Ground that just carried cereal rye or a sorghum cover crop is a poor seedbed for small seeds for a few weeks
Ground that just carried cereal rye or a sorghum cover crop is a poor seedbed for small seeds for a few weeks. As the residue breaks down it releases chemicals that stop small seeds germinating, and the smaller the seed the worse it fares; transplants and big seeds largely shrug it off.
Promising A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Rye and sorghum roots and residue release allelochemicals (benzoxazinoids from rye, sorgoleone from sorghum) that persist in the surface soil for weeks after the cover is killed. A small seed sown into that layer has little reserve to push through the inhibition; a large seed or a transplant is already past the germination window.
What to do about it
Wait about two weeks after killing rye and about four after sorghum before sowing small seeds, water an inch to leach the surface, or plant transplants. Big seeds and transplants can go in sooner.
How much it matters: Georgia saw very poor carrot germination after a sorghum-family cover and asks for four weeks and an inch of water; Iowa asks for ten to fourteen days after rye before corn and says susceptibility runs with seed size. No garden figure is given; the two lead times are the sources' own, and the rye figure rests on corn and transplant timings plus the seed-size principle. No rye-specific small-seed trial is behind it.
The evidence
Evidence: checked against 4 sources in the research-based literature.
The 4 sources
Research has shown that susceptibility to allelochemicals is indirectly related to seed size - the smaller the seed the more susceptible the plant
Small-seeded and slow germinating crops are more likely to be adversely effected by allelopathic cover crops than are large-seeded, rapidly germinating crops (Weston, 1996)
A direct-seeded crop with small seeds is particularly vulnerable to allelopathic chemicals
It needs to be terminated in mid to late spring before it goes to seed, about 2 weeks before the desired transplant date.
Plants it applies to · 2
Plant teams that rest on it
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Sources updated
- Wording made clearer
- Added to the rules
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Milpa Gardens. “Ground that just carried cereal rye or a sorghum cover crop is a poor seedbed for small seeds for a few weeks. As the residue breaks down it releases chemicals that stop small seeds germinating, and the smaller the seed the worse it fares; transplants and big seeds largely shrug it off.” Evidence grade: Promising. https://milpa.garden/rules/ground-that-just-carried-cereal-rye-sorghum/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.