What Is Green Manure? Cover Crops That Feed Garden Soil

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Green manure is a crop grown mainly for the soil it improves, not for a harvest. What is green manure in practice is a plant that is sown into a bed, allowed to grow for a period, and then cut and worked back into the soil so it can break down. The term overlaps with cover crop, and the two are often used interchangeably. The difference is in how the plant is finished: a green manure is generally incorporated into the soil, while other cover crops may be left on the surface or mowed. This article explains how green manure works, why legumes are a common choice, and why timing matters.

Quick answer

• Green manure is a cover crop grown to increase soil organic matter and nutrients, and it is usually worked into the soil rather than harvested.
• Legumes such as peas and beans are a common choice because they fix nitrogen from the air through bacteria in their roots.
• Extension guidance recommends cutting the crop before it flowers and then incorporating it so it can decompose.

What green manure is

The University of Wisconsin Extension describes the category in a direct way. It says: “Green manures [oftentimes plants in the pea (legume), mustard and grass families] are a subset of cover crops that are grown specifically to increase soil organic matter and nutrients.” The key phrase is subset. Every green manure is a cover crop, but not every cover crop is grown as a green manure. The green manure is the part of the cover crop group that is chosen for its soil-building role and then turned into the soil.

That role is the reason the plant is grown at all. A garden bed that is empty for part of the year can lose structure and nutrients, and a cover crop covers the soil while it grows. The same extension page notes that the plants help improve soil in a physical as well as a chemical sense. Growing a green manure is therefore less about feeding the crop directly and more about building the soil for the crop that follows.

How the plant feeds the soil

As the plant grows, it takes up nutrients and holds them in its tissue. The Wisconsin guidance puts it this way: “As cover crops grow, they become reservoirs for important plant nutrients such as nitrogen, phosphorus and potassium, as well as micronutrients.” The nutrients stored in the leaves and stems are not lost while the crop is growing. When the plant is cut and worked into the bed, the material breaks down and releases some of those nutrients back to the soil.

The process also adds organic matter. Plant material decomposes and becomes part of the soil’s organic content. The extension source describes the purpose of the crop as increasing organic matter and nutrients, so the organic matter is the point of the exercise, not a side effect. Gardeners who work green manure into a bed are adding the plant itself as a source of material for the soil.

Why legumes are a common choice

Legumes are a popular green manure because of nitrogen. The Wisconsin page explains: “Pea family green manures are unique in that they increase soil nitrogen levels due to bacteria (Rhizobium spp.) in their roots that convert (i.e., fix) nitrogen gas from the air into a form of nitrogen that can be used by plants.” Nitrogen is one of the nutrients that most garden crops need in large amounts, so a bed that gains nitrogen from a legume crop has an advantage over a bed that does not.

The Wisconsin page groups green manures into pea, mustard and grass families, and it singles out the pea family for the nitrogen point. A gardener who wants the nitrogen benefit described on that page should therefore look to the pea family first.

Other benefits

The same extension source lists further benefits. It says: “Cover crops also help prevent soil erosion, reduce weed problems, and provide a habitat for beneficial” insects. The erosion and weed points are the ones the extension page lists first, and the habitat point is listed alongside them without further detail.

These benefits are not guaranteed for every bed. A thin or failed stand covers less soil, and a green manure that is not cut in time does not give the same return. The extension source presents these as typical effects of a cover crop rather than promised results, and readers should treat them the same way.

Timing: why cutting before flowering matters

Timing is one of the most important parts of green manure practice. The Wisconsin guidance states: “Cutting before flowering not only prevents the cover crop from going to seed, but also stops the plant from taking up nutrients from the soil to store in its seed.” A plant that sets seed diverts nutrients into the seed, and those nutrients are then not available to the soil when the plant is worked in. Cutting before flowering keeps the nutrients in the plant tissue that will break down.

The same timing point also limits the risk of unwanted seed. A green manure that goes to seed can become a weed in later seasons. Cutting the crop before flowering addresses both problems at once. Readers who plan the crop around the local season can use the schedule in how to plant peas and oats cover crop as a related example of timing a cover crop.

How to work green manure into the bed

The incorporation step follows the cut. The Wisconsin extension page says: “Once plants have been cut, incorporate the plants into the soil (using a shovel, pitch fork or rototiller) where they can more readily decompose.” The page lists a shovel, a pitch fork or a rototiller as acceptable tools, so the choice depends on the size of the bed.

Cover crops and green manures can also be planned into a rotation. The Wisconsin guidance says they “can be incorporated into a garden as part of an annual vegetable crop rotation, or planted simply to improve the soil quality in a fallow garden area.” That gives gardeners two uses: a short green manure between two vegetable crops, or a longer soil-building period in a bed that is resting. The guide on how to make garden soil better covers other ways to build soil in a bed.

After incorporation, the bed needs time for the material to break down before the next crop goes in. The sources reviewed for this article do not give a single waiting period for every crop, so the timing for a particular vegetable should be checked against a regional extension guide. Keeping a note of when each green manure was cut and worked in makes that check easier the following season.

Frequently asked questions

Is green manure the same as a cover crop?

Green manure is a subset of cover crops grown specifically to increase soil organic matter and nutrients. Other cover crops may be mowed or left on the surface, while a green manure is usually worked into the soil.

Why do gardeners use peas or beans as green manure?

Pea family plants host bacteria in their roots that fix nitrogen gas from the air into a form plants can use. The Wisconsin guidance says this increases soil nitrogen levels.

When should green manure be cut?

The Wisconsin guidance says to cut before flowering. Cutting before the plant sets seed keeps nutrients in the plant tissue and prevents the crop from becoming a later weed.

How is green manure worked into the soil?

After cutting, the plants are incorporated into the soil with a shovel, pitch fork or rototiller so they can decompose more readily, according to the Wisconsin extension page.

Does green manure give a harvest?

Green manure is grown mainly for the soil, not for a harvest. Its purpose is to build organic matter and nutrients that the next crop can use.

What we could not verify

What the sources do not say

The sources reviewed for this article describe the purpose and general method of green manure, but they do not give a fixed waiting period between incorporation and planting, and they do not give exact nitrogen amounts for each legume. The University of Tennessee Extension publication on cover crops and green manures was opened and returned HTTP 200, but the quotations in this article come from the Wisconsin page. Readers who want species-by-species timing should check a regional extension publication for their area.

How we chose

The sources were selected from search results that pointed to university extension publications. Each page was opened directly and checked for an HTTP 200 response on 2026-10-27. Quotations were copied from the fetched text. Pages that returned errors were not counted, and commercial pages were not used for definitions.

Sources

• University of Wisconsin-Madison Extension, Horticulture, “Using Cover Crops and Green Manures in the Home Vegetable Garden,” https://hort.extension.wisc.edu/articles/using-cover-crops-and-green-manures-home-vegetable-garden/ (HTTP 200, opened 2026-10-27)
• University of Tennessee Extension, “Cover Crops and Green Manures,” https://solutions.tennessee.edu/publication/W235G (HTTP 200, opened 2026-10-27)