I have been growing mammoth grey stripe sunflowers for a while, but I want to know if there is a better producer for my climate. The area I would plant them in is a heavy, clay base soil. The average yearly rainfall for this area is forty inches. Is there a better producer that will grow under these conditions?
I have grown sunflowers for seed for several seasons now in central Iowa. That should be pretty close to your climate with a bit less rain and slightly more extreme temperatures. (We typically see -15F in winter and 95F summer and 35in total precipitation).
I plant enough to harvest a half dozen to a dozen flowers (2-3qt yield dried with hulls). So this is totally unscientific anecdotal data :). The best performing variety for me has been Azteca. Azteca is a Medium height (6'-7') single flower variety. 6'-7' is short enough that I can harvest easily and the flowers are very large. The multiflower varieties that I have tried have had much smaller flowers which is more effort to harvest and seem to produce less seed. I have tried mammoth grey stripe but with them I had over 1/2 my plants blown over in a summer thunderstorm.