Introduction

Living mulches are annual or perennial cover crops grown during the growing season of the main (cash) crop. Numerous studies have documented the capacity of living mulches to diminish the need for intensive tillage, reduce soil erosion and nitrate leaching, and improve soil health (Andrews et al. 2020; Hartwig and Ammon 2002; Leary and DeFrank 2000; Qi et al. 2011; Siller et al. 2016). For example, Hall et al. (1984) reported that two legume living mulches, crownvetch [Securigera varia (L.) Lassen] and birdsfoot trefoil (Lotus corniculatus L.), generally reduced erosion in no-till corn (Zea mays L.) relative to a dead mulch of corn stover residue. Living mulches also increase agroecosystem biodiversity, which is frequently associated with improved disease, insect, and weed management (Malézieux et al. 2009; Petit et al. 2018).

Weed suppression is among the most important functions of living mulches. Living mulches tend to provide improved weed control relative to terminated cover crops because they can suppress weeds through multiple mechanisms and throughout weed life cycles (Teasdale et al. 2007). Mechanisms of weed suppression by living mulches include inhibition of weed seed germination by shading, competition for light and belowground resources, and allelopathy (Médiène et al. 2011; Petit et al. 2018; Teasdale 1996; Weston 1996). Harvested intercrops may perform similar functions, but living mulches selected primarily for weed suppression often achieve this goal more effectively than intercrops selected primarily for harvest benefits (Liebman and Dyck 1993). Although synthetic mulches can provide effective weed suppression, they may be associated with environmental concerns (e.g., persistence and disposal) and may be prohibitively expensive for extensively grown field crops (Grundy and Bond 2007; Norsworthy et al. 2012).

The primary drawback associated with living mulches is their tendency to suppress main crops. Most living mulches that have the ability to suppress weeds also have the ability to suppress crops (Teasdale 1996). Crop suppression may involve allelopathy (Walters and Young 2008) but typically occurs through competition for resources (Liebman et al. 2001; Teasdale 1998). When living mulches compete excessively with main crops, they may cause unacceptable yield losses. In an extreme example, Eberlein et al. (1992) reported that an unsuppressed alfalfa (Medicago sativa L.) living mulch could cause corn yield losses greater than 96% under nonirrigated conditions. White and Scott (1991) reported that second-year legume living mulches reduced winter wheat (Triticum aestivum L.) yield by approximately 70% (average of six perennial legumes, spring top-dressed nitrogen treatment). However, living mulches do not always reduce main crop yield (Teasdale 1998) and may even increase yield. In one study, herbicide-suppressed living mulches of white clover or ladino clover (both Trifolium repens L.) increased marketable sweet corn yield by 75% (Vrabel et al. 1981). These divergent outcomes are partially explained by differences in living mulch species and management practices. It is possible to promote both weed suppression and main crop yield by selecting appropriate living mulches and managing them in ways that capitalize on the morphological, physiological, and developmental differences among main crops, living mulches, and weeds (De Haan et al. 1994; Liebman et al. 2001; Verret et al. 2017).

Research on the management of living mulches began as early as the 1960s and intensified in the 1970s and 1980s (Hughes and Sweet 1979; Paine and Harrison 1993). In subsequent decades, this research continued without leading to widespread adoption, perhaps because no-till planting of herbicide-resistant crops into killed sods provided some of the same benefits with reduced risks to yield. However, excessive use of herbicides carries risks of its own, including environmental harm and the emergence of resistant weeds (Mortensen et al. 2012; Norsworthy et al. 2012). In recent years, interest in living mulches has again increased because living mulch systems can contribute to agricultural sustainability through reduced herbicide inputs and improved soil health (Bartel et al. 2020; Moore et al. 2019; Robačer et al. 2016). Despite these benefits, the widespread use of living mulches remains limited by several barriers (Sheaffer and Moncada 2012 p. 354; Vincent-Caboud et al. 2017; Wezel et al. 2014), including ongoing uncertainty about best management practices.

This review focuses on the management of living mulches grown alongside field or vegetable crops in temperate environments for the primary purpose of weed suppression. Our goal is not to describe the benefits of living mulches, many of which are summarized in articles cited above. Instead, we evaluate cultural, mechanical, and chemical methods of maximizing weed suppression and minimizing mulch-crop competition (Figure 1). The purpose of this review is to characterize management practices that can increase the likelihood of positive outcomes (good weed suppression and main crop yield) in living mulch systems. Although few generalizations apply to every living mulch system, we seek to identify emerging trends in the management literature and draw attention to remaining knowledge gaps.

Living Mulch Species

In this review, we define living mulches as plants grown alongside main crops for noncommercial benefits that occur during the main crop growing season. Living mulch biomass is returned to the soil rather than being harvested. This functional definition does not include morphological, physiological, or developmental traits, which vary widely among successful living mulches. However, living mulches useful for weed control do share some general characteristics. Living mulches that provide dense ground cover early in the growing season can prevent weed establishment (Nicholson and Wien 1983; Teasdale 1998). For this reason, many living mulches are either perennial species or annual species with rapid initial growth. Because competition for light is largely asymmetric (Weiner 1990), it is important that living mulches remain short to prevent excessive competition against the main crop (De Haan et al. 1994; Echtenkamp and Moomaw 1989; Leoni et al. 2020). If mechanical or chemical tactics are used to limit living mulch height, living mulches must recover from these control measures more quickly than weeds. The remainder of this section provides more detail on living mulch traits, then notes that different living mulches are appropriate for different cropping systems.

Most living mulches are legumes or grasses, although brassicas and other species, such as purslane (Portulaca oleracea L.), are also used (Ellis et al. 2000; Masiunas 1998; Teasdale 1998). Legume living mulches are notable for their ability to add nitrogen to the cropping system, but the in-season and post-season availability of fixed nitrogen to crops may vary (Germeier 2000; Hartwig and Ammon 2002; Liebman and Davis 2000; Sanders et al. 2017; Singer and Pedersen 2005; Triplett 1962). Although nitrogen fixation represents a useful benefit, it is possible for legume living mulches to increase nitrogen availability to weeds and thereby exacerbate weed issues over time (Sjursen et al. 2012). Grasses may offer advantages such as ease of maintenance (Elkins et al. 1983). Cool-season grasses lose vigor in the summer, potentially reducing the need for or difficulty of additional suppression (Adams et al. 1970; Elkins et al. 1979). Because weed control improves with cropping system diversity (Liebman and Dyck 1993), some research has tested mixtures of living mulch species (Echtenkamp and Moomaw 1989; Hartwig and Hoffman 1975). However, such mixtures may be difficult to manage.

Annual and perennial living mulches should be selected according to different criteria and offer different advantages. Ideotypes for spring-seeded living mulches often specify that establishment should be rapid (Buhler et al. 1998; De Haan et al. 1994). In contrast, the ideotype of a perennial living mulch could include delayed green-up in the spring to reduce mulch-crop competition (Flynn et al. 2013). Leoni et al. (2020) tested 11 commercial cultivars of legumes in Italy and concluded that self-seeding annuals offer rapid and complete establishment, but perennial mulches might provide the best weed control. Perennial living mulches are particularly suitable for no-till or low tillage systems, although annual species can also reduce tillage by eliminating the need for interrow cultivation during the growing season. Once established, perennial living mulches may be more difficult to suppress. For example, Cardina and Hartwig (1980) found that a crownvetch living mulch became more tolerant of herbicides with age. In addition to being easier to suppress, relative to established perennial living mulches, annual living mulches give the grower more control over planting and termination times. Annual living mulches are sometimes intended to reseed themselves (Teasdale 1996), but can be terminated before seed set to prevent volunteer plants from emerging in the next season.

When choosing living mulch species and management programs, growers should consider the main crop's ability to tolerate competition. For example, Ziyomo et al. (2013) reported an interaction between corn hybrid and kura clover (Trifolium ambiguum M. Bieb.) control on grain yield in Minnesota and Wisconsin, USA. Drought-susceptible hybrids showed yield reductions in a living mulch relative to a killed mulch. Moynihan et al. (1996) tested multiple medic species (Medicago spp.) as intercrops in barley (Hordeum vulgare L.) in Minnesota and concluded that this system was more successful with a conventional-height barley cultivar than a semi-dwarf cultivar. Uchino et al. (2016) found substantial differences among eight soybean [Glycine max (L.) Merr.] varieties grown for forage in an Italian ryegrass (Lolium multiflorum Lam.) living mulch in Japan. The living mulch usually reduced whole-plant yield, but this reduction was larger in early-maturing than late-maturing soybean varieties. Taken together, these findings indicate that main crops vary in their yield responses to competition from living mulches. Whenever possible, main crops for living mulch systems should be competitive (e.g., tall-statured) and tolerant of resource limitation (e.g., drought resistant).

Figure 1.

Purpose and methods of living mulch management. (A) Managers seek to reduce mulch and weed suppression of crop growth (red arrows) without eliminating the positive effects of the living mulch. (B–D) Management practices have both direct effects (green arrows) and indirect effects (black arrows). (B) Increasing the competitiveness of a living mulch strengthens its effects, both positive and negative. (C) Fertilization and irrigation may benefit any plant species, although unequal benefits often affect competitive dynamics. When resources are less limited, interspecific competition may be weaker (dotted lines). (D) Mechanical and chemical tactics are used to suppress the living mulch and provide supplemental weed control. In graphics, the main crop is shown in blue, the living mulch as the green clover, and the weed in red.

Comprehensive field screening of living mulch species and cultivars is a good way to identify suitable candidates for a particular cropping system and geographic area. For example, studies in Connecticut, USA, revealed that an annual grass, field brome (Bromus arvensis L.), yielded more marketable snap bean (Phaseolus vulgaris L.) and sweet corn than other living mulch candidates selected from an initial group of 57 possibilities (DeGregorio and Ashley 1985, 1986). In New York, USA, Nicholson and Wien (1983) tested five turfgrasses and three white clover cultivars from an initial group of 82 grasses and legumes. They found that Chewing's fescue (Festuca rubra L.), Kentucky bluegrass (Poa pratensis L.), and the white clover cultivar ‘Kent’ provided some ground cover without reducing sweet corn or cabbage (Brassica oleracea L.) yield. In Norway, subterranean clover (Trifolium subterraneum L.) exhibited traits desirable in a living mulch, although hairy vetch (Vicia villosa Roth) may be preferred for its superior frost resistance in that region (Brandsæter et al. 2000; Brandsæter and Netland 1999). In southern Sweden, Bergkvist (2003a) reported substantial differences between white clover varieties established in barley and maintained in winter wheat or winter oilseed rape (Brassica napus L.). In the first year, the clover variety ‘AberCrest’ did not reduce wheat or high-density rape yields, but the variety ‘Sonja’ reduced wheat yield by approximately one third and nearly eliminated the rape yield.

The choice of living mulch species should reflect several factors, including cropping system, climate, and grower priorities (e.g., willingness to use tillage or herbicides). In general, living mulches should be capable of quick initial growth. They should be competitive against weeds and recover well from field management operations. A manageable, short-statured growth habit should not interfere with the main crop canopy. Ideally, growers should choose living mulch species and main crops from different families to promote pest control and functional diversity.

Mechanical Management

Mechanical or chemical management of living mulches can serve several purposes. It is often necessary to kill strips of a pre-established ground cover to permit main crop planting and establishment. Management practices can also decrease the severity of mulch-crop competition and/or provide supplemental weed control (Figure 1D). In early studies on living mulches, herbicides were often considered essential (Teasdale 1996). However, trends toward herbicide rate reduction and organic farming have increased the demand for mechanical management strategies (see the Supplementary Table). These strategies are often effective but may not kill weeds occurring in crop rows or gaps in living mulch stands. In weedy situations, the presence of living mulches sometimes complicates weed suppression by limiting the number of available mechanical tools (e.g., eliminating in-season cultivation options). For this reason, it is desirable to plant competitive main crops and living mulches. Competitive living mulches typically require management to reduce mulch-crop competition.

Strip tillage can be an effective means of suppressing living mulches. In a factorial study of living mulch species, tillage, and herbicide treatments, Wiggans et al. (2012) found that corn grain yield losses could be prevented if Kentucky bluegrass was strip-tilled in the fall, then treated with paraquat (0.84 kg ai ha^–1 over the entire plot) and 25-cm bands of glyphosate (1.0 kg ai ha^–1, applied twice over the row) around planting time. This result echoed an earlier suggestion that mechanical and chemical methods should be combined for row establishment in a mixed-species living mulch (Martin et al. 1999). When only one method is used, strip tillage sometimes outperforms chemical treatments. Adams et al. (1970) found that strip tillage could be a better method for suppressing coastal bermudagrass than a growth retardant, maleic hydrazide (4.5 or 9 kg ha^–1), which also delayed corn development. More recently, Pearson et al. (2014) reported that strip tillage of well-established kura clover shortly before corn planting resulted in higher corn yields than herbicide bands in one of two years. Ginakes et al. (2018) compared glyphosate banding (4 kg ae ha^–1, 30-cm bands), shank tillage (traditional strip till unit), zone tillage (rotary zone tiller with power take-off), and shank plus zone tillage as methods of establishing corn in kura clover. Relative to the herbicide bands, shank plus zone tillage resulted in reduced kura clover encroachment into the crop rows and higher nitrogen availability (see also Alexander et al. 2019b on strip tillage and nitrogen availability). In another study of corn grown in a kura clover living mulch, Dobbratz et al. (2019) observed a yield advantage of zone tillage over shank tillage or herbicides in one of two years. Hooks et al. (2013) reported that eggplant (Solanum melongena L.) yield was reduced when the crop was transplanted into a mowed red clover stand (first year), but there was little yield reduction when the red clover was instead strip-tilled (second year).

Although tillage can be a powerful tool for reducing mulch-crop competition, main crop yield does not always increase with increased soil disturbance. Beale and Langdale (1964) compared tillage treatments that disturbed 100% (turnplow), 50% (rip plant), or 33% (lister plant) of an established coastal bermudagrass sod in advance of corn planting. Corn yields were similar between treatments and postharvest grass stands were improved by less aggressive tillage. Hartwig and Loughran (1989) found that a crownvetch living mulch had little effect on either corn or summer annual weeds, regardless of tillage treatment (no-till or primary tillage with a moldboard plow, heavy offset disk, or chisel plow, followed by secondary tillage with a tandem disk). In sweet corn, Mohler (1991) found that strip tillage of a white clover living mulch did not increase marketable ear weight relative to a no-till treatment.

Living mulches may be suppressed with mechanical tactics other than preplant tillage. In white cabbage, Brandsæter et al. (1998) found that mowing did not reduce yield losses due to competition with living mulches of subterranean clover or white clover. However, these yield losses were reduced by rototilling between the rows 6 wk after transplanting. Rototillage also improved weed suppression. Similarly, Grubinger and Minotti (1990) reported that white clover suppression by partial rototilling resulted in higher corn yields than mowing in the clover establishment year. Chase and Mbuya (2008) found that mowing living mulches failed to improve yield in broccoli. Graham and Crabtree (1987) and Vrabel et al. (1981) reported that mowing was often inferior to chemical control for living mulch suppression. In zucchini (Cucurbita pepo L.) grown with a living mulch of sunnhemp (Crotalaria juncea L.), yield losses were reduced when the living mulch was cut to a height of 20 cm rather than 45 cm (Hinds et al. 2016). Mechanical control of living mulches could also be improved by the introduction of novel methods. Båth et al. (2008) were able to increase the aboveground biomass of white cabbage by pruning the roots of living mulches (custom equipment with horizontal blades at 0.2 m depth). The increase in cabbage biomass was most dramatic with a living mulch of winter rye, sown shortly before cabbage planting, but it was also significant with red clover, birdsfoot trefoil, and salad burnet (Sanguisorba minor Scop.), all sown in the year prior to the experiment.

Mechanical practices may improve crop nitrogen uptake. Thorsted et al. (2006b) used a weed brusher on a white clover living mulch (cut at the soil surface in 11-cm bands between crop rows) and left cut material on the ground to release nitrogen. This brushing method increased wheat nitrogen uptake and grain yield relative to an unbrushed control. Another strategy intended to increase nitrogen availability to the crop involves depositing legume clippings onto the crop row after mowing. It is not clear that this strategy is effective (Thériault et al. 2009), but removing clippings from the system could have a negative effect. In a study of soil nitrogen beneath an alfalfa sod, Varco et al. (1991) found that nitrogen levels at 0 to 10 cm were lower in a cut-and-remove treatment than in a cut-and-return treatment after 14 d.

Future research on the mechanical management of living mulches may uncover strategies that balance soil fertility, weed control, and crop yield without much soil disturbance or damage to the living mulches. For now, the most reliable methods of reducing competition against main crops are fairly intense (e.g., strip tillage rather than mowing) or combine mechanical control with other approaches, such as chemical control.

Chemical Management

Like mechanical control, chemical control may promote several goals in living mulch systems, including better crop establishment, less mulch-crop competition, and greater weed suppression. At the same time, living mulches are valued for their potential to facilitate herbicide rate reductions (Moore et al. 2019; Norsworthy et al. 2012). Herbicide rate reductions are desirable for environmental reasons and may promote long-term sod persistence, but herbicides applied at reduced rates may not adequately reduce competition against the main crop (Bennett et al. 1976; Buck 2018; Kosinski et al. 2011; Williams and Hayes 1991). In contrast, herbicide use at high rates tends to reduce mulch-crop competition at the expense of ground cover, biomass accumulation, and weed suppression by the living mulch. Early research on living mulches sometimes failed to identify herbicide treatments that achieved good main crop yields without killing the living mulches or severely reducing ground cover (Hughes and Sweet 1979; Linscott and Hagin 1975). Subsequent research has focused on developing treatment programs that protect yields without excessive mulch kill. These goals may require herbicide choice, rate, application method, and/or timing to be tailored to the living mulch system (Supplementary Table).

One way to manage the tradeoff between protecting the main crop and maintaining the living mulch focuses on spatial variation in herbicide applications. For instance, using herbicides to kill strips of a living mulch prior to crop planting or emergence can decrease interspecific competition. Zemenchik et al. (2000) found that killing 61-cm bands of kura clover with glyphosate (4.0 kg ae ha^–1) and postemergence dicamba (0.7 kg ae ha^–1) resulted in improved corn yields in one of two years over a treatment with glyphosate (1.7 kg ae ha^–1) and postemergence bromoxynil (0.4 kg ai ha^–1) applied to the entire field. In contrast, Eberlein et al. (1992) reported that unirrigated corn yields were sometimes greater with broadcast applications of atrazine (1.68 kg ha^–1) than with band applications (3.36 kg ha^–1, 38 cm) in an alfalfa living mulch. This difference was not observed in an irrigated treatment. Strip width may influence the efficacy of band applications. Kumwenda et al. (1993) demonstrated that killing crimson clover (Trifolium incarnatum L.) in strips covering 60% to 80% of total field area could prevent corn yield losses while still allowing clover reseeding. Similarly, Wilkinson et al. (1987) increased corn yields by doubling the width of a killed strip of tall fescue (0.20 m to 0.41 m). It is also possible to use band applications after main crop emergence. Reddy and Koger (2004) found little difference in yield between corn planted into a live hairy vetch and corn planted into 38-cm bands killed with paraquat (1.1 kg ai ha^–1). However, postemergence applications of glyphosate (0.84 kg ae ha^–1) were more effective when broadcast than applied in 38-cm bands over the row, partially because the broadcast treatment contributed more to weed control.

Weed suppression can be improved by selecting appropriate herbicides and considering their interactions with other management decisions. In a study of both cultural and chemical factors, Rajalahti et al. (1999) reported that interseeding living mulches 3 wk after planting could facilitate a 70% reduction in herbicide use in potato (Solanum tuberosum L.). Nurse et al. (2018) tested three living mulches in sweet corn, both independently and matched with herbicides appropriate for grass control: adzuki bean [Vigna angularis (Willd.) Ohwi & H. Ohashi] with linuron plus S-metolachlor (0.55 plus 1.14 kg ai ha^–1), cereal rye with saflufenacil (0.075 kg ai ha^–1), and oilseed radish (Raphanus sativus L.) with pendimethalin (1.68 kg ai ha^–1). Adzuki bean provided poor grass control in the absence of herbicides, but the combination of adzuki bean, linuron, and S-metolachlor was highly effective. In many cases, the weed control achieved by living mulches depends on the identity of problem weeds. Lightly to moderately suppressed crownvetch improved control of dandelion (Taraxacum officinale F.H. Wigg.) and yellow nutsedge (Cyperus esculentus L.) in corn but was not helpful in controlling redroot pigweed (Amaranthus retroflexus L.; Hartwig 1977, 1989; Hartwig and Loughran 1989).

A different approach to weed suppression involves planting herbicide-resistant crops. In an early test of this strategy, Affeldt et al. (2004) found that glyphosate- or glufosinate-resistant corn could be grown in a kura clover living mulch. The kura clover did not reduce yield relative to monocrop corn when it was suppressed with a preplant application of glyphosate plus dicamba (1.66 plus 0.14 kg ae ha^–1) and postplant banding of dicamba plus clopyralid (0.56 plus 0.05 kg ae ha^–1, 25 cm) before the in-season application of glyphosate (0.83 kg ae ha^–1) or glufosinate (0.37 kg ae ha^–1) between the corn V3 and V5 stages. In contrast to the more standard practice of using glyphosate to terminate a living cover crop by the time of main crop emergence (Petersen and Röver 2005), this system allows significant kura clover regrowth. It has increased in popularity since the study published by Affeldt et al. (2004). However, it is also worth noting that living mulches can be managed with chemical treatments other than popular herbicides. For example, Elkins et al. (1979) tested several growth retardants [fluoridamid (4.5 to 9 kg ha^–1), maleic hydrazide (4.5 to 9 kg ha^–1), and mefluidide (0.6 to 1.1 kg ha^–1)] or herbicide treatments [dalapon (2.2 to 4.5 kg ha^–1), glyphosate (1.1 to 2.2 kg ha^–1), glyphosate plus atrazine (1.7 to 2.2 plus 1.1 kg ha^–1), metolachlor (4.5 to 9 kg ha^–1), metolachlor plus atrazine (6.7 to 9 plus 0.6 to 1.1 kg ha^–1), and paraquat plus atrazine (0.6 to 1.1 plus 1.1 kg ha^–1)] on a tall fescue or Kentucky bluegrass sod for corn. With the exception of paraquat plus atrazine, which largely killed the forage grasses, most growth retardants or herbicides achieved some success in promoting good corn yields while maintaining sufficient ground cover to prevent erosion.

Application timing and method account for considerable variation in herbicide efficacy. Bergkvist (2003c) tested various rates of isoproturon plus diflufenican in a third consecutive crop of winter wheat sown in a white clover living mulch. All autumn applications (0.375 to 1.625 plus 0.075 to 0.15 kg ha^–1) were effective in reducing annual weed biomass and increasing wheat grain yield without permanently damaging the living mulch, whereas a spring application of isoproturon only (1.25 kg ha^–1) reduced white clover biomass and ground cover without effectively suppressing weeds or increasing yield. Within the spring season, Cardina and Hartwig (1980) found that preemergence applications could be superior to preplant incorporated applications for control of a crownvetch living mulch in corn, likely because the preplant incorporated applications occurred before the crownvetch began spring growth. Their results agree with other suggestions that living mulches should be treated while actively growing (Rinehold 1987) and that crownvetch is a good target for chemical manipulation (Hartwig and Hoffman 1975; Hartwig 1976). Teasdale (1993) showed that the weed control benefit of hairy vetch was greatest early in the corn season and suggested that this living mulch could contribute to a weed control program including only postemergence herbicides.

Future Directions

Given the pressing needs to diversify weed control programs, limit land degradation, and address environmental issues associated with intensive farming (Godfray et al. 2010; Mortensen et al. 2012), we believe that living mulches merit increased attention. Living mulches are particularly appropriate for growers willing to accept minor yield losses in exchange for ecosystem services. More than 50 yr of research on living mulches have revealed that cultural, mechanical, and chemical management practices can increase main crop yields and promote services such as weed control. Best management practices vary across living mulch systems, so further research is needed to develop system-specific recommendations. General research priorities include refining the list of effective living mulch species, improving the options available for organic systems, identifying low-input chemical control strategies, and adopting a more holistic approach to living mulch management.

Candidate living mulches have been selected from the enormous diversity of species and cultivars used as classic (terminated) cover crops and forages. Most of this diversity remains unexplored in the context of living mulch systems. However, the ideal living mulch would meet criteria not always required of other cover crops, such as complementarity with main crop species. Therefore, the widespread commercial use of living mulches may be facilitated by breeding programs (Moore et al. 2019) following the living mulch ideotypes that have already been proposed (Buhler et al. 1998; De Haan et al. 1994; Flynn et al. 2013). Another strategy involves the introduction of living mulch species to different regions. Radicetti et al. (2018) observed that intercropped subterranean clover reduced wheat yields in temperate agroenvironmental zones (Mediterranean North and Continental) but caused little yield reduction under colder temperatures (Atlantic North) or drier conditions (Mediterranean South). On a larger scale, (sub)tropical species may offer management advantages as temperate living mulches because the onset of cool temperatures could automatically terminate growth and prevent seed set. Bhaskar et al. (2020) tested two (sub)tropical species, sesbania [Sesbania sesban (L.) Merr.] and sunnhemp, as living mulches for fresh-market field tomato in New York, USA. Although sesbania did not successfully establish, the sunnhemp results were promising: sunnhemp established and grew well until growth was arrested by tomato harvesting and cool fall temperatures. In addition to being easier to terminate, non-native species are less likely to suffer from or carry native pests and pathogens. Despite these possible benefits, species introductions involve risks that should be evaluated on a case-by-case basis. Notably, introduced living mulch species might become invasive weeds, although this outcome is less likely for species that cannot set seed in the introduced range. Lastly, living mulches have been considered as components of agroecological strategies to mitigate the effects of climate change (Diacono et al. 2017), a goal that might require the identification of living mulches with broader climatic requirements.

Because herbicides are among the most effective tools for living mulch management, incorporating living mulches into organic systems may require additional planning. Such planning is often worthwhile: living mulches make valuable contributions to weed management, soil health, and other aspects of sustainability in organic systems (Leary and DeFrank 2000; Montemurro et al. 2020; Vincent-Caboud et al. 2017). In organic systems (even more than non-organic ones), it is prudent to use cultural methods to maximize the main crop's competitiveness relative to the living mulch rather than relying exclusively on mechanical control. Such methods may include identifying highly competitive crop cultivars and less competitive living mulches (including annuals) or adjusting relative planting times and densities. Fertility amendments can also modify competitive dynamics. Mechanical management programs for organic systems might involve preseason strip tillage, in-season rototilling, and/or mowing to short heights.

Chemical control provides additional options but can be challenging to implement. A single, high-rate herbicide application often eliminates the benefits of a living mulch, whereas low-rate applications tend to allow excessive mulch-crop competition. Improved control of living mulches and weeds may come from herbicide combinations, which have been common practice from the outset of living mulch research (Cardina and Hartwig 1980; Elkins et al. 1979, 1982, 1983; Hartwig 1977; Linscott and Hagin 1975). Repeated herbicide applications have also been evaluated. For instance, Pedersen et al. (2009) found an increase (generally insignificant) in soybean yield with more glyphosate applications over a kura clover living mulch. Some recent work has focused on combining these two strategies. In the study of tomato grown with a sunnhemp living mulch (Bhaskar et al. 2020) and a concurrent study of sunnhemp in monoculture (Bhaskar et al. 2021), two herbicides were applied sequentially at reduced rates. Applying a herbicide with soil residual activity followed by a herbicide with greater postemergence activity helped balance living mulch performance, weed control, and (in Bhaskar et al. 2020) tomato yield. Both reduced-rate applications included surfactants, which increased the postemergence injury caused by (primarily preemergence) residual herbicides. These findings demonstrate the potential for improved application techniques to enable herbicide rate reductions. Further research must evaluate how chemical control practices interact with decisions about planting and mechanical control.

Taken as a whole, the literature on living mulches suggests that high-precision, multipronged management approaches are most likely to result in good weed control and yield outcomes. Ideally, many aspects of cropping system management would contribute to competitive environments in which living mulches promote weed suppression without outcompeting main crops. The design of integrated management programs should reflect a long-term perspective. For example, weed suppression is relevant not only to main crop yield, but also to weed seed production. Living mulches can reduce weed seed production more effectively than cover crop residues (Teasdale et al. 2007). However, living mulches are unlikely to completely prevent weed seed production, especially if they are strongly suppressed to reduce mulch-crop competition. Management practices that kill weeds while providing milder living mulch suppression could limit additions to weed seedbanks. Trends in weed seed production and seedbanks have been studied less frequently than trends in end-of-season weed biomass (but see Brainard and Bellinder 2004; Gibson et al. 2011; Uchino et al. 2009) and represent an important area for future studies.

Weed control and main crop yield can vary widely in living mulch systems. Other benefits and drawbacks of these systems, such as long-term impacts on soil health or arthropod communities, are harder to observe. Management practices that influence one aspect of cropping system function can also influence other aspects. For all these reasons, it is difficult to determine when and how living mulches should be adopted. These knowledge gaps can be reduced by research projects that adopt standard practices and holistic, long-term perspectives. Specifically, we suggest that future experiments seek to accomplish one or more of the following goals:

Research following these guidelines could form the foundation for more quantitative cost-benefit analyses. A long-term goal should be the creation of data-driven decision-support tools identifying key challenges and opportunities associated with living mulches.

Supplementary material.

To view supplementary material for this article, please visit  https://doi.org/10.1017/wet.2021.52