My soil is heavy clay too. Well, you can make nice bricks out of my soil actually. lol
If you are getting snow in winter, the rot shouldn't affect them because of how cold it is. It is the warm times of the year where excessive rain can destroy the roots. I have all that in spades here and no one can grow rosemary in the normal soils. A 1st year plant will grow slowly and during that time, it is more susceptible to problems. The 2nd year plants do a lot better since they get properly hooked up to a mycelial network to help prevent diseases. The more rosemary shrubs you have together the better the net affect of disease prevention from the mycelial network.