Seems like an easy enough DIY "collars fit snugly around the stems of cabbages, cauliflowers, broccoli and brussels sprouts to prevent the cabbage root fly laying its eggs beside the roots of the plants. Collars expand as the stems grow.
Cabbage is a cool-weather crop. Seedlings can be transplanted into the garden when they are 3 to 4 inches tall as early as 3 to 4 weeks before the last frost. In mild-winter regions, start seed in late summer for a winter or spring harvest.