Kailan Pests and Problems: Flea Beetle, Cabbage White and Woody Stems

Net the bed from the day you sow

Almost everything that goes wrong with kailan arrives from the air and can be excluded. Fine insect mesh laid over the bed from the day of sowing, edges buried or weighted, keeps out cabbage white butterflies, cabbage root fly, most flea beetle, and the moths whose caterpillars burrow into bud clusters. It is the single most effective thing you can do.

Use mesh with holes of 0.03 inches (0.8 mm) or finer if flea beetle is your main problem, and no coarser than a quarter of an inch (6 mm) if you only care about butterflies. Support it on hoops so it does not rest on the plants, since a butterfly will happily lay through mesh that is touching a leaf. Check the edges weekly; the commonest failure is a gap at one corner rather than the mesh itself.

Flea beetle: the seedling killer

Flea beetles are tiny, shiny, and jump when disturbed, and they punch dozens of small round holes through brassica seedling leaves. On a mature kailan plant the damage is cosmetic. On a seedling with two true leaves it is fatal, because a leaf that is half holes cannot photosynthesise enough to grow the next leaf. Warm dry weather is when they are worst, and warm dry weather is exactly when kailan is most often sown.

The strategy is to get the seedlings through the vulnerable fortnight. Mesh keeps the beetles off. Failing that, keeping the soil surface damp discourages them, since they prefer dry dusty conditions. Sowing thicker than you need and thinning to the survivors is sensible insurance. Once plants have five or six true leaves you can stop worrying, since damage on older leaves does not affect the stem you are going to eat.

1. Mesh from the day of sowing Not from emergence. Beetles find seedlings within hours of them appearing, and a net thrown over a row that already has damage is closing the door too late.
2. Keep the surface damp through emergence A light evening watering over the drill each day until the seedlings have two true leaves. Flea beetles avoid damp surfaces and the seedlings grow away faster.
3. Sow thicker and thin to the survivors Sowing at an inch (2.5 cm) spacing rather than at final spacing means that losing a third of the seedlings still leaves you a full row.

Caterpillars hide in the bud cluster, not on the leaf

Cabbage white caterpillars are easy to see on a cabbage leaf and very hard to see in a kailan bud cluster, which is exactly where they end up. By the time the stem is ready to cut, a green caterpillar the same color as the buds can be sitting in the middle of it, and you find it in the cooking pot rather than in the garden. Check every bud cluster before you cut and again before you cook.

The right time to act is at the egg stage. Cabbage white lays clusters of yellow, ribbed, upright eggs on the underside of leaves, and the small white lays single eggs. Turning leaves over twice a week and rubbing off egg clusters with a thumb prevents the damage entirely. For an outbreak that has got away from you, a bacterial spray based on Bacillus thuringiensis is specific to caterpillars and harms nothing else in the bed; apply it in the evening and repeat after rain.

1. Turn leaves over twice a week Look on the underside for yellow egg clusters standing on end. Rub them off with a thumb. Ten minutes a week removes the entire problem before it starts.
2. Open the bud cluster before cutting Part the buds with a finger and check for a green caterpillar and for dark grains of frass. Frass in the cluster means a caterpillar is in there even if you cannot see it.
3. Soak cut stems in salted water for ten minutes A tablespoon of salt in a bowl of cold water floats out anything you missed. Rinse before cooking.

Aphids, slugs and root problems

Cabbage aphid is the pest most likely to make a kailan crop inedible rather than merely damaged. It forms dense gray, waxy colonies packed into growing tips and bud clusters, and the wax means it does not wash off easily. Because it hides exactly where you want to eat, one missed week can write off a flush of side shoots. Check tips at every picking and squash small colonies with finger and thumb. Slugs are the other surface pest, worst on seedlings and on freshly cut stubs in wet weather.

Below ground, cabbage root fly maggots eat the fine roots and give you a plant that wilts in sun, grows slowly and lifts easily with a chewed root. Clubroot swells the roots into distorted lumps and stunts the plant permanently. Neither can be cured once present. Root fly is prevented by mesh or collars; clubroot is managed by liming to raise pH toward 7.0 to 7.2, by long rotations, and by never composting an infected root.

Woody, hollow and thin stems are growing faults, not diseases

Most of the disappointments with kailan have nothing to do with a pest. A woody stem is almost always a stem cut two or three days after the flowers started opening, or a stem from a plant that had its growth checked by drought or cold. A hollow, pithy center comes from drought during stem elongation followed by a sudden flush of water and nitrogen. A thin stem comes from crowding, late thinning, or a starved bed.

The useful thing about these faults is that, unlike clubroot, they are entirely within your control. Even and adequate water, one good nitrogen feed, thinning on time, and cutting on the day the first bud shows color eliminate all of them. If you have a whole row of woody stems the answer is not a treatment, it is a change to one of those four habits.

Keep a note of the date you first see each pest each year, along with which end of the garden it appeared in. Pest arrival dates repeat closely from season to season and the note tells you when to have the mesh on next time.