Carrot fly flies low, so a fine mesh cover keeps it out

The surest way to grow carrots without carrot fly is to keep the flies away from them. Cover the row with fine insect mesh from the day you sow until the day you pull them. Carrot fly can’t get through, and nothing else works nearly as well.

Carrot fly is a small black fly that lays eggs at the base of carrot plants. The maggots tunnel into the roots, leaving rusty brown tracks. It’s found across the UK, and it also attacks parsnips, celery and parsley.

Cover the crop

Use fine insect-proof mesh, with holes of about 1 mm or less, over hoops. Bury or weigh down the edges all the way round, so flies can’t crawl underneath. Put it on straight after sowing.

Mesh also keeps the soil a little warmer and damper, which carrots like, and you can water straight through it. Only lift it to weed and thin, ideally on a still day, and put it back straight away.

Barriers

Carrot fly tends to fly low, close to the ground. A barrier of fine mesh or polythene about 60 cm high around the bed stops many of them. It’s less reliable than a full cover, as some flies get over, but it works well alongside other measures and needs no hoops.

Sow thinly

Carrot fly finds carrots by smell, and thinning releases a strong carrot scent. Sow seed thinly, aiming for one every 2 to 5 cm, so you barely need to thin. If you do thin, do it in the evening on a still day, remove the thinnings from the garden, and water afterwards to settle the soil.

Pelleted seed is easier to space accurately.

Time your sowings

The first generation of carrot fly is busiest from late April to June. Carrots sown in late May or early June often miss the worst of it. Earlies sown in March or early April can be pulled before the second generation arrives in late summer.

Maincrop carrots left in the ground into autumn are the most likely to suffer. Lift them by October, or keep them covered.

Resistant varieties

Varieties such as Flyaway, Resistafly and Maestro are less attractive to carrot fly. They still get some damage, but much less, and they’re a good choice alongside mesh.

Grow in containers

Carrots grown in tall containers or raised beds above about 60 cm are often missed by the low-flying females. A deep pot or a dustbin filled with sandy compost grows short-rooted carrots well.

Spotting the damage

Leaves may turn reddish or yellow and wilt in dry weather. Pull a carrot and you’ll see rusty brown tunnels near the surface of the root. Once you see damage, there’s nothing to do for that crop. Lift and use the carrots, and cover the next sowing.

For sowing dates, see carrots every three weeks, and set your own frost dates on the carrot page.