Why Is My Broccoli Wilting for No Obvious Reason?

A broccoli or cauliflower plant that wilts during the day despite the soil being adequately moist — and does not fully recover overnight — has root damage. When the roots cannot supply water fast enough the plant wilts, and when the roots are badly damaged recovery does not happen. The most common cause of this pattern in brassicas is cabbage root fly (Delia radicum), whose white maggot larvae feed on the roots just below the soil surface.

Confirming cabbage root fly

Gently dig around the base of the wilting plant and examine the roots. Cabbage root fly larvae are white, legless maggots about 8mm long, found feeding on the root surface and in tunnels through the root tissue. The main taproot may be severed or extensively tunnelled. You may also see small grey flies around brassica plants in spring and early summer — these are the adults laying eggs at the soil surface around the stem base.

Can affected plants be saved?

Lightly affected plants that are large and established may survive and produce a head if the remaining roots are healthy. Water them carefully to reduce stress, firm the soil around the stem, and hill up compost around the base to encourage new root development above the damaged zone. Plants that are severely wilted and have lost the majority of their root system are unlikely to recover and are better removed to reduce the risk of adults emerging and attacking neighbouring plants.

Prevention — brassica collars

Brassica root fly collars are the most effective organic prevention. These are discs of cardboard, carpet underlay or purpose-made foam placed flat on the soil surface around the stem at transplanting. They prevent the adult fly from laying eggs at the stem base. The collar needs to be 15cm in diameter and fit snugly around the stem. They are effective, cheap and reusable.

Other protective measures

Fine mesh netting over the crop excludes adult flies from reaching the plants. Planting through a fleece mulch that covers the soil surface achieves a similar effect. There are no effective organic insecticides available to home gardeners for this pest — prevention is the only reliable control.