How to Plan a Crop Rotation by Plant Family

You rotated the beds carefully — potatoes where the tomatoes were, tomatoes where the potatoes were — and blight arrived anyway.

Those two crops are the same botanical family. They share the same diseases, and swapping them achieved nothing at all.

Rotation works by starving pathogens

Most soil-borne pests and diseases are family-specific. Clubroot attacks brassicas. Onion white rot attacks alliums. Potato cyst nematode attacks potatoes and their relatives.

They persist in soil after the host is removed, and their populations decline over time in the host’s absence. Rotation exploits that: keep a family away from a bed long enough and the pathogens specific to it drop to levels that don’t cause visible damage.

Which means the grouping has to follow botany, not the kitchen. Potatoes and tomatoes are both Solanaceae. Rocket and cabbage are both brassicas. Grouping by how you cook them is the most common rotation mistake and it defeats the entire purpose.

The main families worth knowing:

• Solanaceae — potato, tomato, pepper, aubergine
• Brassicaceae — cabbage, broccoli, kale, swede, turnip, radish, rocket
• Alliaceae — onion, garlic, leek, shallot
• Fabaceae — peas, beans (the nitrogen fixers)
• Apiaceae — carrot, parsnip, celery, parsley
• Cucurbitaceae — courgette, squash, cucumber

Sequence matters too

Beyond separation, the order can work in your favour.

Legumes fix nitrogen through root nodule bacteria and leave some in the soil. Following them with a hungry crop — brassicas especially — uses that directly.

Root crops after brassicas works because brassicas are typically manured and roots grown in freshly manured soil tend to fork.

A common four-year sequence: legumes → brassicas → roots → alliums, then back to legumes.

How long, and what rotation can’t fix

Three years is the practical minimum, four is a common standard, and longer is better where you’ve had a problem.

But some pathogens outlast any realistic rotation. Clubroot in brassicas can persist in soil for many years — well beyond a four-year cycle — which means rotation alone won’t defeat it. Managing clubroot needs liming to raise pH, resistant varieties, raising plants in modules to establish a strong root system, and accepting affected beds are out for a long time.

Onion white rot is similarly persistent. Knowing which problems rotation solves and which it merely slows saves a lot of misplaced confidence.

Common mistakes to avoid

• Grouping by culinary category rather than botanical family.
• Forgetting that a green manure or a cover crop belongs to a family too — mustard is a brassica.
• Rotating the main crop and leaving a perennial herb from the same family in place all year.
• Assuming rotation defeats clubroot or white rot.
• Giving up on rotation because you only have two beds — a two-year gap is better than none.

How to do it with Crop Rotation Planner

The Crop Rotation Planner groups by family and lays out the years.

1. Enter how many beds you have and how long a rotation you can sustain.
2. Group crops by botanical family, not by how you eat them.
3. Sequence so legumes precede hungry crops and roots avoid fresh manure.
4. Note any bed with a persistent problem separately — rotation won’t clear it alone.

The RHS guidance on crop rotation covers the family groupings in detail. Other garden tools are in the tools directory.

Frequently asked questions

Why group by plant family?

Because pests and diseases target families, not culinary categories. Tomatoes and potatoes are both nightshades and share blight, so following one with the other defeats the point.

How long should a rotation be?

Three years is the practical minimum, four a common standard. Some pathogens — clubroot particularly — survive well beyond any realistic rotation and need other measures.

What if I only have two beds?

Rotate anyway; a two-year gap beats none. Pay more attention to soil health, resistant varieties and removing infected material, and consider containers to break the cycle for one family.

Final thought

Write the family beside each crop on your plan. Once the plan is grouped botanically, the rotation almost designs itself — and the mistake that produces blight disappears.

Try the free Crop Rotation Planner