A winter cover crop is sown in autumn a set time before the first fall frost - about six to eight weeks for crimson clover, 30 to 45 days for hairy vetch, four to six weeks for cereal rye - so it roots and grows before winter.
Promising · ignored, it still works, just worse · the planner suggests it
How it works
The plants need time to germinate and grow before the first hard frost and short days force them into dormancy; sown too late, a cover crop neither covers the soil through winter nor survives to grow again in spring. Sown too early, crimson clover can go to seed in the fall and not regrow until the soil warms. Cereal rye tolerates the latest sowing of the three.
What to do about it
Sow within the crop's window before your typical first fall freeze: crimson clover six to eight weeks before it, hairy vetch 30 to 45 days before, cereal rye four to six weeks before (rye can go in later and still overwinter, with less growth before winter).
How much it matters: No yield or biomass figure is stated or invented. One field study (UVM, cereal rye) found that sowing after its optimal window cut ground cover by 57 percent and biomass by 44 percent; that is rye in Vermont, not a general figure. The rule advises and never blocks.
The evidence
Evidence: confirmed against 6 sources in the research-based literature.
the 6 sources
University of Maryland Extension, 'Cover Crops For Gardens', extension.umd.edu/resource/cover-crops-gardens (fetched in-container 2026-09-25, read 2026-09-25): 'Fall-sown cover crops are typically planted in a two-month window, starting in mid-August and ending no later than mid-October. The plants should have enough time to germinate and grow before the first hard frost and short days force them into dormancy.'
USDA SARE, 'Managing Cover Crops Profitably' (3rd ed.), Crimson Clover, sare.org/publications/managing-cover-crops-profitably/legume-cover-crops/crimson-clover/ (fetched in-container 2026-09-25, read 2026-09-25): 'Seed six to eight weeks before the average date of first frost'; 'Don’t plant too early or crimson clover will go to seed in the fall and not regrow in spring until the soil warms up enough to germinate seeds.'
USDA SARE, 'Managing Cover Crops Profitably', Hairy Vetch, sare.org/publications/managing-cover-crops-profitably/legume-cover-crops/hairy-vetch/ (fetched in-container 2026-09-25, read 2026-09-25): 'Plant vetch 30 to 45 days before killing frost for winter annual management'
Clemson Cooperative Extension HGIC, 'Cover Crops for the Fall', hgic.clemson.edu/cover-crops/ (fetched in-container 2026-09-25, read 2026-09-25): 'Legumes should be planted 6 to 8 weeks before a killing frost, and non-legumes should be planted 4 to 6 weeks before a killing frost.'
USDA SARE, 'Managing Cover Crops Profitably', Selecting the Best Cover Crops for Your Farm (fetched in-container 2026-09-25, read 2026-09-25): 'In many regions, seed winter covers at least six weeks before a hard frost. Winter cereals, especially rye, are an exception and can be planted a little later. If ground cover and N recycling needs are minimal, rye can be planted as late as the frost period for successful overwintering.'
University of Vermont Extension, NW Crops & Soils, 'Optimize Your Cereal Rye Cover Crop with the Right Sowing Dates and Seeding Rates', uvm.edu/extension/nwcrops (fetched in-container 2026-09-25, read 2026-09-25): 'the optimal window for sowing cereal rye is during the four-week window around the historic first frost date for that location.'; 'At all sites, sowing after that window decreased ground cover by 57% and biomass by 44%.'
Where this rule stops
A STANDING CULTURAL ADVISORY (trigger.kind always), gated by applies_to to the three fall-sown winter cover crops. Each crop's window is its own sourced field, fall_sow_days_before_frost; the calendar counts it back from the garden's typical first freeze (32 F, the median) and dates one autumn job from it - a heads-up that errs early, since some sources count from the later killing frost. It is the cover-crop sibling of garlic's another rule here. It gates nothing and docks no score.
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- corpus: another rule here - sow a winter cover crop a set time before the first fall frost
Cite this page
Milpa Gardens. "A winter cover crop is sown in autumn a set time before the first fall frost - about six to eight weeks for crimson clover, 30 to 45 days for hairy vetch, four to six weeks for cereal rye - so it roots and grows before winter." Evidence grade: Promising. https://milpa.garden/rules/winter-cover-crop-sown-autumn-set-time/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.