Su Ba my growing area has the same conditions. I sprayed with BT my first year growing crucifers, but then read that BT is bad for our intestinal tracks. My soil is excellent. I solved the problem by planting Catmint near crucifers, including kale. Look into companion planting! The scent of the catmint confuses the cabbage butterflies, and they hover, and then go elsewhere. Very effective. Other mints work, too, but catmint doesn't spread and become a nuisance. Floating row covers are also effective, but involve additional cost and storage. The catmint is perennial. I find it is a lot less work. Since I replenish my soil annually, (I grow my own) I don't worry about planting crucifers in the same area. However, if you must rotate your veggies each year or two, the catmint is easy to transplant.
