Yield and Nutrient Concentrations of Kohlrabi Bulbs and Leaves as Affected by Spring Transplanting Dates

Abstract

Transplanting kohlrabi (Brassica oleracea var. gongylodes), a cool-season vegetable crop, in early spring may provide the opportunity for double cropping in short-season regions such as the Northeastern United States. A two-year field study evaluated the impacts of transplanting dates on yield and nutrient concentration of kohlrabi. Additionally, this study aimed to quantify the nutritional value of kohlrabi leaves. The yield of kohlrabi increased by as much as 307 kg ha⁻¹ for each day transplanting was delayed. Soil temperature increased by 2.8 °C between April 23 and May 14, resulting in the increased accumulation of Ca, Cu, Mn, and Fe in kohlrabi bulbs and Ca and Cu in leaves. The nutrient concentration in leaf and bulb were positively correlated, indicating that the two commodities can be simultaneously harvested for optimum quality. Leaf yield was not significantly different among transplanting dates. However, the number of leaves and total leaf area increased with delayed transplanting. Leaf yield and leaf area were not correlated with bulb yield, suggesting that the reductions in yield and nutrient concentrations were unrelated to photosynthetic efficiency. Although the earlier transplanting of kohlrabi may have provided opportunities for double cropping, the yield and nutrient accumulation of kohlrabi transplanted early in the spring were considerably compromised.

1. Introduction

Kohlrabi (Brassica oleracea var. gongylodes) is a cool-season, biennial vegetable revered for its mildly sweet flavor and many health benefits. Kohlrabi is known by many names around the world and is cultivated as an annual crop in many parts of Europe, Asia, and North America. Like other cruciferous vegetables, kohlrabi is a potent source of essential nutrients and bioactive compounds produced by plant secondary metabolites [1,2]. The regular consumption of kohlrabi can aid in reducing the risk of chronic disease, depressive disorders, and many types of cancer [2,3,4,5].

Kohlrabi is primarily cultivated for its round, thickened stem, often referred to as the ‘bulb’, which resembles a turnip at maturity. Cultivars can be white, pale green, or purple and differ significantly in size and storage ability. Fresh market spring varieties are typically harvested when bulb diameter reaches 6–7 cm, while fall storage varieties can reach up to 20 cm in diameter by the time they are harvested and be stored for up to four months in proper conditions. The bulbs, similar to radish in consistency but slightly sweeter in taste, can be cooked, pickled, or consumed raw in salads [1]. The flavorful leaves of kohlrabi, comparable to kale in appearance and texture, are also highly nutritious and can provide an additional commodity to growers and consumers.

Secondary metabolites found in kohlrabi, including anthocyanins, phenolic compounds, and glucosinolates contribute to its antioxidant and anti-inflammatory properties [4,6]. Unlike pale green varieties, purple kohlrabi cultivars contain anthocyanins, bioactive flavonoids important for human health, as well as higher concentrations of phenolic compounds resulting in greater anti-inflammatory and antidiabetic effects [7,8,9]. While the majority of research has focused on the health benefits of kohlrabi bulbs, the antibacterial and antioxidant potential of kohlrabi leaves have also been documented [10,11]. More recently, kohlrabi sprouts have been found to contain high concentrations of bioactive compounds, including glucosinolates and fatty acids [12].

In northern climates, kohlrabi can be planted in the spring as a fresh market crop or in late summer for fall harvest and winter storage. Seedlings are typically started in the greenhouse and transplanted into the field after 4–5 weeks [13]. The impacts of plant spacing, variety selection, and fertilizer application rates on growth, yield, and quality parameters of kohlrabi have been extensively studied [14,15,16,17,18]. However, the optimum time for transplanting kohlrabi, specifically in the Northern United States, has not been reported. It is well-established that the date of planting can significantly impact crop yield [19,20,21,22,23,24]. The growing season in the Northeastern United States is relatively short; therefore, identifying cold-tolerant crops for early spring planting can extend the growing season and provide an opportunity for double cropping. Although kohlrabi growing guidelines were recently added to the New England Vegetable Management Guide (the primary vegetable crop reference in the Northeast), the optimum time for spring transplanting has not been specified. Thus, it is necessary to evaluate the impacts of the spring transplanting date on the yield and quality of kohlrabi in order to optimize land use efficiency and crop productivity. Kohlrabi is a cool-season vegetable, and we hypothesized that the early spring planting of kohlrabi may result in earlier harvest, thus extending the growing season by several weeks.

The goals of this study were to (1) evaluate the impacts of spring transplanting dates on kohlrabi yield and nutrient accumulation in its bulbs, and (2) assess the nutritional value of kohlrabi leaves as an additional commodity by comparing their nutrient content to kale (Brassica oleracea var. sabellica), a popular fresh leafy vegetable.

2. Materials and Methods

2.1. Experimental Site and Weather Conditions

A two-year field experiment (2020–2021) was conducted at the University of Massachusetts Crop, Animal, Research, and Education Center, located in South Deerfield, MA, USA (42° N, 73° W). The soil at this location is characterized as a coarse–silty, mixed, non-acid, mesic Typic Udifluvent (Hadley series). In both years, a composite soil sample was taken at a depth of 15 cm from the field site to ensure that P, K, Ca, and Mg levels were in the optimum range for kohlrabi production. Relevant weather conditions, including annual precipitation and growing degree days during the experiment period and the norm for the experimental site, are presented in the results section.

2.2. Field Experiment Design and Implementation

Four spring dates of transplanting (DOT) were evaluated in this experiment: 23/4, 30/4, 07/5, and 14/5. Experimental plots were laid out in a randomized complete block design with four replications. An early white variety of kohlrabi, Beas, was seeded into plastic trays and transplanted after four weeks in the greenhouse into heavyweight, organic, certified paper mulch (Figure 1). Plants were spaced 15 cm apart, with 30 cm spacing between the rows. All plots were irrigated throughout the season using drip irrigation. Rows of kohlrabi were covered with heavyweight, transparent row cover at the time of planting to prevent insect damage. 180 kg ha⁻¹ of N fertilizer was applied in the form of urea ammonium nitrate (UAN, 32%) as a split application, with all plots receiving 112 kg ha⁻¹ at the time of planting and the remainder receiving 112 kg ha⁻¹ two weeks later.

2.3. Data Collection

Kohlrabi plants were harvested when 50% of the bulbs in each DOT treatment reached 6.4 cm in diameter. Yield was determined by measuring the fresh leaf and bulb weight (kg) of ten randomly chosen plants per plot immediately after harvest. Two plants from each plot were randomly selected for further analysis. Leaves were removed from bulbs, and fresh weight of bulbs and leaves were separately determined. The leaf area (cm²) of each plant was measured using a LI-3100C Area Meter (LI-COR, Lincoln, NE, USA). Total leaf area was calculated by adding the areas of all leaves per plant. Leaf and bulb samples were dried in a forced air oven at 109 °C until they maintained a constant weight. Moisture content of leaves and bulbs was calculated by subtracting the dry weight value from the fresh weight.

2.4. Nutrient Analysis

The nutrient content of kohlrabi bulbs was determined using a dry ashing procedure. Dried bulb and leaf tissue samples were ground in a stainless steel container using a Vitamix 5200 high-power blender to pass through a 20-mesh sieve (Dual Manufacturing Company, Inc., Franklin Park, IL, USA) and homogenized. An amount of 0.2 g of the powdered samples were weighed into porcelain crucibles and placed into a combustion oven at a temperature of 500 °C for 6 hours. Afterwards, the crucibles were allowed to cool to room temperature and 15 ml of 10% HCl was added to each sample. The resulting mixture was filtered through Whatman #2 filter paper. Finally, the Cu, Mn, Fe, Ca, K, and Mg concentrations of each sample were quantified using microwave plasma-atomic emission spectroscopy (4210 MP-AES, Agilent Technologies, Santa Clara, CA, USA).

2.5. Statistical Analysis

Statistical analyses were performed using the GLM and CORR procedures in SAS, version 9.4 (SAS Institute, Cary, NC, USA). The two experimental years were combined for analysis, resulting in eight total replications. Effects that were significant at the p < 0.05 level were fitted to regression curves.

3. Results

3.1. Statistical Analysis

The analysis of variance results for bulb yield and nutrient content as affected by the date of transplanting are shown in

Table 1. Analysis of variance results for yield and nutrient content of kohlrabi bulbs, as affected by date of transplanting.

	Yield	Nutrient
		Cu	Mn	Fe	Ca	K	Mg
DOT	<0.0001	<0.0001	<0.0001	0.0026	0.0118	0.1236	0.1150

DOT = date of transplanting.

. The analysis of variance results for leaf yield, total leaf area, leaf number, and leaf nutrient content as affected by the date of transplanting are shown in

Table 2. Analysis of variance results for yield, leaf number, total leaf area and nutrient content of kohlrabi leaves, as affected by date of transplanting.

	Yield	Leaf Number	Leaf Area	Nutrient
				Cu	Mn	Fe	Ca	K	Mg
DOT	0.1056	0.0007	0.0083	0.1311	0.0104	0.1216	0.0196	0.9495	0.2386

DOT = date of transplanting.

.

3.2. Weather Conditions

Growing degree days (GDD) and total precipitation during the growing periods in 2020 and 2021 are presented in

Table 3. Weather conditions and 20-year norm at the experimental site.

	GDD (Base 4.4 °C)			Precipitation (cm)
	2020	2021	Norm	2020	2021	Norm
23–30 April	46	81	62	4.1	5.4	2.2
May	556	570	493	5.6	11.8	8.0
June	870	955	733	4.4	5.5	11.7
1–13 July	461	--	368	2.6	--	3.1
Total	1933	1606	1656	16.7	22.7	25

Norm values were calculated based on monthly averages measured at the Orange Municipal Airport weather station in Orange, MA, USA; GDD = growing degree days.

, alongside twenty-year averages (2000–2020) to represent the regional norm. The presented norm values were obtained from the Orange Municipal Airport weather station in Orange, MA, USA, approximately 32 km from the experimental site.

Overall, the 2021 growing season was hotter and wetter than 2020 and the 20-year norm for the experimental site. As a result, plants were harvested approximately two weeks earlier than in 2020. The seeding, transplanting, and harvesting dates for both growing seasons are presented in

Table 4. Time of seeding, transplanting, and harvesting of kohlrabi during the two growing seasons.

	2020				2021
	Seeded	Transplanted	Harvested	DTH	Seeded	Transplanted	Harvested	DTH
DOT 1	25 March	23 April	22 June	60	25 March	23 April	21 June	59
DOT 2	4 April	30 April	22 June	53	4 April	30 April	21 June	52
DOT 3	14 April	7 May	1 July	55	14 April	7 May	24 June	48
DOT 4	24 April	14 May	13 July	60	24 April	15 May	30 June	47

DOT = date of transplanting; DTH = days to harvest.

.

Soil temperature at the time of transplanting is presented in

Table 5. Regional soil temperatures for Deerfield, MA.

Soil Temperature °C
DOT	2021	5-Year Average	10-Year Average
23/4	12.2	12.0	11.0
30/4	13.3	11.3	11.8
07/5	13.3	14.0	14.5
14/5	15.0	12.11	13.9

DOT = date of transplanting.

. In 2021, the soil temperature at DOT 4 was 1.1 °C warmer than DOT 2 and DOT 3, and 2.8 °C warmer than the first date of transplanting.

3.3. Bulb and Leaf Yield

The date of transplanting had significant effects on both the total yield and bulb yield of kohlrabi. However, leaf yield was not statistically significantly different among DOT. Bulb yield, and consequently total yield, increased with later transplanting. Kohlrabi transplanted on 14 May produced the highest bulb yields, equal to 20,297 kg ha⁻¹ (Table 4 and Figure 2).

3.4. Leaf Number and Total Leaf Area

The date of transplanting had a significant effect on the average number of leaves per plant, as well as the total leaf area and the cumulative area of all leaves per plant (Figure 3). While the number of leaves increased as DOT was delayed, the total leaf area demonstrated a parabolic response and reached its highest value in DOT 3 (7 May). Correlations between bulb yield, leaf yield, leaf area, and leaf number are presented in

Table 6. Pearson correlation matrix of bulb yield, leaf yield, leaf area and leaf number. Results represent the averages of two growing seasons.

	Leaf Yield	Leaf Area	Leaf Number
Bulb Yield	−0.016 ns	0.047 ns	0.470 **
Leaf Yield		0.560 ***	0.310 ns
Leaf Area			0.653 ***

**, *** Significant at the 0.01 and 0.001 probability levels, respectively; ns = non-significant.

.

3.5. Nutrient Concentration

The date of transplanting significantly impacted kohlrabi nutrient concentrations, including the Mn, Fe, Cu, and Ca in bulbs, as well as Ca and Mn concentrations in leaves. The mean concentrations of nutrients in kohlrabi bulbs and leaves for each DOT are shown in

Table 7. The influence of DOT on nutrient concentrations of kohlrabi bulbs and leaves.

	Bulb Nutrient Concentration mg g−1						Leaf Nutrient Concentration mg g−1
DOT	Ca	K	Mg	Cu	Fe	Mn	Ca	K	Mg	Cu	Fe	Mn
23/4	7.136	55.098	2.450	0.004	0.033	0.011	32.216	32.682	3.284	0.005	0.068	0.033
30/4	7.277	59.311	2.580	0.005	0.039	0.013	30.862	31.803	3.587	0.005	0.068	0.039
07/5	7.233	56.025	2.568	0.005	0.043	0.013	33.115	32.836	3.720	0.005	0.061	0.043
14/5	7.993	60.367	2.827	0.006	0.057	0.016	35.595	32.729	3.670	0.005	0.074	0.057
Trend	L **	NS	NS	L **	L **	C **	L *	NS	NS	NS	NS	L **

DOT = date of transplanting; L = linear, C = cubic; *, **, significant at the 0.05, and 0.01 probability level, respectively; ns = non-significant.

. Among the measured nutrients, Mg and K concentrations in both bulbs and leaves were not affected by DOT. For all other nutrients in which DOT had a significant effect, higher concentrations were detected in the later DOTs. Correlations among the nutrients in bulbs and leaves are presented in

Table 8. Pearson correlation matrix of nutrients in bulbs and leaves of kohlrabi. Results represent the average of two growing seasons.

	Bulb K	Bulb Mg	Bulb Cu	Bulb Fe	Bulb Mn	Leaf Ca	Leaf K	Leaf Mg	Leaf Cu	Leaf Fe	Leaf Mn
Bulb Ca	0.725 ***	0.786 ***	0.636 ***	0.562 ***	0.403 *	0.600 ***	0.149 ns	−0.007 ns	0.565 ***	0.040 ns	−0.002 ns
Bulb K		0.733 ***	0.381 *	0.508 **	0.196 ns	0.416 *	0.443 *	−0.127 ns	0.283 ns	−0.121 ns	−0.199 ns
Bulb Mg			0.640 ***	0.405 *	0.477 **	0.550 **	0.319 ns	0.373 *	0.457 **	0.144 ns	0.054 ns
Bulb Cu				0.620 ***	0.664 ***	0.488 **	0.112 ns	0.168 ns	0.495 **	0.041 ns	0.041 ns
Bulb Fe					0.467 **	0.388 *	0.029 ns	−0.217 ns	0.237 ns	0.202 ns	0.192 ns
Bulb Mn						0.379 *	−0.042 ns	0.308 ns	0.449 **	0.297 ns	0.605 ***
Leaf Ca							0.224 ns	0.273 ns	0.257 ns	0.073 ns	0.243 ns
Leaf K								−0.082 ns	0.221 ns	−0.420 *	−0.290 ns
Leaf Mg									0.263 ns	0.236 ns	0.276 ns
Leaf Cu										0.091 ns	0.104 ns
Leaf Fe											0.485 **

*, **, *** Significant at the 0.05, 0.01, and 0.001 probability level, respectively; ns = non-significant.

.

4. Discussion

The growing season in the Northeastern United States is relatively short, resulting in limited opportunities for double cropping. As a cool-season vegetable crop, kohlrabi can potentially be planted as early as the beginning of April, making it a promising candidate for double cropping. This study evaluated the impacts of spring transplanting dates on the yield and nutrient concentration of kohlrabi bulbs and leaves. Higher yields and nutrient concentrations were observed in kohlrabi transplanted in May rather than in April. Earlier spring transplanting allowed for an earlier kohlrabi harvest but resulted in a considerable yield penalty. Averaging two growing seasons, we found bulb yield increased by 307 kg ha⁻¹ for each day that transplanting was delayed, beginning 23 April. This delay in transplanting also resulted in a greater accumulation of nutrients in both the bulbs and leaves of kohlrabi. Among the measured nutrients, the accumulation of Ca, Cu, Fe, and Mn in bulbs, as well as Ca and Mn in leaves, increased with the delay in transplanting time.

Although the number of leaves per plant increased with the time of transplanting, the total leaf area was highest in the two intermediate transplanting dates: 30 April and 7 May. Leaf yield was not significantly different among transplanting dates and showed no significant correlation with leaf number. This suggests that, although there are more kohlrabi leaves per plant at later dates of transplanting, they are generally smaller and lighter than the leaves of those transplanted earlier in the spring. The number of leaves showed a moderate correlation with bulb yield, suggesting that leaf position may play a greater role in photosynthetic efficiency than the total leaf area.

Lower yields and nutrient concentrations of kohlrabi transplanted earlier in the spring can likely be explained, at least in part, by cooler soil temperatures in earlier DOTs (Table 3). Low root zone temperatures have been shown to negatively impact yield, growth rate, and nutrient accumulation in vegetable crops, including cucumber, red leaf lettuce, tomato, and several brassica species [25,26,27,28,29,30]. Chinese broccoli (Brassica oleracea var. alboglabra) grown at 10 °C root zone temperature, as compared to 20 °C, resulted in a 26% reduction in yield and accumulated less K, Ca, and Mg in the leaves [30].

It has been well-documented that low soil temperatures reduce water absorption by crop roots and hinder plant growth by limiting respiration, and thus, the metabolic activity of root cells [31,32,33,34]. However, the specific mechanisms by which low soil temperature impacts plant physiology vary by crop species. In red leaf lettuce, decreased root oxygen consumption caused by low root zone temperatures (10 °C) led to oxidative stress in the leaves, resulting in reduced final yield [20]. Low root zone temperatures have been shown to cause damage to photosystem II in African snake tomato, resulting in photochemical inhibition and a decreasing net photosynthetic efficiency [35]. By contrast, root zone temperature has been shown to have no direct effect on the photosynthesis of Brassica rapa, despite negatively impacting crop growth rate and biomass accumulation [36,37,38]. The negative impacts of low root zone temperatures on Brassica species can likely be explained by a decreased nitrate uptake efficiency, and water and solute flow rates through the roots [31,39]. In the present study, neither kohlrabi leaf yield nor leaf area were significantly correlated with bulb yield, indicating that photosynthetic efficiency was sufficient across all transplanting dates. Additionally, there were no statistically significant differences in specific leaf area among transplanting dates, further suggesting that the reductions in yield and nutrient concentrations in earlier transplanting dates were unrelated to photosynthetic efficiency.

Although kohlrabi is primarily grown for its enlarged, rounded stems, the leaves of kohlrabi can provide an additional commodity to growers, reduce processing labor, and increase the variety of nutrient-dense leafy greens available to consumers.

Table 9. Comparison of selected nutrient content of leaves of kohlrabi (Brassica oleracea var. sabellica) and kale (Brassica oleracea var. sabellica).

Leaf Nutrient Content (mg 100 g−1 Fresh Weight)
	Kohlrabi	Kale *
Ca	427.00	254.00
K	393.00	348.00
Mg	44.00	29.00
Cu	0.06	0.05
Fe	0.88	1.60
Mn	0.47	0.92

* Kale nutrient content was obtained from the FoodData Central USDA database (USDA, 2019). Kohlrabi nutrient content was obtained from oven-dried leaves and then adjusted to 88% moisture content. The presented nutrient contents of the kohlrabi leaves are based on the values measured in the latest date of transplanting (DOT).

demonstrates a comparison between the nutrient content of fresh kohlrabi leaves and those of kale, which are comparable in taste and texture.

As shown in Table 9, the concentrations of macro minerals (Ca, Mg, K) are higher in kohlrabi leaves, with the concentrations of Ca and Mg over 1.5 times higher than the concentrations found in those of kale. By contrast, kale leaves are generally richer in micronutrients than kohlrabi leaves. These findings suggest that kohlrabi leaves can be consumed as a rich edible source of nutrients and provide an additional commodity to growers who may wish to market both the bulbs and fresh leaves of kohlrabi. Except for Fe, a highly immobile nutrient, concentrations of nutrients in bulbs and leaves were positively correlated, suggesting that nutrients acquired by the plant are consistently distributed throughout the plant and that higher kohlrabi bulb nutrient concentrations correspond to higher nutrient concentrations in the leaves. Therefore, the optimum time of harvest is the same for leaves and bulbs, allowing them to be simultaneously harvested and marketed.

5. Conclusions

The results of the current study revealed that although the early transplanting of kohlrabi provides an opportunity for earlier harvest, and thus double cropping, this advantage comes with significant reductions in the yield and nutritional value of both bulbs and leaves. Therefore, in northern climates, transplanting kohlrabi in May rather than April optimizes crop yield and quality. Further research is needed to better understand the exact mechanisms by which kohlrabi yield responds to a low soil temperature in early DOTs. Additionally, this study confirmed kohlrabi leaves to be a promising fresh market commodity by comparing their nutrient content with related brassica leaves such as kale, a popular leafy vegetable known for its health benefits.