Know What You Are Up Against

Brassicas take months to grow and roughly an afternoon to lose. Pigeons are the most likely culprit in winter and early spring, when hungry flocks strip the young leaves from kale, Brussels sprouts, cabbages and purple sprouting broccoli. They do not eat the whole plant — they shred it, leaving bare stalks and tattered midribs. Cabbage white butterflies work in summer, and their caterpillars can reduce a healthy row of cabbages to lacework within a week.

Two butterflies cause the damage: the large white and the small white. The large white lays clusters of yellow eggs on leaf undersides; the small white lays single pale eggs. Both hatch into velvety green caterpillars that feed on the soft tissue between the veins. Understanding the timing matters — the first generation usually appears from late May or June, with further waves through to September.

Netting: The Single Most Effective Measure

Fine mesh over a rigid frame is the best investment any brassica grower can make. It keeps butterflies out entirely and stops pigeons landing on the crop. The trick is choosing the right mesh and building it properly.

• Mesh size: 7mm or smaller excludes cabbage white butterflies. Anything up to about 20mm will stop pigeons, but small mesh also keeps out flea beetle and root fly, so it earns its keep.
• Frame height: netting must never touch the leaves. A butterfly can lay through the mesh if the leaves press against it, and pigeons will peck through as well. Allow at least 30cm clearance above the plants — 45cm for taller crops like Brussels sprouts.
• Sturdy supports: hoops made from blue water pipe or thick canes with bottles over the tops work well. Push them in firmly so winter gales do not collapse the net onto the crop.
• Tension and burial: pull the net taut and weigh or bury the edges with soil, bricks or pegs. Pigeons will walk in through a gap and then panic.

Put the netting on at planting and leave it on until harvest. Taking it off "just to weed" is how most crops get attacked.

Collars and Barriers at Planting Time

Netting does most of the work, but a few cheap barriers improve your odds further. Brassica collars are discs of cardboard, felt or spare membrane about 10cm across, slit to the middle and fitted snugly around the stem at planting. They stop cabbage root fly from laying eggs at the base of the plant, which causes seedlings to wilt and die. Homemade collars from cereal boxes work perfectly well and cost nothing.

If you cannot net a whole bed, protect individual plants with a clear plastic bottle cloche for the first few weeks, or surround the patch with a low barrier of fleece. On windy allotment sites, a temporary windbreak of mesh also stops pigeons from settling.

Regular Checks and Hand-Picking

No net is perfect and butterflies are determined. From June onwards, check your plants at least twice a week — more often in warm, still weather.

• Look underneath: eggs are laid on the underside of leaves, so lift the outer leaves and inspect carefully.
• Squash eggs and caterpillars: rub off egg clusters with your thumb and pick caterpillars by hand. A bucket of soapy water is handy for disposal.
• Check the hearts: caterpillars often burrow into the centre of cabbages and cauliflowers where sprays and nets cannot reach.
• Tidy up: clear yellowing leaves and old stalks. They shelter pests and give pigeons a reason to hang around.

Grow Strong Plants

Healthy, fast-growing brassicas shrug off minor damage. Prepare the ground with well-rotted compost, keep plants watered in dry spells and feed hungry crops like sprouts and kale with a nitrogen-rich feed in midsummer. Space them according to the seed packet — cramped plants are weaker and easier for caterpillars to strip. Firm planting helps brassicas anchor well, and a mulch of compost keeps moisture steady.

Autumn and Winter Pigeon Watch

Once caterpillars are gone, pigeons remain the main threat. Hungry flocks move onto allotments in cold spells and will strip kale and sprouts down to the stems in days. Keep netting in place through winter, and consider a double layer of mesh on exposed sites.

Where netting is impractical, try reflective tape, old CDs on string or bird scarers — but be prepared to move them regularly, as pigeons quickly learn to ignore anything static. A well-tensioned net on a sturdy frame remains the most reliable answer. Check it after storms, repair tears promptly, and your brassicas should reach the kitchen intact.