Residue on the soil surface increases soil moisture by increasing infiltration of rainfall and by decreasing evaporative moisture loss. Higher soil moisture under cover crop residue could either benefit or retard weed germination depending on species requirements. Under saturated soil conditions, residue could slow evaporation and reduce germination of species inhibited by excess soil moisture. Under droughty conditions, retention of soil moisture could enhance weed germination and seedling survival. Chemical compounds released from cover crop residue have potential to stimulate or inhibit weed germination and growth.
Nitrates released by legume residue can stimulate germination of selected weed species.
On the other hand, research has demonstrated the presence of toxins known as allelochemicals that inhibit germination and growth of many weed species. In natural environments it is difficult to separate allelopathic effects from the physical effects described above. There are many approaches to incorporating living mulches into cropping systems. A few examples of living mulches include perennial sod-like mulches that are malntained for many years once established, a cover crop into which the cash crop is relay planted, and a cover crop overseeded into a cash crop. In almost every case where living mulches are competitive enough to successfully displace weeds, they also are competitive enough to reduce crop growth and yield as well.