Mustard Seed Meal and Mulches for Weed Control in Greenhouses¹ Open Access

Effective weed control is needed during production of ornamental plants to reduce competition for water, nutrients, light, and space. Organic weed control options are especially needed in greenhouses where chemical tools are restricted. This work explores the novel combination of two organic weed control methods, mulches and mustard seed meal (MSM). Individual effects and interactions between the two weed control methods on efficacy and safety were assessed on slow-growing woody plants, revealing additive benefits from use of the two methods. This research also reveals some of the difficulties in establishing crop tolerances for MSM and shows greater weed control efficacy from hazelnut shell mulch when compared to MSM. This work also suggests potential new areas for research in the effect of watering frequency on weed control and MSM efficacy.

Ornamental plant production is a significant economic activity globally, accounting for US $38 billion in 2020, and it is expected to steadily increase over the next few years, as reviewed by Gabellini and Scaramuzzi (2022). The sustainable production of ornamental plants depends on optimizing inputs of energy and resources and integrated pest management (IPM) (Darras 2020). Biological pest control has been extremely successful in greenhouses as part of IPM, reducing pesticide use (Parrella and Lewis 2017). However, the success of an IPM program can be undermined by weed presence since weeds can provide a habitat for pests and potential pathogens such as fungus gnats (e.g., Bradysia sp.), snails (e.g., Helix sp.), slugs (e.g., Deroceras sp.), basal rot (Fusarium oxysporum Schltdl.) and damping-off (Pythium aphanidermatum [Edson] Fitzp.) (Svenson et al. 1997). Diverse weed species infest greenhouses, including broadleaves (dicots), grasses (monocots), algae (Chlorophyta, Charophyta), liverworts (Marchantia polymorpha L.), and mosses (Bryophyta) (Fausey 2003, Sidhu et al. 2020).

Ornamental plants are typically propagated in small containers [less than 10 cm (4 in) in diameter], where weeds can rapidly colonize and threaten growth and liner quality. Weeds compete with crops for essential resources, including water, nutrients, light, and space (Sidhu et al. 2020). Effective weed management is therefore critical, which means preventing weed establishment. Sanitation practices can help reduce weed density. These include using weed-free container substrates, keeping surrounding areas clear of weeds, and removing any weeds before they flower. Research has shown that using preemergence herbicides can further improve control (Fausey 2003, Hester et al. 2012, Newby et al. 2007); nonetheless, preemergence herbicides are unavailable for use in containers maintained in greenhouses. Other cultural practices such as altered lighting conditions, irrigation scheduling, fertilization management, and hand weeding can all reduce weed presence (Altland and Krause 2014, Mache and Loiseaux 1973, Saha et al. 2019, Sidhu et al. 2020). These practices are labor-intensive and may not provide an adequate level of control.

Mulches can also provide highly effective weed control and may be more cost-effective than herbicide treatments when factoring in reduced labor costs for hand weeding (Marble et al. 2017). For instance, parboiled rice hulls effectively controlled Cardamine flexuosa With. and Marchantia polymorpha L. in greenhouses when maintained at 1.3 to 2.5 cm (0.5 to 1 in) depth (Altland and Krause 2014). Effective weed control with mulches is based on light suppression and reduced water availability for seed germination (Altland et al. 2016). Light exclusion by mulches may not be sufficient to eliminate seed germination, and the reduced water availability is overcome by frequent irrigation in greenhouses. Furthermore, mulches effectively prevent infestation of weed seeds and spores that establish in the surface of the container’s growing medium. Still, they may not be as effective when weeds germinate from vegetative propagules in the growing media (Altland and Krause 2014). Local availability is an important factor in selecting natural mulches (Bartley et al. 2017). In Oregon, hazelnut shell mulch (HM) has been utilized for potted ornamentals with good results because of ample availability, reducing herbicide applications by half in some cases (Miller 2018). In other cases, sprayable, biodegradable liquid mulch (LM) made from corn, potato, wheat and cellulose has also shown some promise as an option for weed control, particularly when applied at 1.25 kg·m² (2.3 lb·yd²), capable of reducing weed counts by 97% (Shen and Zheng 2017). Although effective, mulches often require additional weed control strategies to achieve desired weed control levels.

Natural herbicides could improve weed control in greenhouse environments where synthetic herbicides are not available. Corn gluten meal (CGM) is one product that has been extensively studied as a preemergence herbicide. CGM can be effective for weed control in a variety of situations (Bingaman and Christians 1995, Gardner et al. 1997), but its performance has not been consistent. This is particularly true in the West, where very poor weed control has been observed by researchers in Oregon, Washington, and California (Hilgert 2003, Miller et al. 2013, Wilen and Shaw 1999). Another promising product for use as an organic preemergence herbicide is mustard seed meal (MSM). Mustard seed meal (MSM) is a byproduct of oil extraction from white mustard (Sinapis alba L.) and contains glucosinolates, compounds with herbicidal (Stevens et al. 2009, Vaughn et al. 2006) and fungicidal activity (Plaszkó et al. 2021). Commercial MSM fertilizer products are typically incorporated in the soil. Still, if applied to the soil surface, they can act as a preemergence herbicide with comparable efficacy to several synthetic herbicides (Miller et al. 2013). This use of MSM has been tested for weed control efficacy and safety in potted ornamentals, and MSM controlled emerged liverwort up to 6 weeks after treatment with no crop injury in potted Rosa, Phlox, or Coreopsis (Boydston et al. 2008). MSM also shows preemergence control of various weeds at rates from 113-450 g·m⁻² (3-13 oz·yd⁻²) (Boydston et al. 2008, Nagila et al. 2021, Yu and Morishita 2014).

Combining mulches with MSM could improve weed control in greenhouses and eliminate the need for hand-weeding (Khamare and Marble 2023). To our knowledge, no research has yet been conducted to assess the interaction between mulch and MSM in potted ornamentals. Our objective for this study was to identify a natural weed control option that is effective and safe. We hypothesized that MSM, in combination with hazelnut and liquid mulches, would be more effective than the stand-alone treatments.

Three greenhouse experiments were conducted between 2020 and 2021 in Oregon, USA. In the first protocol, newly rooted Buxus x ‘Glencoe’ (Buxus sinica var. insularis (Nakai) M.Cheng × B. sempervirens L.) liners were transplanted to 12.7 × 15.2 cm square pots in March 2020, and plants were grown in a greenhouse at North Willamette Research and Extension Center in Aurora, OR, USA. The plants were about 5 to 7 cm (2-3 in) in height. The second study protocol was initiated in October 2020 and conducted in Corvallis, OR, USA. Liners of Buxus x ‘Green Velvet’ (B. sempervirens L. x Buxus sinica var. insularis (Nakai) M.Cheng), cutting propagated in March 2018, and gardenia (Gardenia jasminoides J. Ellis), cutting propagated in August 2019, were transplanted into 15.8 cm (6.2 in) diameter pots in October 2020, fertilized with 5 ml (1 tsp) 16-16-16 Endure fertilizer (J.R. Simplot Company, Boise, ID, USA). The plants were about 7 to 10 cm (3-4 in) in height. Greenhouse temperature was set to 21 C (70 F) days and 18 C (65 F) nights, and plants were irrigated frequently to promote liverwort infestation. Flats infested with liverwort and hairy bittercress (Cardamine hirsuta L.) were kept near the studies throughout their duration to promote infestation. A commercial sphagnum moss plus perlite potting mix was used for all trials (Sunshine Mix number 4; Sun Gro Horticulture, Agawam, MA, USA).

A commercial MSM product (Pescadero Gold, Farm Fuel Inc., Watsonville, CA; NPK 4.5-1.5-1.15) was applied as a powder in this study. The hazelnut shell mulch (HM) was sourced locally (Decorative Bark Products Inc., Tualatin, OR, USA), and a biodegradable liquid mulch (LM) made from corn, potato, wheat and cellulose (Advanced Bio Products, Milton, ON, CA) was also included as a treatment. Mulches and MSM were applied manually to the container surface around the plants and MSM was measured and deposited on the soil surface before the mulch application.

In experiment 1, MSM was measured and manually deposited on the soil surface at 0, 125, 250, 450, and 900 g·m⁻² (0, 3.6, 7.4, 13.2, and 26.5 oz·yd⁻²) with and without HM at a 1.25 cm (0.5 in) depth. A single treatment with liquid mulch was also included (2:1 water to LM concentrate; 4 L·m⁻² [12.56 fl oz·ft²]). This study was a two-factor factorial organized as a randomized complete block design (RCBD) with six replicates. A pot was considered an experimental unit. The study was conducted once. Liverwort was the predominant weed.

In the second study protocol, three mulch treatments were applied: no mulch, HM (1.25 cm depth [0.5 in]), or biodegradable LM (2:1 water to LM concentrate; 4 L·m⁻²). Each mulch was tested with four MSM application rates (0, 113, 225, and 450 g·m⁻² [0, 3.3, 6.6, and13.2 oz·yd⁻²]). A final treatment with LM topped with HM (LM + HM) at 1.25 cm (0.5 in) depth was included. The study was conducted twice, first in November 2020 with four replicates and repeated in December 2020 with six replicates. Both trials were arranged as an RCBD, and each plot consisted of a single pot. Northern willowherb (Epilobium ciliatum Raf.) and hairy bittercress (Cardamine hirsuta L.) were the predominant weeds in these studies.

The third study protocol evaluated increasing rates of MSM on potted boxwood in December 2020. MSM at 0, 100, 200, 400, 800, 1,600, 3,200, 6,400, or 12,800 g·m⁻² (0, 3, 6, 12, 24, 47, 94, 187, and 377 oz·yd⁻²) was deposited on the soil surface after transplanting. The study was arranged as a RCBD with six replicates, and each plot consisted of a single pot.

Weed coverage and crop injury were assessed at 30, 60, 90, and 120 days. Assessments were based on a scale of 0-100% scale. Weed coverage of 0% means the pot was completely free of weeds, and 100% was full weed coverage of the pot surface. Crop injury ratings of 0% were used when there was no evidence of plant stress (chlorosis, necrosis, epinasty, leaf cupping, etc.) and 100% for plant death. At end of the first study, crop dry weight was recorded. At the termination of the other two studies, several measurements were recorded: weeds were clipped at the soil level, dried, and their weights recorded; crop height and width were recorded and used to estimate canopy volume using the formula for the volume of a cylinder; the crop was sectioned at the soil level and separate weights for shoot and root tissue were recorded; and root length and a visual estimate of root injury were recorded. All studies were terminated at 120 days.

The qualitative data recorded using a percent scale were analyzed using a generalized linear mixed model in the R package glmmTMB (Brooks et al. 2017). The quantitative data were analyzed with a linear mixed model using R package LME4 with experimental blocks and their interactions as random factors while treatments were fixed factors in the analysis (Bates et al. 2014). The data met heteroscedasticity and normality assumptions. A combined analysis was performed because there were no interactions between the two experimental runs. Treatment means separation was done with the emmeans R package (Lenth 2019). The models produced from the mulch trials were also analyzed using contrasts with Bonferroni’s multiple-test correction. This was done to compare MSM and mulch effects separately when the model showed a significant effect from a given treatment and the absence of significant interaction terms. For the dose-response trial, post-hoc analysis was done using Dunnett’s test.

MSM suppressed liverwort coverage in containers without any mulches at all levels of MSM (Table 1). The pots receiving no mulch and no MSM had 40% liverwort coverage, while all pots treated with MSM had 25% coverage or less. Adding HM nearly eliminated liverwort, and MSM addition did not affect liverwort coverage beyond what the mulch treatment provided (Table 1). LM addition promoted liverwort growth, likely a result of increased moisture availability. An interaction between MSM and mulch levels was identified for liverwort and algae coverage, because MSM effects were only present in treatments without mulch.

Boxwood liners suffered significant injury at all rates of MSM, and 98% of plants died when treated with MSM, regardless of HM presence. Plants without MSM grew well, and of these, the plants treated with LM were the largest at the end of the trial (Table 1). Algae growth was promoted by MSM application in pots without HM, likely due to MSM’s nutrient content (Table 1).

MSM treatments reduced weed coverage up to 90 DAT with no differences in treatment effect between gardenia and boxwood pots (Fig. 1), while mulch effects on weed coverage were noted up to 120 DAT in HM-treated pots in both gardenia and boxwood (Fig. 2 and 3). Weed coverage was reduced by LM + HM only in boxwood (Fig. 2). Though the MSM level did not reduce weed coverage through the end of the studies, weed biomass showed a ∼40% reduction relative to the untreated control at 450 g·m⁻² MSM (Fig. 1). All mulches reduced weed biomass in boxwood (Fig. 2), and both treatments containing HM reduced weed biomass in gardenia (Fig. 3).

Canopy volume, crop biomass, and root measurements were not taken for gardenia plants due to high mortality (>80%) unrelated to the treatments. These plants were likely overwatered to promote liverwort and weed growth. Injury developed in the boxwood plants 120 days after treatment, ranging from 5-59% across all treatments (Table 2). Interactions between mulch type and MSM level were nearly absent, except for root injury ratings, where 113 g·m⁻² MSM application caused higher root injury in LM-treated pots than the other mulch types (Table 2) Untreated plants also showed some reduced root quality, possibly due to weed competition, but this effect was relatively low compared to MSM-treated plants. Though root injury data produced the only statistically significant interaction, LM-treated plants with the 113 g·m⁻² MSM treatment also had numerically lower canopy volume, aboveground biomass, and root dry weight as well as higher canopy injury compared to pots grown in bare soil or in HM plus MSM at 113 g·m⁻² (Table 2). Contrasts never showed a significant effect growth or injury when MSM was applied at 113 g·m⁻² when averaged across mulch levels. Still, contrasts showed increases in canopy injury at the 113 and 225 g·m⁻² rates of MSM were associated with p-values of 0.098 and 0.058, respectively (Table 2). Canopy injury was statistically higher in the plants treated with 450 g·m⁻² MSM than in the nontreated control. MSM treatments reduced crop above-ground biomass and plant height and increased root injury at 225 and 450 g·m⁻² MSM. Root weight was reduced by 450 g·m⁻² MSM compared to treatments without MSM (Table 2).

Boxwood plants were injured less by mulches than by MSM, but 59% canopy injury was observed at 120 DAT in the treatment of LM + HM. The LM + HM treatment also resulted in greater root injury and lower root and aboveground biomass relative to the untreated control (Table 2). LM treatments reduced height and increased root injury. In contrast, HM treatments slightly reduced above-ground biomass relative to the untreated control (p=0.06; Table 2). Shoot biomass, root dry weight, and root length were numerically highest in the HM treatment with no MSM. Canopy volume was numerically highest in the treatments with HM and either no MSM or 113 g·m⁻² (Table 2).

MSM injury to boxwood plants in the dose-response trial was first noticed 30 DAT at the rates of 3,200 g·m⁻² and greater, followed by 100% plant death in the subsequent evaluations. Boxwood treated with 1,600 g·m⁻² MSM developed injury from 60 DAT through to the end of the study (Fig. 4). No differences in crop injury were noted with MSM up to 800 g·m⁻², though all plants were moderately stressed (14-25% injury).

Treatments with MSM at 1,600 g·m⁻² or greater reduced boxwood root length and shoot biomass and increased root injury (Fig. 4). MSM treatment at 800 g·m⁻² slightly increased root injury and reduced values for canopy volume, height, above-ground biomass, and root biomass though not at a statistically significant level. MSM at 400 g·m⁻² was associated with the highest values for canopy volume, above-ground biomass, and root biomass (Fig. 4).

To our knowledge, this is the first study that evaluated the effects of HM, LM, and MSM for weed control in greenhouse-grown plants. In general, mulches and MSM controlled weeds in this study, and their combined use affects weed control depending on the mulch type used. HM reduced liverwort surface coverage by over 95% (Table 1) and weed biomass by 40-80% (Fig. 1 and 2). HM also reduced coverage of northern willowherb and hairy bittercress for 120 days and did not affect crop growth. Most measured crop growth metrics did not show differences between HM and no mulch treatments, but contrasts from the mulch trial show that boxwood above-ground biomass was slightly increased in HM plots (p=0.059). These results agree with the previous report of a 90% reduction in liverwort coverage from HM (Svenson 1998). HM was reported to suppress weeds in field conditions as well, with a 43% reduction in Calafate fruit (Berberis microphylla G. Forst.) (Betancur et al. 2023), and 83% in hazelnut orchards for 180 days (Mennan and Ngouajio 2012).

Results with LM were inconsistent. LM improved liverwort growth in experiment 1 relative to those treated with HM (Table 1). LM reduced weed biomass in boxwood pots but not gardenia, and some data suggested that MSM treatments were more injurious when applied in combination with LM. Published research indicates LM is very effective in container nurseries (Shen and Zheng 2017), but contradictory results were obtained here. We believe that excess moisture in our trials reduced LM efficacy and may have increased injury from MSM and LM applied concurrently due to increased hydrolysis, as discussed in the next paragraph. This assertion is correlated with the poor gardenia health in the mulch trial, as gardenia is known to be sensitive to overwatering. The LM manufacturer’s suggestion of a 24-hour drying period was followed, but longer may be necessary in greenhouse environments with high humidity. Efficacy will likely be improved through an altered watering regime or placement, but this may also affect the phytotoxicity of MSM. Therefore, based on this study, LM with MSM was not safe for the crop and is not an effective treatment for weed control.

The MSM treatments at 113-450 g·m⁻² reduced weed coverage, but the effects only lasted for 60 to 90 days (Fig. 1). A similar result was observed with MSM applied in containers, with control of annual bluegrass (Poa annua L.), common chickweed (Stellaria media L.), and creeping woodsorrel (Oxalis corniculate L.) (Boydston et al. 2008). In field conditions, MSM also suppresses weed germination and growth by 50 to 90%, with better performance observed in drier conditions (Shrestha et al. 2015). High soil moisture promotes hydrolysis of glucosinolates into isothiocyanates, the bioactive compound that is volatile and short-lived in the soil (Gimsing and Kirkegaard, 2009). High moisture in this study may have promoted glucosinolate hydrolysis into isothiocyanates, leading to high isothiocyanate levels in the soil soon after application while shorting MSM’s weed control longevity.

MSM treatments caused injury to boxwood at different rates in all three trials. First, all boxwood plants treated with MSM (125-900 g·m⁻²) were killed by treatments in the liverwort trial. These newly rooted liners had poorly developed root systems and were more vulnerable to MSM toxicity. Secondly, we identified that MSM treatments of 800 g·m⁻² and higher were also injurious to larger boxwood plants used in the dose-response trial. These larger plants had well-developed root balls at transplanting and were only sensitive to the higher rates of MSM. Thirdly, we identified consistent injury symptoms and growth reductions associated with MSM treatments at 450 g·m⁻² regardless of mulch treatment. Several metrics identified less consistent, but still significant, injury at 225 g·m⁻² MSM. The plants used in the mulch trial were from the same source and of similar vigor (before transplanting) as those used in the dose-response trial.

MSM effects on crop injury and growth were experiment-specific. No significant increases in plant growth were observed despite the added nutrients provided by MSM. In experiment 3, slightly larger canopies and improved root quality were observed in plants treated with 400 g·m⁻² MSM compared to higher rates. In experiment 2, the no-mulch treatments with 113 g·m⁻² MSM resulted in the numerically greatest crop canopy volume, above-ground biomass, and root quality of the MSM treatments (Table 2). Fertilizers placed on top of the mulch may limit the nutrient availability to the plants, as described with synthetic fertilizers (Altland and Lanthier 2007). However, in this study, MSM was placed below the HM and was likely available to plants.

Interactions of mulches with MSM treatments may also explain the differences in crop growth. Overwatering may have been a problem in the LM treatments, and the same may also be true for the HM treatments. Mulches reduce the irrigation needs in container-grown plants (Lohr and Pearson-Mims 2001). Thus, the excess water maybe be associated with some of the responses observed.

In conclusion, MSM provided a low level of suppression of liverwort and other weeds at 225-250 g·m⁻² that was not adequate as a stand-alone treatment. There are also many crop safety concerns for MSM use in the production of container-grown ornamentals. The size of the transplant and the quality of its root system are important factors determining the effect that MSM will have on the plant. Combining MSM with mulches can increase the risk of injury to container-grown ornamentals. MSM also leaves an unattractive residue on the surface of the growing media from algal and fungal growth that may be problematic in ornamentals. However, when MSM is applied under a mulch layer, any residue is effectively hidden. HM effectively suppressed weeds and did not seem to have negative interactions with MSM treatments. LM did not suppress weed populations and was associated with reduced crop health. Better water management may improve the crop safety profile of MSM.

Despite the crop safety concerns revealed by this research, there is still value in further exploring MSM as an organic herbicide in container-grown ornamentals. Applications of MSM at low rates suppressed weeds but also improved plant growth in the dose-response trial. Granular formulations of MSM may make convenient and consistent application of these products more economical and allow for repeat applications at lower rates. Future work on mulches, especially LM and MSM, ought to carefully address the different water needs of mulched pants. Testing other ornamental species and crop ages is needed before MSM can be recommended for greenhouse ornamental production.