I'm in zone 8, southern oregon. I planted them early in the season with my lettuce and peas. and even now, our nights are upper 40's, days have been fairly mild - a few days of 90's, but otherwise the majority of the time we are 70's or 80's. I water every night, have straw around my plants to help retain moisture, although I did not put straw around the radishes.
Tammy, in hot weather I usually plant radishes in with my pole beans, or anything that will give some shade. Also when you work your soil for radishes, put a little 5-10-10 (about 1 quart per 100 square feet and mix it intothe top 2-3 inches. also you might want to sprinkle a handful of bone meal or superphospate (marked 0-20-0 on the bag) covering about 6-8 square feet of seedbed. it won't burn the seeds for it does not contain nitrogen..always remember phosphorus is inportant for any root crop. so this is an extra boost in the row and will promote steady, fast root growth. with this already in the row there will be no need to sidedress any root crop. also water 'DEEP" and not quiet as often. good luck with your radishes..also the white radishes grows deeper like a carrot and takes longer to mature.