I think you are right that bolting is the issue with when to plant. I got two varieties from Wilkinsons - turbo and sturon. The display in the shop said one was for planting from late February and the other from March. Just can't remember which was which and it says nothing on the bag.
Anyway I always plant direct sometime in March. Starting them off always seems like making two jobs out of one. If you plant them before rain the birds don't seem to bother with them at all (in my experience).
