What is club root, and what can I do about it?

Club root is the most serious soil-borne problem a vegetable grower meets, and it arrives silently — usually on a tray of bought brassica plants. The organism infects the roots, causes them to swell into useless galls, and destroys the plant's ability to take up water. Everything you can do is management rather than cure, and knowing that early saves a lot of wasted effort.

Safety

Step by step

1. Dig up a suspect plant and look at the roots.This is the only reliable diagnosis. Club root produces swollen, knobbly, distorted roots fused into lumpy masses, with the fine feeding roots largely gone. Top growth alone is not enough — several other things stunt brassicas.
2. Distinguish it from cabbage root fly and nematode galls.Cabbage root fly leaves roots eaten away with small white maggots present. Nitrogen-fixing nodules on legumes are small, round, discrete and detach easily. Club root galls are part of the root itself and cannot be pulled off.
3. Accept that there is no cure.No chemical, biological or home remedy removes it from soil. The resting spores survive fifteen to twenty years without a host. Any advice promising to clear it is wrong.
4. Raise the soil pH with lime.The organism works best in acid soil and is much less damaging as pH rises towards neutral or slightly alkaline. Test the soil, apply garden lime well ahead of planting — ideally the previous autumn — and re-test periodically.
5. Improve drainage.It spreads through water in soil, so wet ground makes it far worse. Raised beds, added organic matter and avoiding compaction all reduce infection pressure noticeably.
6. Plant out from large pots.This is the most useful practical trick. Raise brassicas in 9cm or one-litre pots of clean compost until they have a substantial root system, then plant out. A large root ball can outgrow the infection enough to crop, where a small transplant is overwhelmed.
7. Grow resistant varieties where they exist.Club-root-resistant varieties are available in several brassicas and are genuinely useful, though resistance is partial and can be eroded if they are grown continuously.
8. Rotate as long as you can, without expecting a cure.A long gap between brassica crops slows the build-up of spore numbers. It will not clear established infection, because the spores outlive any rotation a gardener can run.
9. Keep it out of clean ground.Raise your own plants from seed rather than buying in, clean soil off boots and tools between plots, do not move soil around the garden, and do not compost infected roots.

Tips

• Club root affects every brassica, including swede, turnip, radish, rocket, mustard and wallflowers — so those cannot be used as an alternative on infected ground.
• Growing brassicas in large containers of fresh compost is a legitimate answer on a badly infected plot, and works well for cabbages and calabrese.
• Lime does not kill the organism; it makes the soil a worse environment for it to operate in. That is why the effect is partial and needs maintaining.
• If you buy in plants, buy from a source that raises them in sterile compost, and never accept plants from a neighbouring plot on an allotment site.
• Keep a note of which beds are affected. On an allotment, telling the site secretary is the responsible thing to do — it protects everyone.

Common mistakes

• Composting infected roots — The resting spores survive a domestic heap easily and you then spread them across the whole garden with the compost.
• Buying in brassica plants from a plant sale or a neighbour — It is the single most common way club root enters a clean garden. Raising your own from seed is the reliable defence.
• Relying on rotation alone — A four-year rotation is nowhere near the fifteen to twenty years the spores survive. Rotation slows build-up; it does not clear infection.
• Liming immediately before planting — Lime takes months to change soil pH meaningfully. Apply the previous autumn or well before the crop goes in.

Questions people ask