Did you read the specific planting instructions on the package for your radishes? Most seeds need to be planted at a depth that isn't much more than the seed's height or width, and radish seeds are typically very small. If you plant too deeply, or not deep enough, it will be difficult for the plants to reach optimal growth.

Another thing to keep in mind is that some seeds are very picky about how much light/heat they need to germinate. Spinach is my personal nightmare! :) Your seeds may need some kind of cover until they germinate in order to get the best from them. I know that Mel recommends covering carrot seeds until they germinate because the seeds are planted very close to the surface.

Since you're planting in Mel's Mix, you probably don't have this problem, but try stirring up the soil in the area you wish to plant to make it very loose (and adding fresh compost to the squares when you harvest). This may help your radishes grow better...and be sure that you aren't planting radishes in the same square every time.

Eric, I had the same problem with all the radishes I planted after June, but didn't have that problem with the ones I planted in early to mid spring. I don't think Lissa has the right advice here because the issue isn't germination, it's bulb development. And Mel's mix is so friable that even when it compacts somewhat, that the radishes should definitely be able to form bulbs.

The radishes that I planted in late June under floating row cover had 8-10" of top growth and bulbs that were half the width of a pencil! I think it might be heat or water related.

Can anyone who's experienced and solved this problem chime in?

Radishes are a cool weather crop, and typically don't do well in summer, except maybe in cooler areas. Try planting now, when it's cooler, and see how they do.

I second sb158. plant again now. Mine planted in Aug didn't do squat and were all funky and never bulbed. The ones planted this month are looking awesome.

I had the same trouble this year. But I am sure the hot dry weather was a problem, we had 80's up to the last week of Sept.and no rain all mounth. None of my fall crops were good. I too tried planting deep and shallow. I have grown fall radish before and don't recall any problems. One more thing my spring radishs were fine and they were all the same seed stock.
Curt

Yep, they are early spring or fall crop...not during the heat of summer. They can generally withstand some cool weather...if frost is predicted, just cover them with some loose leaves or straw and then uncover when the sun comes out.

I planted a few square feet of heirloom radishes in early August. The new planting area was somewhat disturbed by squirrels early on, but for whatever reason, the radish bulbs never formed. We had a very cool summer, rarely above 80F, so it wasn't temperature, but I also had to contend with low light conditions and some overgrown tomatoes. The squirrel attack surely damaged the root development, too. I'm going to try radishes next year out in the field, rather than in Mel's mix at home to see how it goes.