I haven't grown those myself, but Johnnys Seeds has a generic suggestion that might or might not work for your area:
MiyashigeRadish
"For July and early August sowing."
Latin Name: Raphanus sativus var. longipinnatus
Days to Maturity or Bloom: 50
http://www.johnnyseeds.com/p-7...
If you plant at the wrong time, sometimes Daikons will bolt to seed. The Daikon radish seed pods, eaten young and raw, are great and much less hot than the roots!
Plus, you can scatter radish seeds onto clay, scratch them in a little, and get some volunteers over the next few years. Sown this way, "they just know when to germinate" to produce roots.
