You may have read previous posts about the virtues of using brassica collars around the base of any cabbages, kales, broccoli etc. Our reason for this is to avoid letting the dreaded cabbage root fly get into the soil.
You will have seen the cabbage root fly many times in your life as there are lots of them out there. They are attracted to all brassicas and will lay their eggs at the base of brassica plants. When the eggs hatch the larvae bury into the soil and feed on the roots of the brassica plants (as seen in the photo here). As they eat the roots, the plants start to struggle. Growth will slow right down and eventually the plant will wilt and die. There’s nothing you can really do to fix it, and worst of all, the pupae can overwinter ready to hatch out the following spring, which makes growing brassicas a bit tricky (if your crops suffered, then it is a very good idea to rotate crops next year and grow brassicas in a different part of the plot).
Prevention is the way forwards, and we would always recommend putting a brassica collar around the base of plants when you plant them out, ideally covering the row with horticultural fleece as well. Better safe than sorry! (Have a look at our previous post about growing brassicas – see below)