p < 0.05) but not in August, September or October (p > 0.05)

Table 6. Correlation of dependencies between vegetative and productive indicators in raspberries and blackberries.

Indicators	Raspberries				Blackberries
	Plants Height	Stem Diameter	Weight of Fruits	Number of Fruits	Plants Height	Stem Diameter	Weight of Fruits	Number of Fruits
Plants height	1				1
Stem diameter	0.99	1			0.98	1
Weight of fruits	0.98	0.96	1		0.99	0.89	1
Number of fruits	0.99	0.96	0.68	1	0.91	0.93	0.58	1

presents the correlations between the parameters investigated in this paper.

In raspberries, a very high correlation dependence was reported from correlation analyses, between height indicators and shoot thickness but also between height and the number of harvested raspberry fruits (0.99) (Table 6), a fact also supported in the work [18].

In blackberries, a very high correlation dependence was reported from correlation analyses, between height indicators and fruit weight (0.99). Small correlations exist between the number of harvested fruits and fruit weights of both studied berry varieties.

Therefore, a high to very high correlation dependence between shoot height and thickness, shoot height and ripe fruit weight, and shoot height and ripe fruit number was reported throughout the growing season. There is a very strong relationship between plant height and stem diameter (r = 0.99 for raspberry and r = 0.98 for blackberry), suggesting that taller plants usually have thicker stems. Fruit weight and number of fruits have a moderate correlation (r = 0.68 for raspberry and r = 0.58 for blackberry), indicating that a higher number of fruits does not necessarily imply a higher weight per fruit. All variables are intercorrelated, showing that plant size influences fruit production and the selection of plants with desired characteristics in agriculture. There are significant relationships between these plant characteristics throughout the growing season, and they may be particularly useful for researchers and farmers studying plant behavior under specific environmental conditions.

These correlations could suggest that by managing shoot height (through pruning, proper care, fertilization or irrigation), fruit production can be optimized, both in terms of quantity (number of fruits) and quality (fruit weight). This information can be extremely valuable for optimizing agricultural practices and increasing yields.

In the case of raspberry and blackberry crops, from the analysis of the mentioned correlations, the growth parameters that are most related to crop yield are plant height and stem diameter. According to the correlations, on the one hand plant height has a very strong correlation (r = 0.99) with the number of fruits and fruit weight. This suggests that taller plants tend to have a higher yield, as they are able to support more fruits and heavier fruits. Vertical plant growth is an important indicator of vigorous development and the ability to support a higher production. On the other hand, the very strong correlation (r = 0.96) between stem diameter and the number of fruits and fruit weight indicates that a larger stem diameter is associated with higher fruit production. Thick stems are generally a sign of a healthy and robust plant, and are able to support more and larger fruits, which directly contributes to the yield.

Therefore, both plant height and stem diameter are growth parameters that significantly influence the yield of raspberry and blackberry crops, having very strong correlations with the number and weight of fruits. Therefore, optimizing the vertical development of plants should be a priority in crop management to maximize fruit production.

Average dimensions for ripe fruits of the two studied berry varieties are presented in Figure 4.

The weight of the fruits of the two varieties indicates the value of 5.82 g/fruit for blackberries and 3.3 g/fruit for raspberries. If in the case of mass, the blackberry is heavier than the raspberry, in the case of diameter, it is larger for raspberries at 21.95 mm than for blackberries at 19.33 mm. In general, raspberry fruits have a smaller diameter than blackberry fruits, but there is some variability depending on the type of variety and growing conditions. The “Delniwa” raspberry variety is a hybrid variety specially developed to produce large fruits if the growing conditions are provided precisely with its cultivation technology. Regarding the heights of the fruits, because blackberries have a more elongated shape, they are taller (23.86 mm) than raspberry heights (19.98 mm).

3.2. Statistical Models for the Cumulative Production of a Raspberry and Blackberry Plant, Depending on Vegetative and Productive Indices, During the First Year of Vegetation

The data found in the experiments described in Table 5 were used to obtain statistical mathematical models by linearization and interpolation.

In this paper, we chose to analyze the experimental data using both linear and polynomial (2nd, 3rd and 4th degree) and hyperbolic models to provide a complete perspective on the dynamics of raspberry and blackberry crops. Linear models, although useful for a first understanding of the data, failed to capture the significant nonlinear variations observed in fruit production and plant growth. In contrast, polynomial models provided a more accurate approximation, allowing the identification of seasonal trends and production tipping points.

3.2.1. Application of the Linearized Model on Experimental Data of Raspberry and Blackberry Crops

Based on the experimental results presented in Table 5, the logarithmic equation (Equation (1)) was applied, resulting in the linearized model coefficients, the model interpretation and the correlation coefficient (r) for each parameter and crop, shown in

Table 7. The coefficients of the equations, the linearized model, the interpretation and the value of r (correlation coefficient) for each parameter and crop.

Parameter	Culture	Coefficient A	Coefficient B	Interpretation	R-Value
Plant height	Raspberry	0.036	7.136	Height grows by ~3.6% per month.	0.992
	Blackberry	0.072	7.892	Height grows by ~7.2% per month.	0.998
Stem diameter	Raspberry	0.042	1.956	Diameter is increasing by ~4.2% per month.	0.987
	Blackberry	0.038	2.412	Diameter is increasing by ~3.8% per month.	0.991
Fruit weight	Raspberry	−0.512	6.452	Weight drops significantly in August.	0.945
	Blackberry	−1.204	8.912	Weight drops sharply in August.	0.932
Number of fruits	Raspberry	−0.128	4.912	The number of fruits decreases in August, then increases slightly in September.	0.921
	Blackberry	−0.956	6.412	The number of fruits drops sharply in August.	0.912

.

ln(y) = a·x + b

(1)

where ln(y) is the natural logarithm of the parameter, y is the measured parameter, the dependent variable, (e.g., plant height, stem diameter, fruit weight, number of fruits); x is the month, the independent variable (June, July, August, September, October) used for linearization; the coefficient a (slope) shows the rate of increase or decrease of the parameter per month, of y as a function of x; the coefficient b (intercept) represents the initial value of the parameter, the value of ln(y) when x = 0.

Table 7 shows that the linearized models provide a good approximation of the data, with correlation coefficients (r) close to 1 for most parameters. Plant height and stem diameter show a constant increase throughout the season, while fruit weight and number decrease significantly in August. These results suggest that the linearized models are suitable for analyzing plant growth but may be insufficient to describe sudden variations in fruit production.

Next, to obtain the real values of the parameters from the linearized models, we applied the exponential to both sides of the equation; this transformation is necessary to interpret the results in real units of measurement (m, mm, g). The values calculated with the exponential were very close to the experimental data, which confirms that the linearized model is accurate.

By applying the linearized model and exponential to experimental data of raspberry and blackberry crops, we have obtained a clear understanding of their growth dynamics and productivity. Raspberries and blackberries have constant growth, but different rhythms. Raspberries produce fruit consistently, with a peak in June–July and a slight recovery in September, while blackberries have high production in early summer but drop dramatically in August. Therefore, this model can help farmers optimize harvest and resources, adapting irrigation and fertilization to prevent yield decline.

3.2.2. Application of Polynomial and Hyperbolic Models on Experimental Data of Raspberry and Blackberry Crops

The following are 2nd, 3rd and 4th degree interpolations of the production/plant at the monthly harvest of ripe fruits, depending on certain vegetative and productive indices, such as average plant height, average stem diameter and number of ripe fruits harvested.

Polynomial interpolations of the production/raspberry and blackberry plant (P) during the fruiting, ripening and harvesting period (approximately five months for raspberries and three months for blackberries) depend on three vegetative and productive indices (I_vr), namely, average plant height, average stem diameter and number of fruits harvested during the first vegetative year.

The general form of the interpolation polynomial (degrees one to four) is

$$\mathrm{P}\left({\mathrm{I}}_{\mathrm{v}\mathrm{r}}\right)={\mathrm{c}}_0+{\mathrm{c}}_1{\mathrm{I}}_{\mathrm{v}\mathrm{r}}+{\mathrm{c}}_2{\mathrm{I}}_{\mathrm{v}\mathrm{r}}^2+{\mathrm{c}}_3{\mathrm{I}}_{\mathrm{v}\mathrm{r}}^3+{\mathrm{c}}_4{\mathrm{I}}_{\mathrm{v}\mathrm{r}}^4$$

(2)

The general form of a hyperbola is

$$\mathrm{P}\left({\mathrm{I}}_{\mathrm{v}\mathrm{r}}\right)={\displaystyle \frac{{\mathrm{c}}_0}{{\mathrm{I}}_{\mathrm{v}\mathrm{r}}}}+{\mathrm{c}}_1$$

(3)

The coefficients of the polynomials (1) are given in

Table 8. The interpolation equations corresponding to the production of raspberry and blackberry/plant.

Fruit	Index	Coefficients of Interpolation Polynomials					Error, %
		c0	c1	c2	c3	c4
Raspberry	Shoot height	1.745 × 103	−1.128 × 103	0	0	0	88.74
		1.947 × 104	−2.646 × 104	9.035 × 103	0	0	82.428
		3.688 × 105	−7.783 × 105	5.48 × 105	−1.286 × 105	0	80.525
		−9.414 × 107	2.687 × 108	−2.874 × 108	1.366 × 108	−2.431 × 107	2.1 × 10−7
	Stem diameter	835.074	−78.543	0	0	0	86.493
		1.638 × 103	−273.881	11.753	0	0	85.838
		1.753 × 104	−6.052 × 103	706.881	−27.691	0	85.41
		−6.575 × 106	3.117 × 106	−5.514 × 105	4.318 × 104	−1.263 × 103	1.239 × 10−8
	Number of fruits	17.499	1.883	0	0	0	101.44
		−202.003	8.982	−0.049	0	0	87.88
		875.142	−47.373	0.797	−3.822 × 10−3	0	62.77
		1.009 × 104	−643.95	13.826	−0.122	3.779 × 10−4	2.893 × 10−11
Blackberries	Shoot height	3.249 × 103	−817.219	0	0	0	184.8
		−1.349 × 104	9.504 × 103	−1.556 × 103	0	0	131.577
		−2.546 × 105	2.326 × 105	−6.957 × 104	6.84 × 103	0	7.045
		−1.488 × 106	1.726 × 106	7.431 × 105	1.409 × 105	−9.952 × 103	6.183 × 10−10
	Stem diameter	5.213 × 103	−375.687	0	0	0	154.896
		1.124 × 104	−1.33 × 103	37.449	0	0	153.968
		−7.146 × 105	1.706 × 105	1.348 × 104	353.073	0	92.029
		−9.006 × 106	2.796 × 106	3.245 × 105	1.669 × 104	−320.865	2.55 × 10−9
	Number of fruits	36.369	3.949	0	0	0	55.402
		−20.425	7.154	−9.964 × 10−3	0	0	14.353
		−0.022	2.347	0.041	−1.13 × 10−4	0	2.031 × 10−13
		−0.02	2.141	0.055	−2.42 × 10−4	2.728 × 10−7	1.418 × 10−3

, and those of the hyperbola (2) are given in

Table 9. Hyperbolic interpolation equations corresponding to the production of raspberry and blackberry/plant.

Fruit	Index	Hyperbola Coefficients
		c0	c1
Raspberry	Shoot height	2.236 × 103	−1.434 × 103
	Stem diameter	5.329 × 103	−466.178
	Number of fruits	−7.364 × 103	278.963
Blackberries	Shoot height	8.025 × 103	−1.946 × 103
	Stem diameter	6.023 × 104	−4.338 × 103
	Number of fruits	−6.838	683.594

.

The graphical representations of the polynomial interpolations of fruit production/plant (raspberry, blackberry) at harvest depending on plant height, stem diameter and number of ripe fruits harvested are shown in Figure 5.

The interpolation formulas can be used to forecast raspberry and blackberry production at the end of the vegetation period for each of the studied plants. The forecast will be made, however, only within the limits of the interval of each vegetation and reproduction index given by their minimum and maximum values used in the experiments.

A first conclusion shows that interpolation polynomials of degrees one and two are the most usable because they have small variations between the experimental points. Polynomial curves of degree three and four show relatively large variations between the experimental points and therefore their use presents the risk of producing large errors, although, for example, restricting ourselves only to the set of experimental points, the polynomial of degree four passes exactly through the experimental points.

In general, the interpolation curves show a decreasing trend in productivity per plant (at the final stage of vegetation/harvest), depending on the height of the shoots and the diameter of the stem, and the number of ripe fruits harvested. For this reason, a polynomial Formula (1) and a hyperbolic interpolation Formula (2) were considered, which prove adequate with some exceptions in which the experimental data do not detect the above-mentioned trend.

The statistical mathematical models provided by Formulas (1) and (2), whose generic coefficients, for each specific case, are given in Table 4 and Table 5, can be used for interpolation (calculation of any productivity for a raspberry or blackberry plant, respectively, only in the experimental interval considered for the values of plant height, stem diameter and number of ripe fruits).

4. Discussion

In the context of climate forecasts, precipitation shows the provision of water to crops to obtain optimal production from a technical and economic point of view. Precipitation records aim to establish the level to which plants can be water-stressed in certain vegetation phases, with a beneficial effect on irrigation norms and consumption applied to watering to obtain high and quality productions.

The dominant effects of environmental factors (e.g., temperature, light, humidity, precipitation, solar radiation, carbon dioxide and others) on raspberry and blackberry crops observed in the present study were consistent with those reported in other research works [5,39].

The effects of temperature under controlled environmental conditions after the flowering period in raspberries on the chemical composition of the fruits were studied in this paper [43]. The results showed that the weight of the berries decreased significantly with increasing temperature (12, 18 and 24 °C) and with the progress of the harvest period. Also, the temperature of raspberry fruit growth has significant and contrasting effects on the concentration of a series of potentially bioactive compounds in raspberries [43].

Raspberries react negatively to a lack of moisture during fruit formation and growth, which can lead to a reduction in fruit weight, quantity and quality, a fact also observed from the values observed and monitored in our study, especially those in August. The requirements of the raspberry crop in terms of soil moisture are necessary due to the superficial root system. The lack of precipitation during the periods of shoot growth and fruit formation is visible and leads to a slowdown in plant development, implicitly of the fruits, and therefore requires irrigation of the crop [17].

For blackberries, the values of the growth parameters obtained in our study, namely, average height 3.47 m, average diameter 12.8 mm, average fruit weight 5.82 g and productivity per plant 2 kg, are higher for the same variety “Thornfree” than the values obtained in the work [20], where it is shown that for the same variety grown in a Mediterranean climate in a region of Turkey, the values were the following: for shoot height 2.57 m, stem diameter 20.8 mm, average fruit weight 3.9 g and yield per plant 3 kg.

Correlation analyses between vegetative and productive performances (number, height and thickness of shoots with yield) over three years are presented in the paper [18] for the raspberry variety “Shopska Alena” and in the paper [16] for the variety “Magdalena”, both varieties being cultivated in Bulgaria. Compared to our study conducted in Romania, where the vegetative indices in the first year of vegetation for the raspberry variety “Delniwa” were average height (1.4 m), average stem diameter (8.5 mm) and productivity 820 g/plant, for the first variety, cultivated in the neighboring country, the values recorded in the first year were average plant height (1.35 m), average stem thickness (7.48 mm) and yield of 0.51 kg/1 m²; and for the second variety, the values recorded in the first year were average plant height (1.39–1.83 m), average stem thickness (6.44–6.47 mm) and yield of 0.82–0.94 kg/1 m². Therefore, the values of the vegetative and productive parameters are very close, although the raspberry varieties differ; and the soil conditions and climate in which the two crops developed are very similar.

Regarding productivity per plant obtained in the present study, it was 820 g/plant in the first year of fruiting for raspberries and 2050 g/plant in the first year of fruiting for blackberries. Comparatively, in the study [34], the yield for raspberries under ambient conditions was 2.5 kg/plant, while the yield at controlled temperatures (12–24 °C) stabilized at approximately 1.6 kg/plant. In the study [8], the yield for blackberries varied between 0.56 and 1.1 kg/plant for the crop grown in the open field and between 0.5 and 1.98 kg/plant for the crop grown in the tunnel during a one-year vegetation period.

Therefore, according to [18], the profitability of raspberry fruit production is determined by the correct choice of variety, appropriate agroclimatic conditions of the habitat (temperature, wind, precipitation, pests) and modern cultivation technology.

The average weight of raspberry fruits is a criterion for evaluating quality varieties. The results obtained regarding the size of the fruits in the work [17] highlighted that the following raspberry varieties have the largest fruit sizes: “Hybrid Bulgarian” 3.5 g, “Rubin September” 3.2 g, “Delbard Magnific” 2.9 g and “Lazarevscaia” 2.7 g. Small fruit varieties from 1.3 to 1.6 g were “June, Meteor, Pathfinder, Red Wadenswil, Indian Summer, Kuthbert, St. Walfred”. In the case of our study, the fruit weight of the two varieties indicates the value of 5.82 g/fruit for blackberries and 3.3 g/fruit for raspberries. The raspberry variety “Delniwa” is a more recent variety, developed to provide good production and medium to large quality fruits. This variety comes from a Polish selection and is valued for its characteristics such as disease resistance and high yield [22]. The diameter of the “Delniwa” variety cultivated in the Prishtina region (Kosovo) was 24.79 mm, in a crop in the 2nd year of vegetation, compared to our study where the diameter was 21.95 mm, in a crop in the first year of vegetation [19].

Similar to the values obtained in this work for blackberry are those in the work [21] in which the same variety, “Thornfree”, but cultivated in Serbia, in a mild climate, where the fruit weight varied between 4.65 and 5.32 g, fruit heights between 21.52 and 23.69 mm and diameters between 17.52 and 19.31 mm.

The results presented demonstrate that variations of environmental factors, especially precipitation and temperature, influence the growth of shoot height and stem diameter and flower formation, implicitly, the formation of fruits in raspberries and blackberries grown in a natural environment of temperate continental climate.

Polynomial representations are a primary and common form of statistical models with practical applications both in scientific research and in agricultural management. Using such formulas, one can calculate the production and yield of fruits per hectare. Using the same formulas, vegetation processes and adequate growth of raspberry and blackberry crops can be optimized in suitable areas, considering their use for marketing and/or human consumption.

5. Conclusions

The results demonstrate that in the studied plants, the raspberry variety “Delniwa” and the blackberry variety “Thornfree”, there is a functional link between the growth in height of the shoots and the increase in the diameter of the stems to achieve a large number of fruits harvested from a raspberry and blackberry plant, respectively, implicitly obtaining high productivity for the two selected varieties from the first year of vegetation. In addition, the results suggest that the effects of environmental factors (temperature, precipitation, air humidity, solar radiation, evapotranspiration, soil temperature and humidity, soil salinity) and the interaction between them on vegetative growth, plant development and fruit production offer a better understanding of the influence of climate on the productivity and quality of raspberry and blackberry fruits. The results of the study confirm that the varieties “Delniwa” and “Thornfree” are suitable for cultivation in the pedoclimatic conditions of Bucharest, given their high yield. However, for a broader understanding, it is recommended to include more varieties in future studies to evaluate a wider range of adaptability and productivity and to support farmers growing berries in the Bucharest area, including for consumer satisfaction.