Move crops by plant family so soil-borne pests and diseases can't settle in - what to rotate, how far apart, and what rotation can and can't do.
Crop rotation is not planting the same kind of vegetable in the same ground two years running. That is the whole idea in one sentence. The reason it matters is the part worth understanding, because it decides how you do it.
Why it works - and it is not one reason but three
A plant's specialist pests and diseases build up in the ground it grows in. Rotate that plant away and its specialists lose their host and decline; put a plant from a different family in its place and they have nothing to eat.
- Soil-borne disease. Many fungi and bacteria overwinter in the soil and in last year's plant debris, and each prefers certain plants. Growing the same crop in the same spot feeds them.
- Insects and nematodes. Crops in the same family share pests. Moving the crop breaks the pest's life cycle before it can settle.
- Nutrients. Different crops draw a different mix of nutrients at a different pace, and legumes hand nitrogen back to the ground for whatever follows them.
The planner tracks the first of these for you, and it tracks it precisely, by family:
PromisingDo not plant nightshades in ground that held nightshades within 3 years.
costly if ignored → warn
Mechanism: Alternaria solani (early blight) and Septoria lycopersici overwinter on tomato debris and solanaceous weeds; a 2-3 year break plus debris removal starves this inoculum. Fusarium has a very narrow host range, so rotating away from the host is effective. Verticillium is the exception: microsclerotia persist ~10 years and it infects 300+ host families, so a garden-scale rotation does not control it.
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
Clemson HGIC, 'Tomato Diseases & Disorders', hgic.clemson.edu/factsheet/tomato-diseases-disorders (read 2026-07-11): 'tomatoes and related vegetables, such as potatoes, peppers, and eggplants, should not be planted on the same land more than once in three years.'
NC State Extension, 'Early Blight of Tomato', content.ces.ncsu.edu/early-blight-of-tomato (read 2026-07-11): 'Rotate every 2-3 years.'
NC State Extension, 'Septoria Leaf Spot of Tomato', content.ces.ncsu.edu/septoria-leaf-spot-of-tomato (read 2026-07-11): 'The fungus is known to overwinter on infected tomato debris and solanaceous weed hosts'; 'Rotate away from tomato for 2 or more years.'
NC State Extension, 'Verticillium Wilt of Tomato and Eggplant', content.ces.ncsu.edu/verticillium-wilt-of-tomato-and-eggplant (read 2026-07-11): 'The pathogen can survive in the soil without a host for up to 10 years in the form of structures called microsclerotia'; 'affects more than 300 host families.'
UMass Extension, 'Solanaceous, Verticillium Wilt', umass.edu/agriculture-food-environment/vegetable/fact-sheets/solanaceous-verticillium-wilt (read 2026-07-11): 'Verticillium wilt tends to affect a wide range of hosts and can infect many unrelated plants. Fusarium spp., on the other hand, have very narrow host ranges.'
remedy
Plant nightshades in different ground, or wait until 3 years have passed since this ground last held them. Clear and remove the old debris either way — the inoculum overwinters on it.
Read the full rule → · open in the planner ↗
PromisingDo not plant cucurbits in ground that held cucurbits within 2-3 years.
costly if ignored → warn
Mechanism: Melittia cucurbitae overwinters as a pupa in the soil where its host grew and emerges the next summer; Anasa tristis overwinters as an adult in sheltered sites (plant debris, under boards or rocks) and flies to cucurbits in spring. A host-free break cuts local carryover, but both are strong fliers that re-invade from a distance, so rotation is only partial and works best paired with fall sanitation (destroy spent vines).
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
UMN Extension, 'Squash vine borers', extension.umn.edu/yard-and-garden-insects/squash-vine-borers (read 2026-07-11): 'squash vine borers spend the winter in the soil near their host plants ... Practice rotation ... planting cucurbits in different areas of your garden if possible, or alternate seasons'; 'rotate away from the area and destroy crop residue.'
Ohioline (Ohio State Univ.), 'Squash Vine Borer', ohioline.osu.edu/factsheet/ent-0106 (read 2026-07-11): 'Crop rotation away from previously infested fields helps reduce local populations, as overwintering pupae remain in the soil. However ... fields often need to be rotated more than 5 miles away as SVB can fly into fields from long distances.'
UMN Extension, 'Squash bugs', extension.umn.edu/yard-and-garden-insects/squash-bugs (read 2026-07-11): 'Squash bugs can live through the winter as adults in sheltered places, such as under plant debris, around buildings, or under rocks. When adults come out in the spring, they fly to growing cucurbit plants.'
remedy
Plant cucurbits in different ground, or wait 3 years. Destroy the spent vines in autumn and clear debris and boards nearby — the pupae overwinter in the soil where the crop grew, the adults in the shelter around it. Rotation is only partial; both pests fly in from a distance.
Read the full rule → · open in the planner ↗
PromisingDo not plant brassicas in ground that held brassicas within 4 years (extension recommends 4-7, and even that is not a cure).
costly if ignored → warn
Mechanism: Plasmodiophora brassicae forms thick-walled resting spores that survive in soil more than 10 years. They germinate in wet, acidic soil (pH < 7); liming above ~7.2 suppresses infection. Spores outlast any garden rotation, so a host-free break reduces but does not clear inoculum.
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
NC State Extension, 'Clubroot of Brassicas', content.ces.ncsu.edu/clubroot-of-brassicas (read 2026-07-11): 'Plasmodiophora brassicae is capable of surviving in the soil for more than 10 years by producing thick-walled, resting spores'; 'Infested fields should be rotated away from brassica for a minimum of 4-7 years, but this is not guaranteed to prevent disease'; 'Liming to raise soil pH to 7.[2]'.
Cornell Vegetables, 'Clubroot of Crucifers', vegetables.cornell.edu/pest-management/disease-factsheets/clubroot-of-crucifers (read 2026-07-11): 'the pathogen survives for many years even in the absence of host crops'; 'rotation of at least 7 years out of susceptible crucifers may be effective.'
UMass Extension, 'Brassicas, Clubroot', ag.umass.edu/vegetable/fact-sheets/brassicas-clubroot (read 2026-07-11): 'Long (5-7 years) rotations between brassica crops'; 'Maintain a high pH by regular applications of lime.'
remedy
Plant brassicas in different ground, or wait at least 4 years — extension recommends 4 to 7. Rotation alone is not a cure: the resting spores outlast it, and raising soil pH above ~7.2 suppresses infection.
Read the full rule → · open in the planner ↗
Alliums (onion, garlic) and the carrot family are unrelated to those three, so they can sit between them in a rotation - but they are not a free pass, and they need their own rotation too:
A good hunchAlliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles — but they are not "low-pressure" and need their own rotation.
suboptimal if ignored → advise
Mechanism: They host none of the big-three families' soilborne pathogens, so following those crops with an allium or apiaceous crop breaks the host cycle. But they are not inherently low-pressure: allium white rot (Sclerotium cepivorum) is allium-specific, its sclerotia persist 15+ years, and it cannot be eradicated once established; apiaceous crops carry their own soilborne diseases.
Evidence: confirmed against 1 source in the research-based literature.
the source
UMass Extension, 'Alliums, White Rot', ag.umass.edu/vegetable/fact-sheets/alliums-white-rot (read 2026-07-11): 'Sclerotium cepivorum is specific to allium species'; sclerotia 'may remain viable in soil for 15 years or more without allium hosts'; 'Currently, there is no way to eradicate white rot from a field where it has become established.'
Read the full rule → · open in the planner ↗
Rotate by family, not by crop
The pest does not know that a tomato and a potato are sold under different names - they are both Solanaceae, and a disease of one is a disease of the other. So you rotate whole families. The ones worth tracking in a vegetable garden:
- Nightshades (Solanaceae): tomato, pepper, eggplant, potato, tomatillo
- Legumes (Fabaceae): beans, peas - the nitrogen-fixers
- Brassicas (Brassicaceae): cabbage, broccoli, cauliflower, kale, radish, turnip
- Cucurbits (Cucurbitaceae): cucumber, squash, pumpkin, melon
- Alliums (Amaryllidaceae): onion, garlic, leek, chive
- Carrot family (Apiaceae): carrot, parsnip, celery, parsley, dill
The standard advice is to grow a family in the same ground at most once every three years, and four to five years is better for the pests that are slow to fade. The exact interval is a rule of thumb, not a constant - it is the corpus's own rotation windows above that the planner actually enforces, family by family.
If you plant a team of crops together - a Three Sisters bed, a salsa garden - the rotation clock is set by every family in it, across the whole area it covered:
PromisingA guild deposits family history across its whole footprint.
costly if ignored → warn
Mechanism: Soilborne inoculum and overwintering pest stages disperse across the bed. A guild dominated by one family forecloses that ground for that family's rotation interval.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Same soilborne-persistence basis as other rules here (read 2026-07-11): NC State Extension (Verticillium microsclerotia 'can survive in the soil without a host for up to 10 years'; Septoria/early blight 'overwinter on infected tomato debris and solanaceous weed hosts') and UMN/Ohioline (squash bug overwinters 'as adults in sheltered places ... they fly to growing cucurbit plants'; squash vine borer 'overwinters in the soil as a pupa').
Those sources establish that inoculum and pest stages persist in the ground and are not confined to the exact spot a plant occupied; this rule applies that persistence across the whole guild footprint. The whole-footprint attribution is the corpus's rotation geometry (history attaches to ground, not to the individual plant — see the data model), not a claim from an external source.
remedy
Rotate the whole bed the guild occupied, not just the rows that family grew in.
Read the full rule → · open in the planner ↗
This is why rotation history in this planner attaches to the ground, not to the bed. When you rearrange the yard, the record of what grew where survives, because it was never pinned to a bed that no longer exists.
What rotation can't do - the honest limit
Rotation only helps against pests that stay in the soil where you left them. It does little against insects that fly in from a neighbour's garden, against spores that blow on the wind, or against the broad-appetite diseases that attack hundreds of unrelated plants. Southern blight, for instance, hits more than 500 species across many families - no rotation partner is safe from it, though a grass like sweet corn is a better bet than another broadleaf crop.
So rotate for the pests it works on, and lean on the other honest tools - resistant varieties, clean debris removal, good spacing and airflow - for the ones it doesn't. Rotation is one lever, not the only one.
Does this apply in a raised bed?
Yes - the pathogens live in the soil, not in the wooden frame, so a raised bed needs rotating exactly as the open ground does. And no, the answer is not to dig out and replace the soil every year: that is the expensive way to do what rotation does for free. There is a guide to that question of its own; this one is the why behind it.
