Preemergent Liverwort Control by Organic Mulching in Containerized Ornamental Production

Abstract

Liverwort (Marchantia polymorpha) competes for resources within containers, resulting in a reduction in the quality and market value of ornamentals. Therefore, the objective of this study was (1) to assess the impact of different mulch types, depths, and their moisture-holding-capacity on liverwort control and (2) to quantify if phytotoxicities develop on ornamental plants due to the mulches. The percent water retention of four different organic mulches [rice hull (RH), cocoa hull (CH), pine bark (PB), or red hardwood (HW)] was determined in a laboratory experiment. In a greenhouse experiment, the Hosta plantaginea (Plantain Lily) varieties ‘Curly Fries’ and ‘Pandora’s Box’ were mulched with either RH, HW, CH, or PB at a depth of 0.63, 1.27, 2.54, or 5.08 cm. Liverwort thalli coverage on the container surface was visually estimated bi-weekly, and the fresh weight of the thalli was recorded at the end of the experiment. The results indicated that CH mulch retained the highest amount of moisture (86%). The RH and HW mulches, at depths of 1.27 cm or more, provided excellent (>80%) liverwort control in ‘Curly Fries’. All mulches at depths of 1.27 cm or more showed excellent (100%) liverwort control for ‘Pandora’s Box’. Overall, RH and PB mulches at all depths provided excellent liverwort control and no reduction in the growth of ‘Curly Fries’ and ‘Pandora’s Box’.

1. Introduction

Common thalloid liverwort (Marchantia polymorpha) is a difficult weed that has spread throughout nursery and greenhouse container production systems in the United States [1,2]. Liverwort is particularly problematic at low temperatures (10–15 °C for reproductive growth and 17–22 °C for vegetative growth), under low ultraviolet (UV-B) radiation, high moisture, and high fertility [3]. It can rapidly reproduce, both sexually, by spores (male anthrediophores and female archegoniophores), and asexually, by gemmae that are produced in specialized structures known as gemmae cups, as well as by fragmentation [1,4]. Liverwort colonies form a mat-like structure on the substrate surface, impeding water and nutrient flow to the root zone of ornamentals. As a result, the growth and quality of ornamental plants are affected, reducing their market value [1]. Liverwort control with herbicides may result in phytotoxicity to sensitive ornamental plants and can have residual effects on the environment. The hand removal of common liverwort is a laborious, time-consuming, and costly operation. In addition, weeding by hand can result in the removal of the upper substrate and top-dressed fertilizer from the container. This disrupts plant root growth in the upper layers and adds to production costs.

An alternative approach is the use of organic mulches. Various experiments in the past have reported weed control by mulching with different materials at varying depths. Non-living mulches may be organic or inorganic and are either the by-product of industrial manufacturing processes or are specifically manufactured for the purpose [5]. Particle mulches, such as bark chips, finer wood particles, and crop wastes, help in weed cover reduction and are non-phytotoxic to ornamentals and other crops [6]. Top dressing the substrate in containers with organic mulches such as pine bark (PB), pine straw, cocoa hull (CH), cereal straw, and hardwood (HW) chips is a common practice in nursery production to reduce weed growth [7,8,9]. Mulching has several additional benefits, such as increasing substrate moisture, reducing substrate loss and compaction, moderating substrate temperatures, improving substrate nutrition, reducing salt and pesticide contamination, improving plant establishment and growth, reducing the occurrence of diseases, and improving the aesthetics of landscapes [10]. However, some mulches may lead to soil acidification, allelopathic activity on plants, competition from living mulches such as grasses, flammability of mulch materials, contamination from pathogens or weed seeds, and nutrient deficiency [10].

In nursery plants, such as highbush blueberry (Vaccinium corymbosum), black currant (Ribes nigrum), and rhododendron (Rhododendron arboreum), liverwort was effectively controlled using Sphagnum moss and blackcurrant stem pieces [11]. Liverwort control by Sphagnum moss alone ranged from 78 to 99%, while blackcurrant stem pieces provided complete control. For roses (Rosa spp.), a complete liverwort control was obtained for 8 weeks with a 1.3 or 2.5 cm (0.5 or 1.0 in) depth of parboiled rice hulls (RHs), with no adverse impact on plant growth [12]. Rice hulls applied at a depth of 0.6 cm showed 2.5% and 20% liverwort coverage on the substrate at 4 and 8 weeks after potting (WAP), respectively. Similarly, the application of fast-drying mulches such as European hazelnut (Corylus avellana) shells, rice (Oryza sativa) hulls, and pumice on the container media surface suppressed liverwort growth. A depth of 1.3 cm of hazelnut and oyster shell mulches resulted in reduced liverwort growth [13]. Another study conducted by [8] quantified the effect of different herbicide combinations with organic mulch materials such as pine bark, pine straw, and HW chips on weed control in container production systems. There was a reduction of from 88% to 100% in large crabgrass (Digitaria sanguinalis) and garden spurge (Euphorbia hirta) in containers with mulch depths of 2.5 cm or greater along with herbicide combination.

Limited research has been conducted so far to determine the moisture-holding capabilities of different organic mulches and how different organic mulch types and depths can impact common liverwort and ornamental plant growth in containerized production. Therefore, the main objectives of this experiment were to (a) assess the moisture-holding capacity of different organic mulch materials RH, CH, PB, or red HW and (b) to evaluate the impact of organic mulches on preemergent liverwort control and their phytotoxicity on container-grown ornamentals.

2. Materials and Methods

Laboratory experiment: The moisture-holding capacity in terms of ‘percent water retention’ of four mulch materials was determined in a laboratory experiment conducted at Michigan State University in the summer of 2020. Following the methods described in [14], two-piece plastic Buchner funnels (12.7 cm inner diameter, 6.6 cm tall) were filled with 5 cm of either RH, CH, PB, or red HW mulch materials (Figure 1). The weight of the mulch was first determined, and their weights were used to uniformly apply the same mass of each mulch material to replicate funnels. The volume of water added was determined in advance based on an irrigation depth of 1.02 cm. Each mulch-filled funnel was weighed (W_i) and placed over a 900 mL glass jar, and 171 mL of water (equivalent to 1.02 cm of irrigation) was added. The water passing through the mulch layer was collected in a glass jar. The funnels were weighed after the irrigation ceased and the funnels stopped dripping (W₀), and 1 h, 4 h, and 24 h after irrigation (W₁, W₄, and W₂₄, respectively). The volume of water passing through the mulch was measured (V). The following formulas were used to calculate water retention (%) [14]:

(a)

The percentage of water not retained by the mulch was calculated as [V ÷ (W₀ − W_i + V)] × 100.

(b)

The amount of water retained in the mulch layer at 1 h, 4 h, and 24 h was calculated as W₁ − W_i, W₄ − W_i, and W₂₄ − W_i, respectively.

(c)

The percentage of water retained by the mulch at 1 h, 4 h, and 24 h was calculated by the formula [(W_{1 to 24} − W_i) ÷ W_i] × 100.

The experiment was conducted in a completely randomized design with four mulch types and three time intervals. There were four replications per mulch material. All data were analyzed by PROC GLIMMIX in SAS (Ver. 9.4, SAS Institute, Cary, NC, USA) to conduct the Analysis of Variance (ANOVA) to determine the effects of mulch type for their effect on water retention (%). Before analysis, the data were inspected to ensure that the assumptions of ANOVA were met, and the data were log-transformed because they did not meet the normality assumptions. Where ANOVA results revealed significant effects, mean comparisons were performed using Tukey’s honest significant differences (HSD) test. All the effects were considered significant at alpha = 0.05. The experiment was repeated, and the combined data were analyzed to separate out the means.

Greenhouse experiment: Greenhouse experiments were conducted at the Horticulture Teaching and Research Center, Michigan State University, Holt, MI, USA, in the summer and fall of 2020 to assess the impact of organic mulch type and depth on preemergent control of liverwort and phytotoxicity on container-grown ornamentals.

In this experiment, 3.78 L nursery containers were filled with standard commercial soilless media containing 70% peat moss, 21% perlite, and 9% vermiculite (Suremix, Michigan Grower Products Inc., Galesburg, MI, USA). Controlled release fertilizer (CRF) Osmocote^® [N:P:K 17-5-11 (from 8 to 9 months)] (ICL Specialty Fertilizers, Dublin, OH, USA) was incorporated at the highest labeled rate according to the manufacturer’s recommendation of 35 g per 3.78 L container. The experiment was conducted in two rounds using Hosta plantaginea (Plantain Lily) ‘Curly Fries’ and ‘Pandora’s Box’, respectively. Ornamental plants were obtained from a commercial nursery (Walter Gardens, Zeeland, MI, USA). For the first round of the experiment, ‘Curly Fries’ was potted in 3.78 L pots. One day after potting, RH, HW, CH, or PB mulch (Figure 1) was applied on top of the substrate in each container at a depth of 0.63, 1.27, 2.54, or 5.08 cm. A control set without mulch was also included. Containers were irrigated with approximately 1.02 cm of water via overhead sprinklers. The same method was followed for a second round with ‘Pandora’s Box’ plants.

After 1 or 2 days, liverwort gemmae were applied over the mulch or the substrate (for control) in each container. For gemmae collection, gemmae cups were scraped off vigorous liverwort stock plants and put into a bowl of tap water, thus releasing gemmae upon separation from their clumps [12]. A plastic spoon was used to apply approximately 5 mL of water from the bowl, which contained gemmae, across the surface of each container. The containers were completely randomized after gemmae application. Gemmae were applied bi-weekly to each container until the end of the experiment. All containers received irrigation daily of approximately 1.02 cm via an overhead sprinkler.

The initial growth indices of the Hosta plants, calculated as an average of the plant height and two widths, were recorded 1 day after the initial gemmae application. The percentage of container surface covered by liverwort thalli was visually estimated at 2, 4, 6, 8, 10, and 12 weeks after treatment (WAT). The fresh weight of the thalli was also recorded at 12 WAT. The final growth indices of the Hosta plants were recorded at 12 WAT (end of the experiment).

The percent increase in growth index of plants was calculated by the following formula:

$$\%\mathrm{Increase}\mathrm{in}\mathrm{growth}\mathrm{index}=100\times {\displaystyle \frac{(\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x})}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}}}$$

The experiment was conducted in a completely randomized design as a 4 × 4 factorial treatment arrangement with four mulch types and four depths for each Hosta variety. There were four single-container replications per treatment. All data were analyzed by PROC GLIMMIX in SAS (Ver. 9.4, SAS Institute, Cary, NC, USA) to conduct the Analysis of Variance (ANOVA) to determine the effects of mulch types and depths and the interactions of these variables on data collected for various experimental parameters. Before analysis, the data were inspected to ensure that the assumptions of ANOVA were met. The replications were considered random effects, while mulch type and depth were considered fixed effects. Where ANOVA results revealed significant effects at alpha = 0.05, mean comparisons for fixed factors were performed using Tukey’s honest significant differences (HSD) test.

3. Results

3.1. Laboratory Experiment

There was a significant difference in water retention at different time points (p < 0.05) for all the mulch types studied. The analysis of variance for percent water retention at 1, 4, and 24 h has been presented in

Table 1. Analysis of Variance of the percent water retention at 1, 4, and 24 h.

Effect	Degrees of Freedom	F Value	p Value
Time	3	1353.71	<0.0001
Mulch type	2	63.41	<0.0001
Time × Mulch type	6	13.02	<0.0001

. There was a significant interaction between mulch types and time for percent water retention. After 1 h of initiation of the experiment (adding water to the mulches), CS mulch retained the highest amount of water (91%), while PB allowed the maximum amount of water to infiltrate and retained only 9% of the water applied. Similarly, at 4 and 24 h, CS retained the maximum quantity of water (90% and 86%, respectively), and PB retained the least amount of water (8% and 4%, respectively) (

Table 2. Percent water retention in organic mulches (pine bark, cocoa shell, red hardwood, and rice hull) at 1, 4, and 24 h.

Water Retention (%)
Mulch Type	1 h	4 h	24 h	p Value
Pine bark (PB)	9.1 Ca *	7.7 Cab	3.8 Cb	0.0057
Cocoa shell (CS)	91.2 Aa	89.7 Aa	85.8 Ab	0.0045
Red hardwood (HW)	17.2 Ba	16.2 Bab	12.4 Bb	0.0110
Rice hull (RH)	15.1 B	14.0 B	11.3 B	NS
p value	<0.0001	<0.0001	<0.0001

* Means followed by different capital letters within the column for time intervals for each mulch or small letters within the row which compare a specific mulch material across different time intervals are significantly different according to Tukey’s honest significant differences (HSD) test, at alpha = 0.05. NS: Non-significant at a = 0.05, respectively.

). For all the mulch types, out of the whole amount lost through infiltration, the maximum was observed within the first hour, and it was slower for other time points observed. For example, within 1 h, 91% of water passed through the PB mulch, and only 1% and 4% infiltrated by 4 h and 24 h, respectively. Similarly, for HW and RH, the amount of water infiltrating through the mulches by one hour was 83% and 85%, respectively.

3.2. Greenhouse Experiment

The growth index (%) of Hosta ‘Curly Fries’ was significantly affected by the interaction of mulch type and depth (p < 0.05). For example, the growth index was higher when RH (324%) and HW (316%) mulches were applied at a depth of 5.08 cm. The growth index of ‘Curly Fries’ was suppressed (153%) when CS mulch was applied at the thickest layer of 5.08 cm. In contrast, the highest growth index was recorded when RH mulch was applied at a depth of 5.08 cm, while it was suppressed at a depth of 2.54 cm (

Table 3. Percent increase in growth index of Hosta ‘Curly Fries’ growing in the presence of liverwort, as affected by application of organic mulches (pine bark, cocoa shell, red hardwood, and rice hull) at different depths (0.63 cm, 1.27 cm, 2.54 cm, or 5.08 cm).

Percent Increase in Growth Index of Hosta ‘Curly Fries’
Mulch Type	Depth of Mulch Applied
	0.63 cm	1.27 cm	2.54 cm	5.08 cm	p Value
Pine bark (PB)	185.5	279.18	223.37	252.55 AB	NS
Cocoa shell (CS)	230.9 ab *	313.8 b	303.0 b	153.0 Aa	0.0111
Red hardwood (HW)	224.5	241.4	271.7	316.1 B	NS
Rice hull (RH)	238.1 ab	269.2 ab	162.1 a	324.3 Bb	0.0253
Control	163.2	163.2	163.2	163.2 A	NS
p value	NS	NS	NS	0.0020

* Means followed by different capital letters within the column for different depths for each mulch type or small letters within the row which compare a specific mulch material across different depths of mulches applied are significantly different according to Tukey’s honest significant differences (HSD) test, at alpha = 0.05. NS: Non-significant at a = 0.05, respectively.

).

Mulching Hosta ‘Curly Fries’ significantly limited liverwort thallus coverage from 2 to 12 WAT. From 2 to 10 WAT, liverwort thallus coverage was significantly different across all the mulch treatments compared to the control but not different amongst themselves. All the mulches were able to limit liverwort container coverage to under 15% until 10 WAT, as compared to the control (76%). The RH, HW, and PB mulches provided the best control of liverwort (5, 7, and 12% liverwort coverage, respectively) at 12 WAT as compared to the control, which had 81% of the container surface covered with liverwort thallus (

Table 4. Liverwort coverage (%) on top of container surface from 2 to 12 weeks after treatment (WAT) as affected by application of different organic mulches (pine bark, cocoa shell, red hardwood, and rice hull) in containers having Hosta ‘Curly Fries’.

Liverwort Coverage (%) on the Top of Container Surface bi-Weekly
Treatment	2 WAT	4 WAT	6 WAT	8 WAT	10 WAT	12 WAT
Control	16 a *	22 a	41 a	64 a	76 a	81 a
Pine bark (PB)	0 b	2 b	4 b	7 b	9 b	12 bc
Cocoa shell (CS)	0 b	0 b	2 b	5 b	15 b	29 b
Red hardwood (HW)	0 b	1 b	3 b	4 b	6 b	7 c
Rice hull (RH)	0 b	0 b	1 b	2 b	3 b	5 c
p value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

* Percentages followed by the same letter are not significantly different within a column at p = 0.05.

). All the depths of mulching performed equally until 10 WAT, but they always provided significantly better control than the untreated containers. There was a significant difference in the depths of mulches applied at 12 WAT (

Table 5. Liverwort coverage (%) on top of container surface from 2 to 12 weeks after treatment (WAT) as affected by application of organic mulches at different depths (0.63 cm, 1.27 cm, 2.54 cm, or 5.08 cm) in containers having Hosta ‘Curly Fries’.

Liverwort Coverage (%) on the Top of Container Surface bi-Weekly
Treatment	2 WAT	4 WAT	6 WAT	8 WAT	10 WAT	12 WAT
Control	16 a *	23 a	41 a	64 a	76 a	81 a
0.63 cm	1 b	3 b	6 b	11 b	18 b	24 b
1.27 cm	0 b	0 b	1 b	6 b	11 b	19 bc
2.54 cm	0 b	0 b	0 b	2 b	4 b	8 bc
5.08 cm	0 b	0 b	0 b	0 b	0 b	1 c
p value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

* Percentages followed by the same letter are not significantly different within a column at p = 0.05.

). The 5.08 cm depth of mulching provided season-long liverwort control and continued to provide excellent (>80%) control without any significant reduction in the growth index of ornamental until the end of this experiment. The 1.27 and 2.54 cm depths of mulching also provided good (>80%) control of liverwort coverage over 12 weeks, whereas the 0.63 cm depth of mulching was the least effective towards the end of the experiment.

For Hosta ‘Pandora’s Box’, neither mulch type nor depth had an effect on the growth indices (p > 0.05). However, liverwort thallus coverage on the top of the container surfaces was significantly affected by mulching from 2 to 12 WAT. All the mulch treatments were significantly different from the control but not amongst themselves from 2 to 12 WAT. At 12 WAT, the liverwort coverage in all the mulch-treated pots was less than 10%, while it was 94% in the untreated control containers (

Table 6. Liverwort coverage (%) on top of container surface from 2 to 12 weeks after treatment (WAT) as affected by application of different organic mulches (pine bark, cocoa shell, red hardwood, and rice hull) in containers with Hosta ‘Pandora’s Box’.

Liverwort Coverage (%) on the Top of Container Surface bi-Weekly
Treatment	2 WAT	4 WAT	6 WAT	8 WAT	10 WAT	12 WAT
Control	21 a *	29 a	46 a	74 a	85 a	94 a
Pine bark (PB)	0 b	2 b	2 b	4 b	6 b	9 b
Cocoa shell (CS)	0 b	0 b	1 b	2 b	5 b	9 b
Red hardwood (HW)	0 b	1 b	2 b	4 b	4 b	5 b
Rice hull (RH)	0 b	0 b	1 b	3 b	5 b	7 b
p value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

* Percentages followed by the same letter are not significantly different within a column at p = 0.05.

). Various depths of the organic mulch applied were also significantly effective in limiting liverwort thallus coverage on the top of container surfaces from 2 to 12 WAT (p < 0.05) (

Table 7. Liverwort coverage (%) on top of container surface from 2 to 12 weeks after treatment (WAT) as affected by application of organic mulches at different depths (0.63 cm, 1.27 cm, 2.54 cm, or 5.08 cm) in containers with Hosta ‘Pandora’s Box’.

Liverwort Coverage (%) on the Top of Container Surface bi-Weekly
Treatment	2 WAT	4 WAT	6 WAT	8 WAT	10 WAT	12 WAT
0 (Control)	21 a *	29 a	46 a	74 a	85 a	94 a
0.63 cm	0 b	2 b	5 b	11 b	15 b	20 b
1.27 cm	0 b	0 b	0 c	2 c	4 c	8 c
2.54 cm	0 b	0 b	0 c	0 c	0 c	1 c
5.08 cm	0 b	0 b	0 c	0 c	0 c	0 c
p value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

* Percentages followed by the same letter are not significantly different within a column at p = 0.05.

). All depths of mulch application were significantly different from un-mulched containers from 2 to 12 WAT. At 2, 4, and 6 WAT, the liverwort coverage at either of the mulch depths was <10%, while it was 21%, 29%, and 46% for the untreated control pots at these bi-weekly intervals, respectively. At 8, 10, and 12 WAT, the 1.27 cm, 2.54 cm, and 5.08 cm depths provided almost complete inhibition of liverwort, and 0.63 cm depth provided >80% control, in comparison to control pots, which had 74%, 85% and 94% liverwort coverage at 8, 10, and 12 WAT, respectively (Table 7). The growth index for Hosta ‘Pandora’s Box’ was non-significant for the effect of type or depth of mulch applied (p > 0.05).

Liverwort fresh weight obtained from containers at 12 WAT, where it was growing in competition with either ‘Curly Fries’ or ‘Pandora’s Box’, was also significantly affected by the mulch treatments applied over the top of the container surface (p < 0.05). The RH and HW mulches were the most effective in minimizing liverwort establishment in ‘Curly Fries’ containers, as the fresh weights of the liverwort thallus were only 0.8 and 0.9 g, respectively. Similarly, the fresh weights of liverwort thallus in containers with PB and CS mulches (2.9 and 9.2 g, respectively) were compared to the control (24.3 g). For the ‘Pandora’s Box’ containers, the liverwort fresh biomass ranged only from 1 to 2 g across mulch treatments, as compared to the control, which had 26.2 g (

Table 8. Liverwort fresh biomass (grams) recorded at 12 weeks after treatment (WAT) as affected by the application of different organic mulches (pine bark, cocoa shell, red hardwood, and rice hull) in containers with Hosta ‘Curly Fries’ and ‘Pandora’s Box’.

Liverwort Fresh Biomass at 12 WAT (g)
Treatment	Curly Fries *	Pandora’s Box
Control	24.3 a	26.2 a
Pine bark (PB)	2.9 bc	2 b
Cocoa shell (CS)	9.2 b	2 b
Red hardwood (HW)	0.9 c	1 b
Rice hull (RH)	0.8 c	1 b
p value	<0.0001	<0.0001

* Percentages followed by the same letter are not significantly different within a column at p = 0.05.

).

4. Discussion

Our results showed that the application of RH and PB mulches provided significant control of liverwort growth without causing any reduction in the growth of ornamental plants used in this experiment. The organic mulch layers of RH and PB acted as a physical barrier for the liverwort gammae or spore germination. The irrigation water infiltration rate was very high in the case of PB and RH, and most of the water passed down to the ornamental root ball, compared to the CS and HW mulches. The application of the RH, PB, and HW mulches reduced gemmae/spore germination as compared to CS. However, an additional advantage of reduced moisture availability in the upper layers of RH and PB mulch could have possibly caused significant and effective liverwort control in the containers in comparison with the CS and HW mulches.

Similar effects of the RH mulch and its moisture retention on weed control in containerized production were investigated by [15]. Increasing mulch depth reduced weed seed germination and establishment for both bittercress (Cardamine hirsuta) and creeping woodsorrel (Oxalis corniculata). Additionally, the moisture retention ability of RHs was compared to peat moss and PB. It was reported that RHs retained less moisture as compared to other mulches, which turned out to be the primary mechanism of controlling weed seed establishment above the mulch layer. This mechanism could also be applicable to our current research results. In another study, [16] investigated the effects of rice straw and newspaper mulching on soil moisture and temperature regimes, moisture availability, and water-use efficiency in soybean (Glycine max). It was found that mulching improved soil moisture availability, reduced soil water consumption, and improved water-use efficiency by 25–47% in comparison to un-mulched soil. In another study conducted to examine the water-holding capacity and water permeability properties of organic mulches (bran, grass, and newspaper), the application of these mulches had significant effects on moisture retention in soil and crop growth. The bran possessed the highest water retention and proved to be beneficial for water retention under sprinkler or drip irrigation. Also, the water holding capacity was found to be directly related to the amount of water absorbed and the water immersion (application) time [17]. In a study conducted by [18], organic mulching with bran, grass, and newspaper in greenhouse tomato production helped in regulating soil moisture and temperature and water-use efficiency and improved crop yields. Ref. [19] recorded the effectiveness of various organic (pine bark, wheat straw, vine pruning residues, geotextile) and inorganic mulches for evaporation control and found that pine bark had lower evaporation rate than other organic mulches. This could be a beneficial attribute of the mulch for reducing water loss and improving moisture retention for plant growth. Ref. [20] recommended the use of a coarse-textured mulch of particle size 0.63–1.9 cm (0.25–0.75 inches) with a low water-holding capacity for effective weed control in containerized production systems.

The results of the current study show that RH, PB, and HW mulches were effective in controlling liverwort and improving ornamental plant growth. In a study conducted by [9], various organic and inorganic mulches, including pine needles, wood chips, volcanic stone (scoria), and polyethylene, have also been reported to affect the growth characteristics of flowering zinnia (Zinnia elegans). Mulching with pine needles and wood chips helped to attain higher water-use efficiency and improved growth and shoot fresh and dry weights of the plants. The flowering time increased by 6 days, and the time to first flowering was reduced with the application of mulches. The application of biodegradable mulches resulted in improving the growth of container-grown giant arborvitae ‘Martin’ (Thuja plicata), moderating substrate temperature, and improving water content availability, alongside reducing weed growth in containerized production [21]. Another study reported that pine pellet, rice hull, paper pellet, and vermiculite mulching improved the success of propagation of butterfly bush (Buddleja davidii), crape myrtle (Lagerstroemia indica), and hydrangea (Hydrangea paniculata) cuttings. The rice hull mulch resulted in a slight reduction (less than 50%) in root volume and length of crape myrtle cuttings [22]. Organic mulching with pine bark, shredded hardwood, or pine sawdust, applied alone or in combination with plastic film and paper slurry mulch, in container nursery production provided 64–91% weed reduction, which was equivalent to the control provided by plastic mulch. The mulch application significantly reduced the hand-weeding time and weed biomass as compared to a non-treated control [23]. In another experiment, stratified substrates composed of pine bark additionally mulched with rice hull on the top had significant effects on the growth of an ornamental Hibiscus rosa-sinensis and nursery weeds liverwort and bittercress. Mulching the pine bark substrate with rice hull was highly effective for controlling bittercress and liverwort in nursery containers [24]. Pine bark mini-nuggets applied at depths of 0, 1.5, or 3 inches were found to be effective for oxalis and bittercress control in nursery containers planted with Gardenia jasminoides, Lagerstroemia indica, Hydrangea quercifolia, and Ternstroemia gymnanthera. There was no detrimental effect recorded on the growth of the ornamentals with mulching, and mulching at a depth of 3 inches provided season-long control of weeds [25].

Various other studies have reported the importance of organic mulches for weed suppression and improvement of ornamental plant growth. Ref. [26] analyzed a hydro-compacting organic fiber mulch in containerized production and found that it reduced weed presence by 70% and improved the plant performance of camellia (Camellia japonica), cupressus (Cupressus sempervirens), and photinia (Photinia fraser ‘Red Robin’). Rice husk mat applied at 8 mm depth can be used for weed control in nursery polybags as it helps to reduce the coverage, emergence, and biomass of Cyperus distans, Ageratum conyzoides, and Eleusine indica weeds [27]. Different tree-based mulches and their depths effectively controlled eclipta (Eclipta prostrata), spotted spurge (Chamaesyce maculata), and long-stalked Phyllanthus (Phyllantus tenellus) in nursery production. The mulches included ground whole loblolly pine (Pinus taeda), eastern red cedar (Juniperus virginiana), and sweetgum (Liquidambar styraciflua) applied at 1-, 2- or 4-inches depth and were also compared to the effects of herbicide dimethenamid-p. The results showed that mulching at a 1-inch depth reduced the fresh weight of weeds by 82–100% 30 days after application. The effects of mulching were still significant until 168 days after application, providing a 90–100% reduction in spotted spurge fresh weight when the herbicide treatments had lost all its efficacy in comparison [28].

The parboiled rice hull mulch applied at depths of 0, 0.25, 0.5, and 1 inches effectively controlled liverwort and hairy bittercress (Cardamine flexuosa) in container nursery production of single rose (Rosa spp. ‘Radrazz’). There were no adverse effects on the growth and quality parameters of ornamental crops, and the weed establishment was significantly reduced with increasing depth of rice hull mulch. This particular study aligns with our current research result, where rice hull mulch at a depth of 1.27 cm or more provided effective liverwort control without causing any injuries to the Hosta varieties [12]. In another study, sphagnum moss and stem pieces of blackcurrant as mulches were significantly effective in controlling liverwort in blackcurrant, highbush blueberry, and rhododendron production. The liverwort control ranged from 78 to 100% after the application of these mulches, and there was no significant difference observed in the depth or coarseness of mulch layers [11]. Further, pine pellets, rice hull, paper pellet, and vermiculite mulching were effective for controlling large crabgrass (Digitaria sanguinalis), bittercress (Cardamine hirsuta), mulberry weed (Fatoua villosa), and creeping woodsorrel (Oxalis corniculata). It was found that pine pellets and paper pellets applied at a 0.5-inch depth reduced the growth of all four weed species [22].

5. Conclusions

Overall, the results from this experiment indicate that the application of RH or HW mulches at a depth of 1.27 cm or more provided excellent control of liverwort and improved the growth of ‘Curly Fries’. In contrast to other mulch types, the CS mulch provided the least liverwort control and caused a reduction in the growth of Hosta ‘Curly Fries’. However, for ‘Pandora’s Box’, all the mulches provided promising control of liverwort, but the HW mulch caused a reduction in its growth indices. For the water retention capabilities of the mulches, CS retained the maximum amount of water in the laboratory study, which is not suitable for containerized production. Also, in the greenhouse study, CS led to the promotion of liverwort gemmae germination and establishment on the top layer of mulch, as compared to the other mulches. In a nutshell, the RH and PB mulches at depths of 1.27, 2.54, and 5.08 cm are recommendable for excellent liverwort control with no reduction in the growth of the ‘Curly Fries’ and ‘Pandora’s Box’ varieties of Hosta. Growers need to consider mulch costs, the stability in the container (decomposition rate), the availability and quality of the source of the material, and the labor costs for the mulch application, in addition to their weed control benefits.