You must rotate every crop to new ground every single year.
You don't have to move every crop every single year by rule. What matters is keeping each crop family out of the same soil long enough to starve its diseases, commonly three to five years. Rotating yearly is a good habit. It isn't a law, and perennials like asparagus and rhubarb stay put for years.
What is actually known
Rotation works by starving a pathogen out of the soil, so the right interval depends on how long that pathogen survives, and for many diseases that cannot be stated precisely. Extension advice is to keep a crop family out of the same soil for three to five years; clubroot of the mustard family can survive seven years, bacterial spot needs at least two years without a host, and soil-dwelling soft-rot bacteria are not managed by rotation at all. Moving families every year is a sensible habit. It isn't a law, and Milpa's rules carry the intervals that have named mechanisms. Perennials are the exception. Asparagus and rhubarb stay in one spot for years, and cultivating too deep damages asparagus crowns.
The evidence
Evidence: checked against 4 sources in the research-based literature.
The 4 sources
To manage a disease successfully with rotation, one needs to know (1) how long the pathogen can survive in the soil
Rotating crops each year and only growing vegetables in the same family in the same area every 3 to 5 years starves out most pathogens that cause stem and leaf diseases.
Rhubarb is often planted to one side of the garden, where it will not be disturbed if the main garden is plowed or rototilled in the fall.
Cultivating too deep, or at the wrong time in the season, can damage the crowns and emerging spears.
What we can actually defend
- Don't plant nightshades where nightshades grew in the last 3 years
- Do not plant cucurbits in ground that held cucurbits within 2-3 years.
- Do not plant brassicas in ground that held brassicas within 4 years (extension recommends 4-7, and even that is not a cure).
- Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles. They are still not "low-pressure" and need their own rotation.
- Perennial members must not be sited in ground that is annually turned or rotated.
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Milpa Gardens. “You must rotate every crop to new ground every single year.” Evidence grade: Contested. https://milpa.garden/beliefs/rotate-year-crop-rotation-yearly/
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