How to Protect Carrots from Carrot Fly

Carrot fly is the pest that turns a promising row of carrots into a brown, tunnelled mess, and it is the reason so much carrot growing advice begins with a cover. The practical answer is a physical barrier: put insect mesh over the crop so the fly cannot reach the plants to lay its eggs¹. Carrot fly can destroy a crop of carrots, so this is not a cosmetic job¹.

The detail that decides whether a cover works is height. Carrot flies do not fly much above 50cm (19.5 inches) above ground, so a cover that stands clear of the foliage and is sealed along every edge keeps the crop out of reach². The second detail is timing: the Royal Horticultural Society's growing guides say to grow carrots under fleece to protect them from carrot fly in March, and to keep carrots covered with fleece for the same reason as the season goes on³.

Get those two things right, a barrier that clears the crop and edges that stay shut, and you have most of the protection you need. The rest of this page is about choosing between mesh and fleece, where covers fail, and how to look after them so they work again next year.

Why a cover, and not a spray

Carrot fly is a low-flying pest, and the damage it does is done below ground, out of sight, which is why the advice leans on blocking it rather than treating the plant afterwards. The maker guidance is blunt: put up a barrier of insect mesh to stop them getting access to your crop¹. The same approach is recommended for related root crops. For turnips, employing mesh or horticultural fleece can deter cabbage root fly from accessing the roots⁵.

A cover also fits the way most households grow carrots, in a bed or a raised bed rather than a field, where a physical barrier is practical to erect and to keep in place. It has the further advantage of doing several jobs at once. A fleece cover protects plants against frost and stops wind, hail, birds and insects damaging them⁶. A mesh cover stops pests such as carrot fly and cabbage root fly from damaging your plants⁷. Neither needs applying, mixing or repeating after rain.

There are supporting measures worth knowing about, though they are secondary to the barrier. Intercropping leeks with carrots can help manage onion fly, while also protecting the carrots from carrot fly too⁸. Shallots work well alongside carrots and help with carrot fly, while carrots help shallots with onion fly⁹. If you grow celery, note that carrot flies are among the pests to look out for and protect against¹⁰. And if you would rather start from a variety with some resistance built in, Organic 'Flyaway F1' is described by its supplier as carrot fly resistant¹.

What a cover cannot do is rescue carrots already infested, or protect a crop you keep opening up. The sections below deal with both.

Mesh or fleece: how each one protects the crop

Both work by putting something between the fly and the plant, but they are different materials doing different jobs, and it helps to know which you are buying.

Mesh coverings with a fine enough weave can protect not only against birds, slugs and snails but also cabbage caterpillars and cabbage root flies¹². That fine weave is the point: the barrier has to be fine enough to stop the fly, not merely shade the crop.

Fleece earns its place for early sowings because it does two jobs. It protects against frost and stops wind, hail, birds and insects damaging your plants⁶. Cloches or fleece can allow earlier plantings as well as offering the young plants a little extra protection¹³. The same logic appears across crops: celeriac can be covered with cloches or row covers during early growth to keep it warm and reduce bolting risk from cold snaps¹⁴.

The trade-off is straightforward. Fleece is lighter and warmer; mesh is the more single-minded barrier against insects. Many households use fleece for the earliest sowings and mesh for the main crop, and some covers let you swap between the two: the Popadome crop protection system takes a choice of fleece, polyethylene and insect mesh covers⁶.

How high, and how to seal the edges

The height question has a clear answer in the guidance. Carrot flies do not fly much above 50cm (19.5 inches) above ground². A cover that stands at or above that height, with the crop growing underneath it, puts the plants above the fly's working range as well as behind a barrier.

Raised beds do some of this work for you. If you are growing in raised beds at 18 inches high, the risk of carrot root fly is lower, for the same reason². If you want to be absolutely sure your crop is protected, you could erect an anti carrot fly screen around your raised bed². For anyone building raised beds from scratch, the Royal Horticultural Society's guidance on creating raised beds is worth reading before you fix the height¹⁵.

Height alone is not enough. A cover fails at its edges, and the fly will find the gap. Work through these checks:

• Peg or weight every edge, including the ends, so there is no lifted hem for the fly to walk under.
• Keep the cover clear of the leaves so the crop is not pressed against the mesh.
• Repair tears before the season starts, not after you notice damage.
• Water through the cover rather than lifting it, so the seal is never broken.
• Do not lift the cover to weed and leave it open while you work.

The principle that a barrier is only as good as its openings holds well beyond the vegetable bed. On a private water supply, all openings must be protected using a fine mesh screen to prevent the ingress of insects and animals¹⁶, and overflow pipes or vents should be protected by a suitable mesh to prevent small animals entering the tank or chamber¹⁷. Around a roof, gaps in soffits and cladding can be screened with wire mesh, which retains ventilation but helps to prevent embers from getting in¹⁸. The lesson is the same each time: the mesh is only doing its job where it is continuous.

Fleece over March sowings

March is the month the advice names. The Royal Horticultural Society's spring growing guide says to grow carrots under fleece to protect from carrot fly³. Early cultivars can be sown in late winter, in February or March, under cloches¹. Fleece over a March sowing therefore does two things at once: it warms the ground and shelters the seedlings, and it puts a barrier over the crop before the fly is active.

Fleece is well suited to this because it can be laid without hoops. It is a soft, fibrous, translucent material laid over or around plants, hastening their growth and protecting them against weather and pests, usually used without supporting hoops³. That makes it quick to put on a small sowing, though it also means it sits close to the plants and needs weighting along the edges.

The same material is used the same way on other early crops, which is a useful cross-check on technique. Onions can be covered with fleece until the sets have rooted, because birds like to pull freshly planted sets from the soil¹⁹. Shallots can be covered until the sets form roots and are more firmly anchored⁹. Kohlrabi benefits from cloches or fleece to allow earlier plantings and give young plants a little extra protection¹³. In each case the cover is temporary and its job is defined.

There is a second sowing date worth knowing. Carrots can be sown in June for use over winter, which also has the added bonus of missing the first attack of the carrot root fly²⁰. That is a timing strategy rather than a barrier strategy, and the two combine well: a June sowing under mesh, or a March sowing under fleece kept on through the season, both reduce the exposure.

Where a cover fails: gaps, lifting and the end of the season

Most carrot fly damage in covered beds comes down to one of three things: an edge that was never sealed, a cover lifted for a job and not put back properly, or a cover removed too early.

The first is the commonest. A cover pegged at the corners but loose along the sides is an open door. The second is a habit problem: lifting the mesh to weed, thin or water breaks the seal, and the fly is small enough to take advantage. Water through the cover and weed at the edges rather than in the middle.

The third is a judgement call. The summer growing guide says to keep carrots covered with fleece to protect them from carrot fly⁴, which points to leaving the cover on rather than whipping it off once the seedlings are up. If you want to reuse the cover for something else, do it after the carrots are lifted, not before.

Covers also come to the end of their life. Fleece and mesh wear, tear and thin, and a worn patch is a gap. When you take covers in at the end of the season, check them over, and replace anything that no longer forms a continuous barrier.

There is a related point about how long protection is needed. If you save seed, you may leave a carrot plant to over-winter so it flowers the next year²¹. That plant is in the ground for far longer than a cropping row, so plan its protection accordingly.

Reusing fleece and mesh next year

Both materials are reusable, and looking after them is part of getting value from them. The pattern in the guidance is to remove covers once their job is done and store them for the next season. Netting over grapes and pears is used in July to protect against birds and wasps and removed in autumn once picking is finished²². The same discipline applies to crop covers: take them off when the crop is lifted, shake them out, check for damage and store them dry.

There is a second life for some kit. Individual cloche hoops can be converted into garden netting hoops by swapping the polythene for a net cover later in the year, to protect from both airborne and ground-dwelling pests²³. If you already own cloches, that is a way to get mesh protection without buying a separate frame.

Before you put a cover back out, check three things:

• Tears and thin patches, which are gaps however small.
• The edges, which should still take a peg or a weight along their full length.
• The fit, which should still clear the crop rather than press on it.

Fleece that has been used for frost protection through the winter may have stiffened or torn; mesh that has been out all summer may have UV damage. Neither is expensive to replace relative to the crop it protects, but replacing it mid-season, after the fly is about, is the wrong time to discover the problem.

If you grow under cover more widely, whether in a polytunnel, a greenhouse or a cloche system, the same principles of sealing, height and material choice carry across. For the wider picture on managing pests without reaching for a spray, see Vegetable Pests and Diseases: Prevention and Control, and for fitting covers into a longer growing plan, Polytunnels and Greenhouses: Growing Under Cover All Year.