Why are my plants chewed through at soil level?

Losing a row of young plants overnight is a specific and infuriating kind of damage, and it is worth identifying properly because the responses have nothing in common. All three culprits work at night and none is on the plant when you look in the morning.

Step by step

1. Look at the cut end of the stem.A stem chewed through cleanly at or just below soil level, with the top of the plant lying beside it uneaten, is cutworm. Slug damage leaves ragged, rasped tissue and usually eats the leaves as well. A stem cut at a clean angle above ground with the plant gone is more likely a mammal.
2. Dig for the cutworm.Scrape the top few centimetres of soil within a hand's width of the fallen plant. Cutworms are the larvae of several noctuid moths — fat, dull grey-brown or greenish caterpillars up to 4cm long that curl tightly into a C when disturbed. They feed at night and shelter just under the surface by day, and finding one confirms the diagnosis in a minute.
3. Look for slime trails.Silvery dried trails on the soil, the stem, the pot rim or a nearby slab. Slugs and snails also usually take some leaf as well as stem, and lettuce, hostas and young brassicas are the classic targets.
4. Look for disturbance in a seed bed.Seeds and seedlings that vanish completely, with the compost or soil scuffed and small holes, is mice — very common with peas, beans, sweetcorn and sunflower seeds in a greenhouse or a cold frame. Birds pull seedlings out and leave them lying, without eating them.
5. Deal with cutworms by hand and by cultivation.Hand searching around damaged plants is genuinely effective because they stay close. Hoeing and cultivating bare ground exposes them to birds. Nematodes for cutworm are available and work on warm moist soil. Keeping the plot free of weeds removes the plants the moths lay on.
6. Use collars on transplants.A simple ring cut from a plastic bottle or a cardboard tube, pushed a couple of centimetres into the soil around each transplant, physically stops cutworms and slugs reaching the stem. It is cheap, quick and probably the most reliable measure there is for a small number of valuable plants.
7. Protect seed from mice.Sow into modules under cover and plant out as established plants rather than sowing directly, cover seed trays with a sheet of mesh or glass, and keep greenhouses and sheds free of accessible seed packets. Traps work but need placing carefully away from wildlife and pets.
8. Water in the evening and patrol at dusk.All three feed after dark. A torch check an hour after dusk on a damp evening finds slugs, and often finds cutworms on the surface, which is the easiest time to collect them.
9. Replace and protect rather than treating the whole bed.Damage of this kind is patchy and usually short-lived. Replanting with collars in place is more effective than any broad treatment, and by midsummer the plants are large enough that it stops mattering.

Tips

• Cutworm damage is worst on light, dry soils and in warm dry spells, which is when the larvae are most active near the surface.
• A row of transplants lost one at a time along the row, over several nights, is a strong signal for cutworm — it is working its way along.
• Collars made from toilet-roll tubes rot into the soil and need no removal, which makes them ideal for brassicas and sweetcorn.
• Do not blame vine weevil for this. Vine weevil grubs eat roots in pots and never sever a stem at the surface.

Common mistakes

• Scattering slug pellets at cutworm damage — Cutworms are caterpillars, not molluscs, and pellets do nothing to them. Identify before treating.
• Replanting into the same spot with no protection — The culprit is still within a few centimetres of where it fed. A collar takes ten seconds and works.
• Sowing peas and beans directly where mice are active — They will take the lot. Module-sowing under cover and planting out is the standard answer and costs nothing extra.

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