Why Are My Carrots Covered in Knots and Galls?

If your carrots come up knobbly, forked, stubby, and covered in small knots, galls or beads of swollen tissue along the root, the likely culprit is root-knot nematodes — microscopic worms in the soil that attack the roots. The damage is distinctive and the carrots are often unusable, but understanding the pest lets you manage it and protect future crops. Let me explain.

What root-knot nematode damage looks like

Root-knot nematodes cause carrots to develop small galls, knots and swellings along the root, along with excessive forking, stubby growth, and masses of hairy side roots — the root ends up deformed, knobbly and branched rather than smooth and straight. Above ground the plants may be stunted and yellowish and wilt in heat, because the damaged roots cannot take up water and nutrients well. The galls are part of the root tissue (unlike the rounded nodules of beneficial bacteria on legumes), and the overall knobbly, beaded, forked appearance is the giveaway.

What they are and why they build up

Root-knot nematodes are tiny soil-dwelling roundworms that invade roots and cause the plant to form the galls in which they feed and multiply. They thrive in warm, sandy soils and build up where susceptible crops are grown repeatedly in the same ground. They attack a very wide range of vegetables, so a heavily infested bed causes trouble across many crops, not just carrots. Once established in the soil they are difficult to eliminate, so management focuses on reducing their numbers and avoiding building them up.

How to manage them

You cannot cure infested carrots, so remove and destroy badly affected plants and roots (do not compost them), and focus on the soil. Rotation is the most important tool — avoid growing carrots and other susceptible crops in the infested bed, and rotate with less susceptible crops to starve the nematodes down over a few years. Some cover crops and marigolds are used to suppress nematodes. Adding plenty of organic matter supports the beneficial soil life that competes with and preys on pest nematodes, and keeps plants vigorous enough to tolerate some damage. In serious cases, soil solarisation (covering moist soil with clear plastic in hot weather) can reduce populations.

Prevention and clean soil

To keep nematodes from ruining carrots: rotate crops so carrots are not grown repeatedly in the same ground, build healthy, organic-matter-rich soil that supports nematode predators, remove and destroy infested roots, and consider nematode-suppressing cover crops or marigolds in problem beds. Growing carrots in containers of fresh, clean mix is a reliable way to dodge soil nematodes entirely where they are a serious problem. With rotation and healthy soil, root-knot nematodes can be kept below damaging levels and your carrots come up smooth again.