The primary obstacle to the production of a fall crop of broccoli, Brussels sprouts, cabbage or cauliflower is the cabbage butterfly, Pieris rapae. Left unchecked, its caterpillars will ruin the entire crop. For many years, we have used Bt to control cabbage caterpillars organically, with only partial success. In the spring, we brought in a fair crop of broccoli by using Bt and keeping the plants under a floating row cover. However, they seemed to suffer from the additional shading.
We started seeds for fall brassicas on August 19 and transplanted them to the garden on September 12. From past experience, we have learned to start a dozen seeds to obtain three or four healthy transplants. In the past, we tossed the extras into the compost. This year, we left them on the nursery bench as a trap crop. Plants were dusted with Bt from the time they had their first true leaves until transplant time, renewing the Bt after each rain. Newly transplanted plants were also dusted with Bt, but the extras were left untreated. They were heavily damaged by caterpillars, while the plants in the garden seem to have suffered the least damage in our experience.
I hypothesize that the added stress of being confined in the cell trays caused the extra seedlings to signal to the butterflies. However, this was not a controlled experiment, and we may have just gotten lucky. Next year, we will try the same thing in the spring, and see what happens.