Living mulches are cover crops grown simultaneously with and in close proximity to cash crops. Advantages of living mulches over dead cover crops may include increased weed suppression, reduced erosion and leaching, better soil health, and greater resource-use efficiency. Advantages of living mulches over synthetic mulches may include enhanced agroecosystem biodiversity and suitability for a wider range of cropping systems. A major disadvantage of this practice is the potential for competition between living mulches and cash crops. The intensity and outcome of mulch-crop competition depend on agroecosystem management as well as climate and other factors. In this review, we consider the management of living mulches for weed control in field and vegetable cropping systems of temperate environments. More than 50 yr of research have demonstrated that mechanical or chemical suppression of a living mulch can limit mulch-crop competition without killing the mulch and thereby losing its benefits. Such tactics can also contribute to weed suppression. Mechanical and chemical regulation should be combined with cultural practices that give the main crop a competitive advantage over the living mulch, which, in turn, outcompetes the weeds. Promising approaches include crop and mulch cultivar selection; changes to planting time, density, and planting pattern; and changes to fertilization or irrigation regimes. A systems approach to living mulch management, including an increased emphasis on the interactions between management methods, may increase the benefits and lower the risks associated with this practice.

Cotton (Gossypium hirsutum L.) is a major crop in the Vidarbha region of central India. The vertisol soils on which much of the cotton is grown have been severely degraded by the tropical climate, excessive tillage and depletion of organic matter. Living mulches have the ability to mitigate these problems but they can cause crop losses through direct competition with the cotton crop and unreliable weed control. Field experiments were conducted in 2012 and 2013 at four locations in Vidarbha to study the potential for growing living mulches in mono-cropped cotton. Living mulch species evaluated included gliricidia [Gliricidia sepium (Jacq.) Kunth ex Walp.], sesbania [Sesbania sesban (L.) Merr.], sorghum sudan grass [Sorghum bicolor (L.) Moench × Sorghum bicolor (L.) Moench ssp. Drummondii (Nees ex Steud.) de Wet & Harlan] and sunnhemp (Crotalaria juncea L.). Living mulch height was controlled through mowing and herbicides were not used. Living mulches generated 1 to 13 tons ha⁻¹ of dry matter across sites and years. Weed cover was negatively correlated with both living mulch biomass and cover. Where living mulches were vigorous and established quickly, weed cover was as low as 7%, without the use of herbicides, or inter-row tillage. In a dry year, living mulch growth had a negative impact on cotton yield; however, in a year when soil moisture was not limiting, there was a positive relationship between cotton yield and living mulch biomass. Use of living mulches in cotton production in the Vidarbha region of India is feasible and can lead to both effective weed suppression and acceptable cotton yields.

Living mulches can provide many sustainability benefits. However, living mulch–cash crop competition and unreliable weed control are major challenges in living mulch systems. In this study, we evaluated the potential of herbicides used at reduced rates in combination with living mulch to suppress weeds, while simultaneously reducing living mulch vigor. Herbicide treatments were a combination of two POST applications, each consisting of a single, different herbicide. Field trials were conducted in Freeville, NY, USA, using: fresh market field tomato as cash crop; sesbania and sunn hemp as living mulch; and the herbicides fomesafen, halosulfuron, metribuzin, and rimsulfuron. In 2015, when water was not limiting, tomato yield and living mulch biomass were positively correlated. This relationship was negative in 2016, likely because of drought during the growing season. Compared with the untreated living mulch check, using the herbicide treatments in combination with living mulch reduced tomato yield losses by up to 71% in 2015 and 51% in 2016. In these herbicide plus living mulch plots, weed biomass was reduced by up to 97%, compared with the weedy check. Living mulch in herbicide treatments generated up to 2500 kg ha⁻¹ of dry matter during both 2015 and 2016, with an average ground cover of 63% in 2015 and 85% in 2016. A predominantly PRE herbicide with residual soil activity (metribuzin), followed by a herbicide with greater POST activity (halosulfuron/rimsulfuron) was the most effective herbicide application sequence. Results from our study indicate that well-designed herbicide applications may enhance the practicability of living mulch systems.

Striking a balance between the weed control capacity of living mulches and their competition with the main crop is complex. At rates that avoid severe injury to living mulch, herbicides may reduce their vigor while simultaneously contributing towards weed control. In a two-year field study carried out in Freeville, NY, USA, we evaluated the effects of various combinations consisting of two herbicides, applied sequentially at reduced rates, on the growth of a sunn hemp living mulch and weeds (including common lambsquarters, common purslane, hairy galinsoga, and Powell amaranth). When a herbicide with primarily POST activity (Type 1) (e.g., rimsulfuron, 0.005 to 0.007 kg ai ha-1) was applied first, performance of sunn hemp (1.7 to 3.9 tons ha-1 dry biomass; 10 to 88% groundcover) was poor and weed growth (25 to 62% groundcover) was high, likely because sunn hemp, severely injured at a young growth stage, was outcompeted by weeds. A follow-up application (approximately two weeks later) of a herbicide with primarily PRE and residual activities (Type 2) (e.g., metribuzin, 0.05 to 0.15 kg ai ha-1), with a surfactant to enhance its POST activity, had little effect on established weeds. However, because sunn hemp was already 20 cm tall at weed emergence, applying a Type 2 herbicide first, did not cause severe injury to sunn hemp and reduced weed pressure, thereby also enhancing sunn hemp performance (3.8 to 6.1 tons ha-1 dry biomass; 85 to 94% groundcover). Moreover, the follow-up application of a Type 1 herbicide affected the smaller weeds more (4 to 21% groundcover) than the better-established sunn hemp. Our results demonstrate that an appropriate sequence of herbicides at reduced rates may be important to control weeds while maintaining a healthy living mulch stand.