Green manure refers to a cover crop that is specifically grown to improve soil fertility, structure, and overall health. As the name suggests, green manure is made up of plants that are grown, harvested, and then incorporated back into the soil, mimicking the benefits of traditional animal manure. This natural and sustainable practice enhances soil quality without the need for synthetic fertilizers and chemical additives.

Using green manure in your garden brings a multitude of benefits. Firstly, it acts as a natural compost, adding valuable organic matter to your soil. The decomposition process feeds the microorganisms in the soil ensuring their continuation. As the cover crop decomposes, nutrients such as nitrogen are added back to the soil to be available for the next crop.

By growing green manure when other plants would not be growing, it helps improve soil structure by enhancing its ability to retain moisture and preventing erosion. Having active plants growing throughout the year also supports the continuation of microorganism life as it begins to die off in exposed bare soil devoid of plant life.

Another vital advantage of green manure is its ability to suppress weeds. By growing a cover crop, you create a dense layer of vegetation that outcompetes and shades out unwanted weed species, reducing the need for weeding. This not only saves you time and effort but also contributes to a healthier garden ecosystem as weeds are typically heavy feeders reducing the available nutrients in the soil.

So, how can you incorporate green manure into your gardening routine? As the typical growing season comes to an end (or even early in the season before you can plant out more frost-sensitive crops), plant a cover crop such as clover, peas, or vetch in your empty garden beds. These crops are especially rich in nitrogen, the nutrient most often depleted from soils.

Allow the cover crop to grow as long as possible (generally 1-3 month) for ample foliage but not long enough to go to seed. Then cut the green foliage down and incorporate it into the soil disturbing the soil as little as possible. If a killing frost is in the forecast and you don’t think your cover crop will survive, it may be best cut down the cover crop beforehand as the benefits green manure come from incorporating the cover crop into the soil while it’s still green. While it’s recommended to allow several weeks for the cover crop to start decomposing prior to planting main crops, you can plant at any time. Keep in mind, it may take several weeks for the nutrients from the cover crops to be available for your main crops.

To ensure maximum benefits, it’s recommended to rotate your green manure crops regularly. This prevents the buildup of pests and diseases, while also diversifying the nutrients and organic matter being added to the soil. By implementing this practice, you’ll witness the long-term improvements in soil fertility and the overall health of your garden.

The most common types of cover crops grown specifically for that purpose are legumes, grasses, and brassicas:

Legumes: Legumes are nitrogen fixers–plants that form symbiotic relationships with soil microorganisms that convert atmospheric nitrogen into a form of nitrogen that is usable by plants. While this ability is dependent upon whether your soil has the microorganisms capable of producing this task, nitrogen fixing plants allow you to add more nitrogen back into the soil than originally was available for plant use when it decomposes. Note that a common misconception of nitrogen fixers is that it add nitrogen back into the soil while they are growing. Extra nitrogen is only added if those plants are then subsequently added to the soil to decompose.

Additionally, legumes improve soil organic matter and soil structure and prevent erosion. Legumes typically have slower growth than grasses. Some common varieties of legume cover crops are cowpeas, field peas, clover, and hairy vetch.

Grasses: Most grasses scavenge nitrogen–taking up available nitrogen from the soil. While grasses will initially remove nitrogen from the soil, the nitrogen is returned back to the soil when the plant matter is added in to decompose. Nitrogen scavenging prevents nitrogen from running off, and instead holds it in the foliage slated for decomposition for providing nutrients to the next crop.

Additionally, grasses improve soil organic matter and soil structure and prevent erosion. Grasses have relatively quick growth. Some common varieties of grass cover crops are rye, oats, wheat, and barley.

Brassicas: Brassicas scavenge nutrients similar to grasses. They have added benefits of alleviating soil compaction and suppressing soil-borne pests and diseases.  Chemical reactions occur when some brassica varieties are chopped up before being adding to the soil. These chemical reactions have fumigant properties which suppress numerous crop pests including nematodes, disease-causing pathogens, and weeds. Some common varieties of brassica cover crops are mustard, radish, rapeseed.

You can plant more than one of these varieties in the same garden bed for maximum benefits. And despite these being the most popular varieties, nearly any plant can be used as green manure incorporated into the soil. Even the leaves of weeds (excluding seeds or roots, so as to not encourage more weed growth) can be suitable green manure adding nutrients back to the soil. Using what you have is a great way to recycle garden waste and save money.