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Irrigation, Soil & Fertility TimingGreen Manure Timing Calendar
A green manure is grown to be worked back into the soil, and its value depends on cutting it while it is still tender and giving the residue time to break down before you plant.
Key points
- Use green manure to fill a real gap or fix a soil problem.
- Sow by soil temperature and frost dates, not calendar dates.
- Cut before seed set, while tissue is still soft.
- Give residue time to break down before seeding.
- Keep soil testing; do not assume a full nitrogen credit.
What is a green manure and when does it fit?
A green manure is a crop grown mainly to feed the soil, not to harvest. Legumes, grasses, brassicas and mixes are all used, each with different strengths. It fits best in a gap in the rotation, such as after an early crop or before a late planting, or as a full season rest for a worn bed.
Because the crop takes up space and water for weeks, it only pays when the soil benefit matches a real need such as low organic matter, compaction or weed pressure.
Soil moisture can limit how well green manures break down. In dry soils, residue can sit for weeks with little change, and in saturated soils it can ferment and cause odors and problems for seedlings. Water management helps the process along.
When should you sow it?
Sow when soil temperature and moisture suit the species. Cool-season types go in during late summer and early fall or very early spring, while warm-season types need warm soil and are sown after the last frost has passed. Frost dates and soil temperature, not the date on the calendar, set the window.
For example, if your first fall frost usually arrives in a given week, count back the weeks a hardy species needs to establish before it, and sow with some margin. If you are late, switch to a faster species or a winter-hardy one.
Compare green manure with other organic matter sources. Compost brings nutrients but needs hauling, while a green manure grows in place. If the field cannot spare the time, a shorter cover crop may fit better.
When is the right moment to cut and incorporate?
Most guidance favors cutting before seed set and while stems are still soft. For many legumes that means somewhere around early flowering, and for grasses before they turn stemmy. Younger material breaks down faster and has a lower carbon-to-nitrogen ratio, so it releases nutrients sooner and ties up less nitrogen.
Older, woody growth decomposes slowly and can hold nitrogen away from the next crop for a while. If you miss the stage, mow and let the residue sit on the surface, or add it to the compost pile.
Record the sowing date, termination date and weather. After two or three seasons you will see which species and timings pay off in your soil.
How long should you wait before planting?
Residue breaks down faster in warm, moist soil and slower in cold or dry soil. A common recommendation is to wait a couple of weeks or more between turning in and seeding, but the right gap depends on residue amount, species and temperature. Ask your extension office for the interval that fits your crop pairing.
Plant a few seeds in a pot of the treated soil, or a small test row, if you are worried about residue effects on germination.
What mistakes should you avoid?
Letting the crop set seed turns it into a weed. Incorporating on saturated soil damages structure. Expecting a legume to replace all fertilizer is optimistic, because the nitrogen release varies, and a soil test should still guide your plan.
Note too that some species host pests or diseases that affect related cash crops, so check the rotation. Seed sources and noxious weed rules differ by state.
What does a worked example look like?
Suppose you sow a hardy cover in late summer after an early crop. It grows until frost, overwinters and resumes growth in spring. You would cut or turn it before seed set, wait for the residue to start breaking down, test germination if you are concerned and then plant.
In a colder region, the same crop may winter-kill. Then you would simply plant into the residue in spring. The calendar changes with climate, so use your local frost and soil temperature records.
How do you pick species for your rotation?
Think about what the soil needs. If it needs organic matter, a high-biomass grass or mixed cover may suit it. If it needs nitrogen, a legume may help, though the release timing varies. If it needs compaction relief, a deep-rooted type might fit.
Match the species to your window of growth and to the next crop. Avoid species that host pests of the following crop. Your extension office can recommend options with local experience.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.