Types of Worms in Potted Plants — 6 and Which Harm Roots

Tipping a plant out and finding something moving in the compost is alarming, and nearly always harmless. Six things turn up in container compost with any regularity, and only two of them have any interest in a living root.

The harmless six-sevenths

1. Pot worms — Enchytraeidae. Translucent white, thread-thin, a quarter to half an inch, often in dozens around the drainage holes. They eat dead roots and peat. They are the single commonest thing people find and the single least important: they tell you the compost is wetter and more acidic than the plant wants, and nothing more.

2. Fungus gnat larvae — Sciaridae. Clear-bodied with a black head, up to a quarter inch, in the top inch of compost. They feed on fungi and decaying matter. In seedlings and cuttings a heavy population will graze fine root hairs, but on an established plant they are a nuisance rather than a threat — and the adults flying around the room are the real complaint.

3. Earthworms. Occasionally arrive in garden soil or in outdoor pots. A container is a bad home for one: too dry in summer, too wet in winter, and no depth. They do no harm, but they belong in a bed or the compost heap.

4. Millipedes. Hard, many-legged, curl into a flat spiral. Detritivores. They feed on dead tissue and only rasp at living roots when there is nothing else, which in a pot of fresh compost is rare.

The two that matter

5. Vine weevil larvae — Otiorhynchus sulcatus. Creamy-white, legless, about a third of an inch, curled into a C, with a hard orange-brown head. They eat roots from the outside in, and the first symptom is a plant wilting in compost that is plainly moist. By then most of the root system is gone.

6. Root-knot nematodes — Meloidogyne species. Microscopic, so you will never see the animal. What you see is the root: swollen, beaded galls that cannot be rubbed off, on a plant that is stunted and wilts in the afternoon. They arrive in contaminated soil or on infected stock, and for a houseplant the honest answer is to discard the plant and the compost rather than attempt a cure.

Fixing the conditions instead

Pot worms, fungus gnats and millipedes all arrive for the same reason: compost that stays wet long enough for fungi and dead matter to accumulate. Three changes remove all three populations without treating for any of them.

• Let the top inch dry between waterings. This alone ends most fungus gnat problems, because the eggs need damp surface compost.
• Empty the saucer. Standing water keeps the lower third of the pot anaerobic.
• Top-dress with grit or sand, half an inch, which dries fast and blocks egg-laying.

For vine weevil the conditions are irrelevant — it arrives as an adult that walks in and lays eggs on the compost surface — so that is the one case where a treatment, a nematode drench matched to the soil temperature, is the right call.