Hi SAK,
Yes, soil temp should be at least 50 - and 55-60 better if possible. Night time lows are a good " worst case" scenario. Temperatures of soil less that 50, encourage mold growth, and delay germination by several weeks, but the real kicker is " last frost date" for your area.
Even if you temps are OK now, if your area gets a frost in MAY on average, you a playing russian roulette planting now. This is the kind of link you are needing - you need to search for the closest to you
http://205.156.54.206/er/bgm/cli/avp/curavp. In central NY, June 1 is my plant date, I' d guess yours is like May 15 - but again - its a guess.
If you cannot find any better info, plant when the dogwoods bloom.
Good luck