When I first started organic growing an experienced gardener warned me that I would never be able to grow carrots without the use of insecticide. He said that carrot root fly would get the lot and there was no way around this. He could not have been more wrong and the very first season saw a 100% success rate, no carrots were attacked by root fly.
Carrot root flies lay their eggs just below the surface of the soil next to carrots. The eggs hatch and the grubs burrow into the roots. The result is a mess, with grub tunnels all through the carrot. There are several ways to avoid the problem including: companion planting, timing the crop, barriers and using varieties that are resistant to root fly attack.
It is well known that the root fly can be put off the scent by growing strong smelling vegetables next to the carrots. One way to do this is to grow two rows of onions to one of carrots, the smell of the onions confuses the fly which then goes off to lay its eggs elsewhere. This only works when the onion is growing vigorously. It is also said to work the same way with the onion root fly, the smell of the carrots causes confusion and it too goes away.
My preferred method is to cover the crop with horticultural fleece as soon as the first shoots appear, the carrots staying covered until picked. The use of fleece also has another benefit in that a micro-climate is produced which helps protect the plants from wind and cold. A major disadvantage for the organic grower is that unless you make a portable frame it is a real chore to continually lift the fleece to check for slugs. I discovered this last year when I lost two rows in as many days. Now I use a frame similar to the “Geoff Hamilton Cloche” covered with fleece instead of polythene, which is easy to lift.
Some growers use varieties that will overwinter and mature before the root fly is active. You can also plant after the critical period of May-June but you cannot rely on the root fly to follow your timetable.
The varieties I use are “Early Nantes” or “Nantes Tip Top” for the first crop sown in early March under cloches and this year I am trying “Autumn King” for the main crop. There are some new F1 hybrid seeds available which are said to be resistant to root fly attack, “Fly Away F1” is advertised as containing reduced levels of chlorogenic acid which the larvae need to survive. The first reviews suggest that damage is not entirely eliminated.
Some other tips.
- Remember to rotate your crops.
- Do not use ground that has been freshly manured or you will get forked roots.
- Have a reasonable depth of clean soil.
- Keep an eye on the plants and take action to ward off trouble.