I prefer smaller Chines radish/Korean over daikon. To me chines radish is tastier. I grew them last year and they did real well and overwintered here in Atlanta area.
Plant them right around wheat harvest time(in zone 5,= july?) for fall. They love cool weather and do not mind cold. Water them regularly to become juicyer. With hard clay soil, I can imagine it would be difficult to grow daikon. But no matter what you are growing, you need to amend your soil with lots of organic mater for a good dept of 18" suitable for deep rooting crops like radish and carrots.

Sorry, I don't know how to make radishes more flavorfull in your area. In a reference that I have says that PH range for radis is 6 to 7; which is very wide and normal range. so you can grow radishes wherever you can grow other garden vegetable.

Thanks for the follow up. We're a 75% Japanese family here so it's daikon or bust!

Other radishes grow just fine, oddly. We've had no problem with kokabu or the ordinary red radishes. It's just the daikon that does not grow properly. We've softened up the soil considerably this year so we will see if the soil consistency makes a difference. Any other advice is most welcome!

Thanks!

Please post a follow-up with your results.

Do you plant in Spring or Late summer ?

We planted the seeds in late spring (May). I'm quite hopeful at the moment because they aren't ready yet! Last year they matured very quickly.