Are you fertilizing to the plants needs? See if these help.
Hello John John. I'm Cindy. The most important thing to remember when growing brassica's is working the soil. Start digging over your soil in the Fall. Remove any rocks you find and, this is important, work in plenty of fertilizer or manure. Then tread on the soil to remove air pockets and make the surface very firm. It is ideal to add compost 2 or 3 months before planting. Good luck John John.
Growing Bountiful Brassicas - Fifth Season Gardening