The lady in the other room says Radishes grow best in cooler weather AND they are milder tasting. I took most of the day off to watch Nascar from Sears Point Ca and I also wore out 5 refresh buttons on FR! I did put Weed and Feed on the lawns and this application should get the Buttercup in check...
I tried something that has worked very well, not only for radishes but mustard greens and turnips too. I planted in an area near two very large trees ... gets good sun in the morning before it gets scorching hot, then an hour or two of shade before noon. Then sun for an hour or so around noon and then filtered sun for the next couple of hours. By the time that the sun is direct, it is late afternoon and the sun is pretty low and not scorching.
Additionally, I water lightly every day. The radishes are beautiful and just the size they should be -- they are crisp and mild. We had turnip greens with diced turnips last night and they were also mild.
Everybody and their brother told me that I couldn't grow radishes and greens at this time of year that would be worth a darn. I was tickled to be able to provide them with some that they enjoyed, and it has averaged 95 degrees a day for the last 3 weeks.