In DK the cabbage root fly is a pest aswell. When I plant any cabbage I always put a thick layer of grass clipping tight around the roots, in fact I cover the whole bed, with 8-10 cm of grass clipping. I water it down and make sure that it is tight and firm around the roots. The cabage root fly can not get to root.
It is working for me- I do not grow a large numbers of plants- it is not big deal to cover them.
The grass clipping also keep moisture and feed the cabbage-
The only problem with grass clipping is birds turning it to look for insects and earthworms, but a net will solve that.
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