Green mulch is a method of soil management that relies on growing plants, rather than applying inert materials like wood chips or plastic, to cover and protect the ground. This practice uses live, actively growing vegetation, often referred to as a living mulch or cover crop, to perform the functions of traditional mulch. The goal is to create a dense, living canopy that shades the soil, conserves moisture, and suppresses weed growth. Unlike static mulches that simply degrade over time, green mulch is a dynamic, biological system that actively contributes to the health of the entire soil ecosystem. It provides continuous benefits both above and below the surface, making it a sustainable choice for gardeners and farmers.
Defining Green Mulch: Living Ground Covers
Green mulch fundamentally differs from traditional, static mulches because it is a living, biological entity. Instead of being a passive layer applied once or twice a year, this system is constantly growing, respiring, and interacting with the soil environment. The vegetation forms a continuous cover that can be used seasonally, like a fast-growing cover crop that is later terminated, or permanently, such as a low-growing perennial groundcover planted between vegetable rows or in pathways.
This living ground cover offers dynamic benefits that static materials cannot match. Its root systems bind the soil, preventing erosion and runoff, while the above-ground canopy regulates soil temperature more effectively than a non-living material. By keeping the soil covered year-round, green mulch actively supports a thriving microbial community and increases the long-term organic matter content of the soil.
Functional Categories of Green Mulch Plants
Green mulch plants are chosen based on the primary ecological service they provide to the soil and the main crop. These plants can be grouped into distinct categories, each offering specialized benefits.
Nitrogen Fixers
This group includes legumes such as crimson clover, hairy vetch, and common vetch. These plants form a symbiotic relationship with Rhizobium bacteria in their root nodules, converting atmospheric nitrogen into a form usable by plants. This process significantly reduces the need for synthetic nitrogen fertilizers, making the nutrient available to the main crop when the mulch plant is terminated or naturally dies back.
Biomass Producers and Soil Tillers
This category includes plants selected for their ability to generate large amounts of organic matter and physically improve soil structure. Species like cereal rye, sorghum-sudangrass, and oats rapidly produce high volumes of shoots and roots. The dense, fibrous root systems of grasses help to aggregate soil particles, while brassicas like forage radish have a deep taproot that physically penetrates and breaks up compacted layers in the subsoil. This deep rooting action creates macropores that enhance water infiltration and aeration.
Weed Suppressors
Weed Suppressors use both physical and chemical means to inhibit unwanted plant growth. Dense, low-growing plants like white clover or creeping thyme physically shade out weed seedlings, preventing them from accessing necessary light. Furthermore, some plants, like cereal rye and mustard, exhibit allelopathy, releasing biochemical compounds into the soil that naturally inhibit the germination or growth of weed seeds.
Mechanism of Soil and Weed Management
The efficacy of green mulch stems from a complex array of biological and physical mechanisms working below and above the soil surface. Enhanced nutrient cycling is a significant process, especially when legumes are used. When plant residue decomposes, it releases nutrients that were scavenged and held by the living cover. This synchronizes nutrient availability, preventing the leaching of nitrates and making them accessible to the subsequent main crop.
The biological process of allelopathy is a key mechanism for weed management that goes beyond simple competition. Plants like cereal rye release phytotoxic compounds, known as allelochemicals, through root exudation or the decomposition of plant residue. These chemicals, which include phenolic acids and glucosinolates, can inhibit the germination time and root length of competing weed species. This provides a continuous, natural form of weed control.
Below the surface, the dense root mass of the green mulch dramatically improves soil structure. The roots create stable soil aggregates, which increases macropores for better water infiltration and gas exchange. This improved aeration and drainage reduces soil compaction and supports a healthier population of beneficial soil microorganisms. The living canopy also reduces soil temperature fluctuations and minimizes water evaporation.
Establishing and Maintaining Green Mulch Systems
Establishing a green mulch system requires careful planning regarding the timing and method of seeding to ensure the cover crop does not compete too aggressively with the main crop. Seeding can be done by broadcasting the seeds over the soil surface or by drilling them in rows, depending on the scale and species chosen. For annual cover crops, the planting window is dictated by the need to maximize biomass production before the main crop is sown, or before winter terminates the cover naturally.
In perennial systems, the green mulch is often established between the rows of the main crop, a practice called intercropping. This method requires selecting a low-growing species that will not out-compete the cash crop for light or water, such as kura clover or creeping fescue. Maintenance involves managing a living organism rather than simply topping up material, and includes occasional mowing or trimming to prevent the mulch from growing too tall or going to seed.
Termination of the green mulch is a critical management step for seasonal cover crops, designed to convert the living plants into a protective layer of dead organic matter. Mechanical methods like roller-crimping are highly effective in no-till systems, where a specialized tool flattens the plants and crimps the stems without severing them. This action stops the flow of nutrients and water, creating a dense, uniform mat of residue that suppresses weeds and decomposes slowly over the following season. For less vigorous plants, a simple chop-and-drop technique using a mower or scythe can achieve the same result.