In your garden: A fleece will put paid to carrot flies

Lots of pests are just waiting to damage allotment crops. While chemicals can control some of them, organic growers, of course, have to use organic controls, and that is more difficult.

I once asked an organic farmer how he controls carrot root fly. His answer? "I don't grow carrots."

The first carrot crop I grew was a disaster. For the next I used a chemical based on bromine - it worked. However it was withdrawn for health reasons and replaced with one based on chlorine. It was less effective and also withdrawn. Its replacement, also based on chlorine, was almost ineffective.

I then decided to grow organically and over the years have tried most of the recommended methods. Probably the most effective was a high horticultural fleece fence round the carrots. It works because the carrot fly doesn't fly much above ground level, however a gust of wind can blow the fly over the fleece. Once inside, it's trapped and lays its eggs beside the carrots and the larvae eat into them.

I reasoned that only a total barrier would work and grew the carrots in a low fleece tunnel. The hoops are made from old fence wire, anchored with string, giving a height of about 18 inches. Inside there is space for three rows of carrots, six inches apart. The seeds are covered with sifted leaf mould. The fleece is buried on each side of the hoops. Access for thinning, weeding and harvesting is by lifting one side of the fleece. At last, carrots which are almost totally free from damage.

Fleece is cheap but not robust and after a couple of years, holes let in the flies. It has other disadvantages - it sheds the rain, doesn't let in all the light, and you can't see through it. I now use plastic mesh, pictured, it's more expensive but without these disadvantages. It's also easier to wash and after several years is still in good condition.

George Sutherland is a past president of The Federation of Edinburgh and District Allotments and Gardens Association (FEDAGA). He has an organic allotment and is a five-time holder of the Robin Harper Green Trophy for organic vegetables. For more information visit www.fedaga.org.uk.