Across pest categories, weeds are often considered the most problematic due to competitive interactions for light, nutrients, water and space that can severely reduce the yield and marketability of crops. Because of the confined growing environment within containers, weeds have the potential to decrease the growth of ornamental crops by over 60%, subsequently extending the production time (Berchielli-Robertson et al., 1990; Fretz, 1973). Furthermore, weeds can provide habitat for insects and vertebrate pests, serve as alternate hosts for pathogens, and facilitate disease development by altering the physical environment. i.e. weeds do not provide habitat for disease, which is the abnormal condition caused by the pathogen. Even when weed competition is not a primary concern, consumers expect containers to be weed free (Simpson et al., 2002).

Unlike agronomic crops, the container nursery industry has relatively few herbicide options available to use in or around ornamental plants (Fennimore and Doohan, 2008). Container plant production relies heavily on preemergence (PRE) herbicides and supplemental hand weeding to manage weeds. While the use of herbicides presents an easy and effective method for weed control, heavy reliance on them has led to several adverse outcomes, including high chemical costs, potential leaching, runoff, and concerns regarding recycling irrigation water (Poudyal and Cregg, 2019; Wilson et al., 1995). Consumer concerns over the impact of herbicides on human health and the environment are also on the rise. Additionally, the container nursery industry produces thousands of different taxa ranging from succulents, herbaceous annuals, perennials, ornamental grasses, to tropical plants, many of which are highly sensitive to herbicides. Additionally, it has been observed that relying solely on hand weeding for control is cost-prohibitive, with expenses reaching up to $10,000 per hectare per year (Case et al., 2005).

Increased labor costs, environmental worries regarding chemical weed control and the emphasis on sustainability in container-grown crops have motivated numerous growers to adopt non-chemical practices and explore alternative methods of weed control. Various non-chemical methods have been developed to manage weeds including mulching (Chalker-Scott, 2007; Marble et al., 2015), weed discs (Appleton and French, 2000), sub-irrigation (Wilen et al., 1999), substrate stratification (Khamare et al., 2022a), fertilizer placement (Saha et al., 2019; Khamare et al., 2020) and other methods. However, the most widely adopted non-chemical method in recent years is mulching. While mulching has been frequently employed in horticultural crop production and landscaping, recent research has shown their ability to manage weeds in container plant production (Altland and Krause, 2014; Marble et al., 2019; Saha et al., 2020; Poudel and Witcher, 2022). While sanitation and preventive measures such as scouting, using clean soil and sanitized containers, and sourcing weed-free liner sources represent the initial steps in every integrated weed management plan, mulching functions as the primary line of defense against weeds. Numerous reviews have been published in the past decade focusing on weed control practices within container plant production. However, there has been no review dedicated to summarizing research on the use of mulches as a weed management tool, specifically with an emphasis on container plant production. The primary objective of this manuscript is to provide a review of all the research concerning the utilization of various mulch materials as weed management tools in container plant production. The secondary objective is to identify and highlight key knowledge gaps while offering suggestions for potential future research directions.

The word “mulch” is derived from the German word ‘molsch,’ which means soft or decaying matter. This reflects the natural form of mulch in a forest. Functionally mulch is defined as any material placed or applied in a thick layer, coating, or protective covering onto the soil’s surface (Crutchfield et al., 1986). Mulching offers numerous benefits, including minimizing soil erosion (Chalker-Scott, 2007), improving soil moisture retention (Li et al., 2020), regulating soil temperature (Long et al., 2001; Cook et al., 2006; Kader et al., 2019), increasing soil organic matter (Tindall et al., 1991; Duiker and Lal, 1999), promoting plant establishment and growth (Foshee et al., 1996; Cregg and Schutzki, 2009; Maggard et al., 2012a), supporting root development (Patten et al., 1988), providing food and shelter for earthworms (Pelosi et al., 2009), and stimulating microbial activity in the soil (Doran, 1980).

While all these benefits of mulching are notable, one of its most crucial applications in container plant production is weed suppression. The precise mechanism through which different types of mulch control weeds is not yet fully understood and varies depending on the specific mulch material and weed species (Chalker-Scott, 2007). However, research has shown that the main factors contributing to weed suppression include light exclusion (Teasdale and Mohler, 2000), decreasing available air and water for germinating weed seeds (Richardson et al., 2008), leaching of allelochemicals (Saha et al., 2018) and creating a physical barrier (Chalker-Scott, 2007).

The importance of light in the germination of many species is well-researched. Although not universal for all plant species, light responsiveness during germination is particularly significant for small-seeded species (Pons and Fenner, 2000), which are often present in container plant production. Moreover, seed germination necessitates imbibition (Woodstock, 1988). Mulch contributes to enhanced soil moisture retention (Chalker-Scott, 2007), while simultaneously reducing water availability on the mulch surface. The mulch layer generates a top layer that can rapidly dry out, depriving weed seeds of the moisture necessary for germination. This dual action significantly suppresses weed growth. Altland et al., 2016 reported that greater control over flexuous bittercress (Cardamine flexuosa With.) was achieved by placing the seeds on the surface of the mulch, as opposed to positioning them beneath the mulch layer. In another study, pine bark and pine straw mulch retained less water than hardwood mulch and resulted in greater control of garden spurge (Euphorbia hirta L.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) compared to hardwood mulch when the seeds were placed on top of the mulch (Saha et al., 2020). Consequently, the position of the seed relative to the mulch layer dictates whether light exclusion or reduced moisture availability acts as a mechanism of weed control (Ngouajio and Ernest, 2004; Altland et al., 2016; Saha et al., 2020).

Mulches also create a physical barrier by forming a layer near the soil surface that reduces the ability of weed seedlings to photosynthesize (Crutchfield et al., 1986; Chalker-Scott, 2007). The depth of mulch is a critical factor influencing the creation of this physical barrier. Numerous researchers have demonstrated that applying mulch at depths ranging from 2.5 cm to 7.5 cm provides effective and long-term weed control (Penny and Neal, 2003; Richardson et al., 2008; Cochran et al., 2009). The depth of mulch serves a dual purpose, it acts as a physical barrier, preventing weed seedlings from developing roots, and it also blocks light, hindering weed seed germination. However, the effect of the physical barrier is temporary and diminishes as the mulch material starts to degrade (Marble et al., 2015).

Various mulch materials can also suppress weeds through the process of allelopathy. Allelopathy is defined as the direct or indirect effect of plants on neighboring plants through the production of allelochemicals that interfere with their growth (IAS, 2018). Allelochemicals are the secondary metabolites or byproducts of the principal metabolic pathways in plants. These secondary metabolites are non-nutritional and be released through plant parts by leaching from leaves or litter on the ground, root exudation, volatilization from leaves, residue decomposition, and other processes in the natural and agricultural systems (Rice, 1984; Anaya et al., 1990). These allelochemicals can hinder the germination, growth, and establishment of nearby plants. Several studies have established that leachates released from the wood chip mulch of certain allelopathic species can inhibit weed seed germination and seedling growth (Rathinasabapathi et al., 2005). Duryea et al., 1999, compared the chemical, allelopathic, and decomposition characteristics of six mulches that included cypress (Taxodium distichum (L.) Rich.), eucalyptus (Eucalyptus grandis W.Hill), pine bark (Pinus elliottii Engelm.) pine needle, melaleuca (Melaleuca Quinquenervi (Cav.) S.T.Blake) and a utility trimming mulch (pruning from oaks (Quercus laurifolia Michx. and Quercus virginiana Mill) and cherry (Primus serotina Ehrh.), with a small amount of cedar and pine (Juniperus silicicola (Small) Bailey) and southern pines (Pinus spp.). The study reported that eucalyptus and utility trimming mulch had the highest decomposition rates and water extracts from all mulch materials inhibited germination of lettuce seeds. All mulch materials contained hydroxylated aromatic compounds that might have an allelopathic effect on lettuce seeds.

In terms of allelopathy, phenolic compounds encompass a range of substances, including simple aromatic phenols, hydroxy- and substituted benzoic acids and aldehydes, hydroxy- and substituted cinnamic acids, coumarins, tannins, and potentially a select few flavonoids (Zeng et al., 2008). These phenolic compounds have demonstrated to inhibit various plant root elongation, cell division, and reduce the growth and development of the plant. For example, coumarins have been found to significantly reduce the root elongation of lettuce (Li et al., 1993). Similarly, certain phenolic acids such as caffeic acid, coumaric acid, ferulic acid, cinnamic acid, and vanillic acid have been reported to inhibit the photosynthesis and chlorophyll content of soybean (Patterson, 1981). A comprehensive exploration of allelopathy and its practical applications can be found in a recent review (Khamare et al., 2022a).

The ideal mulch materials for container plant production should have several key characteristics. These include having minimal available nutrients, quick drying despite frequent irrigation, being easy to apply, demonstrating slow decomposition rates, being non-toxic to both humans and crops, and possessing aesthetic appeal for customers. In the United States, the most frequently utilized mulch materials in container plant production include pine bark (or other types of bark), rice hulls, and wood chips. ( Figure 1 ). Common mulch materials used in container plant production consist of rice hulls, pine bark, wood chips, wood shavings, coconut coir, nut (peanut, pecan) shells, oyster shells, cacao bean hulls, pelletized newspaper, recycled wastepaper, pine straw, and other materials (Sibley et al., 2004; Ferguson et al., 2008; Richardson et al., 2008; Maggard et al., 2012b; Marble et al., 2015; Altland et al., 2016; Bartley et al., 2017; Marble et al., 2019; Somireddy, 2011; Wilen et al., 1999; Table 1 ). Although there exists a wide range of mulch materials, comprehensive research has been conducted on only a limited selection of these materials. Below, we present an overview of mulch materials that have been extensively studied and thoroughly investigated, with a primary focus on container plant production.

The conventional mulch materials mentioned above are widely used, but several unconventional mulches have been evaluated in landscape and nursery settings, but not extensively in container production. There have been various unconventional materials tested as potential mulch that range from shredded paper, cardboard, corn cobs, spent hops, buckwheat hulls, fiberglass mat, sawdust, cocoa bean hulls, walnut hulls, coconut coir, crushed tires or shredded rubber, yard waste, gravel, and others. Pellet and Heleba, 1995 reported that chopped newspaper mulch applied in nursery crop rows at a depth of 10 cm and 15 cm was able to reduce weed germination for two years and also suppress weed growth. In a field experiment, chopped newspaper mulch at a depth of 7.6 cm was able to control weeds consistently compared to wheat straw, shredded newspaper, black plastic, and plastic landscape fabric (Monks et al., 1997). A study designed to compare shredded newspaper and wheat straw as crop mulch concluded that shredded newspaper was effective in suppressing most annual and some perennial weed species evaluated (Munn, 1992). Smith et al., 1998 evaluated recycle wastepaper crumble and pellets as a non-chemical alternative for weed control in container production ( Table 1 ). Mulching with recycled paper pellets at a depth of 2.5 cm proved effective in reducing the germination of spotted spurge. The authors stated that the paper pellets were able to absorb more water causing the pellets to swell, forming a thick mat with a smooth surface that suppressed weed seed germination. In a landscape experiment, recycled paper crumble and recycled paper pellets provided effective weed control (90% and above) and, in some cases, in some cases, recycled paper crumble and pellets provided weed suppression similar to oxadiazon (Smith et al., 1997). Other products such as PennMulch (Penn State University, State College, PA, USA) which is comprised of pelletized newspaper with 1% nitrogen added, and Wulpak (Wilbro Inc., Norway, SC, USA) which consists of pelletized sweepings from the shearing floors of sheep operation with 5N-0.44P-2.64K added has shown good control of weeds (Wooten and Neal, 2000). Bilderback and Neal, 2004 reported that Wulpak was able to suppress common groundsel (Senecio vulgaris L.), horseweed (Conyza canadensis L.), spotted spurge, and long stalked phyllanthus (Phyllanthus tenellus Roxb.) at mulch depths of 0.6 and 1.3 cm, similar results were observed with PennMulch applied at a depth of 1.3 cm.

Unconventional inorganic materials such as recycled rubber chips, gravel, sand, and polyethylene fabrics have been used in several landscape studies (Martin et al., 1987; Calkins et al., 1996; Hanim et al., 2014; Winkel et al., 1996). However, there is a lack of research on the utilization of these materials as mulch in container production, possibly due to environmental and safety concerns. For instance, the use of recycled rubber mulch has been shown to leach high levels of heavy metals such as zinc, lead, and cadmium and organic chemicals such as benzothiazoles (Kumata et al., 2002; Kanematsu et al., 2009; Crampton et al., 2014; Mohajerani et al., 2022). Several studies have shown that rubber-modified materials have a higher potential of leaching when exposed to seasonal rainwater (Aoki, 2008; Bocca et al., 2009). Consequently, rubber mulches are not ideal for container systems due to daily irrigation requirements. Materials such as polyethylene fabrics or woven polypropylene have higher installation costs, require more time, and need to be replaced once the fabric degrades (Marble et al., 2015).

Mulching provides a range of advantages in container plant production; nonetheless, it is important to acknowledge that there are also several drawbacks. A significant characteristic of numerous organic mulches is their rapid degradation, leading to the need for frequent reapplication for longer-term nursery crops (i.e. trees, large shrubs, etc.). This results in higher material costs and increased demand for labor during the mulching process. Another potential drawback associated with placing mulch against the trunks of container-grown plants is the heightened risk of pest pressure (e.g. pathogens) if irrigation is not managed properly and the potential formation of girdling roots which has been noted in landscape evaluations. In a field study, the plots mulched with pine bark had a dense population of Diptera dominated by Asyndetus spp (Gill et al., 2011). Additionally, applying mulch at a depth of 10 cm or more can adversely affect plant growth due to reduced soil aeration and slower soil warming (Greenly and Rakow, 1995). However, in the context of container plant production, mulch is typically applied at a depth of 2.5 to 7.5 cm. It is important to note that concerns about potential damage from rodents, termites, and other insects associated with mulch have primarily been focused on landscape-grown plants and not within the context of commercial nursery production settings. Another disadvantage linked to mulch materials is the potential loss of material resulting from container blow-over or the susceptibility of lighter materials like rice hulls or sawdust to be carried away by strong winds or heavy rain.

The utilization of mulch materials composed of bark, sawdust, and wood chips from different allelopathic species can also lead to phytotoxicity in the desired crop. The chemical composition of the mulch material can result in nutritional deficiencies, salinity, or metabolic alterations (Ortega et al., 1996). The phytotoxicity of these materials varies depending on the species (Allison, 1965) and the presence of propagating mixture around the root system can reduce or eliminate this effect. The harmful impact of mulch materials has primarily been noticed in the case of young seedlings, plant cuttings, and bare root plants (Allison, 1965; Gruda et al., 2009). Research has shown that mulch materials composed of large portions of wood can result in a high rate of N immobilization (Pickering and Shepherd, 2000). This immobilization usually occurs due to the incorporation of high carbon, low nutrient materials where the nitrogen is extracted by microorganisms during the decomposition process. Nevertheless, the nitrogen rates commonly used in container plant production are sufficient to counteract this nitrogen immobilization (Buamscha et al., 2008). Methods such as aging the bark, composting wood chips, parboiling rice hulls, washing and incorporation of fertilizer eliminate these negative effects (Estaún et al., 1985; Ortega et al., 1996). Additionally, container nursery growers use plant liners with well-established root systems, and only very rarely would seeds be planted directly into a nursery container. These liners typically contain a certain amount of fertilizer and propagation mix within the root ball. This practice further minimizes any potential toxic impact of mulch materials on the intended crop. Research involving different types of bark and wood chips as components of substrates in numerous greenhouse and nursery experiments have indicated that crops can be grown with comparable quality to those grown in peat moss (Boyer et al., 2008; Fain et al., 2008; Jackson and Wright, 2009; Boyer et al., 2009; Khamare et al., 2022b; Khamare et al., 2022c).

In general, research indicates that organic mulches are a valuable addition to weed management programs. A remarkable reduction of 92% in weed growth was observed in containers that utilized mulch compared to those without mulch (Wilen et al., 1999). Research shows that incorporating organic mulch would likely provide benefits in every weed management program for container plant production (Billeaud and Zajicek, 1989; Chalker-Scott, 2007; Cochran et al., 2009; Altland et al., 2016; Bartley et al., 2017). The choice of mulch materials will significantly depend on their availability and cost based upon the region or area, and mulch that is available which can be obtained regularly and be of consistent quality. Several mulch materials, including pine bark, pine tree chips, wood chips, rice hulls, and newspaper, have been shown to have no negative impact on numerous ornamental plants (Lohr and Pearson-Mims, 2001; Richardson et al., 2008; Altland and Krause, 2014; Khamare et al., 2022c; Wilen et al., 1999). Depth of mulch can impact weed control success; research indicates that applying mulch at a depth of 2.5 to 5 cm to achieve efficient weed control while minimizing costs. For example, Bartley et al., 2017 found that applying pine bark nuggets at a 5 cm depth led to a 99.5% control of spotted spurge and eclipta. Furthermore, employing coarse mulches with larger particle sizes that are placed at a depth adequate to cover the container media surface would achieve optimal weed control. These larger particles block more light and facilitate quicker drying compared to smaller particles (Keddy and Constabel, 1986). Additionally, choosing a reputable and certified source of mulch is advisable to reduce the risk of weed contamination. Based upon available research, it is recommended to avoid the use of inorganic mulches like gravel, stones, rocks, sand, and rubber in container plant production. Rubber mulches have been linked to the leaching of heavy metals such as selenium, lead, and cadmium (Kanematsu et al., 2009). Additionally, rocks, gravel, and stones would make the containers heavy and impractical or cost-prohibitive for transport.

The mulch materials available for container plant production are limited. This limitation may be the result of material availability, aesthetic appeal to consumers, effects of crop growth and weed suppressive ability. Consequently, there is a need to investigate new mulch materials for nursery containers. Several potential mulch species can be evaluated for their use in container plant production. Black walnut (Juglans nigra L.) is one of the well-researched allelopathic species. The allelopathic compound present in black walnut is called juglone (Davis, 1928). There have been numerous studies demonstrating its allelopathic effect on vegetables, field crops, ornamental species, and various weed species (Topal et al., 2007; Shrestha, 2009; Strugstad and Despotovski, 2012). Many allelopathic invasive species are problematic to the local environment. Utilizing invasive species, such as the melaleuca mulch used in Florida, as mulch can effectively reduce the control cost of trees and contribute to decreasing the population of invasive species. Moreover, repurposing these invasive species into mulch not only controls costs but also provides an opportunity to generate value from the removal process. Tree of heaven (Ailanthus altissima (Mill.) Swingle) contains an allelochemical called ailanthone that has shown both pre- and postemergence activity (Heisey, 1996). In a container study, Heisey, 1990 reported oven dried root bark of the tree of heaven reduced the germination and growth of garden cress (Lepidium sativum L.). Comprehensive research has been performed on the effect of crop residues on controlling weeds in agricultural production. Crop residues from wheat (Triticum aestivum L.), rice, sorghum (Sorghum bicolor (L.) Moench), alfalfa (Medicago sativa L.), sunflower (Helianthus sp.), and corn (Zea mays L.) have demonstrated to control weed growth through allelopathy (Singh et al., 2003). The incorporation of sorghum stems, roots, and leaves in soil has been shown to decrease weed growth by 25-50% (Cheema et al., 2012). Khaliq et al., 2011 reported that the crop residue of brassica, sunflower, and sorghum reduced the growth of horse purslane (Trianthema portulacastrum L.) better than the sole application of the crop residues. Kenaf (Hibiscus cannabinus) has been shown to suppress weed growth similar to that of black polyethylene plastic mulch (Russo et al., 1997a). In another study, Russo et al., 1997b further demonstrated that extracts from kenaf plant material were able to suppress the germination of redroot pigweed (Amaranthus retroflexus L.), annual ryegrass (Lolium multiflorum Lam.), tomato, and cucumber. Many of the studies mentioned above are carried out in field conditions, and their findings can be applied to container plant production (Singh et al., 2003; Khaliq et al., 2011; Cheema et al., 2012).

The research for container plant production needs to focus on finding new mulch materials, innovative ways of mulch application, and combining mulching with other weed control methods. The agriculture byproducts used in field production can be further processed to use as mulch in nursery containers. The food processing industry produces large and rising amounts of waste each year (Virtanen et al., 2017). Similar to rice hulls, which have garnered significant use on a broad commercial scale, other food processing byproducts may hold potential to be mulch materials and would be inexpensive and widely available, at least in different regions.

Novel techniques for weed control using mulching could also be devised by observing approaches employed in studies related to organic agriculture. As an example, the concept of living mulch could be applied to container plant production. Living mulch comprised of plant species that could cover the container surface and not become overly competitive with the container-grown crop may hold potential. As an example, an annual grass species such as annual ryegrass could be seeded in large containers, provide quick cover, and then be killed by postemergence application of a graminicide, providing a short-term mulch for weed management. Another area which deserves further investigation is the economic aspects of utilizing different mulch types in container production. To date, no study has specifically addressed the cost or return on investment of utilizing different types of mulch in lieu of preemergence herbicides. Knowing how many preemergence herbicide applications can be eliminated with the use of different mulch materials applied at various depths is needed in order for more broad adoption by the industry.

The environmental concerns associated with herbicides, the cost of handweeding, and the availability of mulch material can be addressed by focusing research on the above-mentioned methods. These materials and innovative methods have great potential to improve weed management, reduce the use of herbicides, and overall improve the sustainability of the container plant production. Collaboration between researchers, nursery operators, local government, and manufacturers will be fundamental in creating these effective weed management methods.

YK conducted the literature searching, reviewed and wrote the manuscript with assistance of SM. All authors contributed to the article and approved the submitted version.