Root too pungent

Too high a temperature (above 70˚ F) may cause the flavor of the root to be excessively hot. Lack of water is another possible reason.

Bolting

Radish is a short-lived annual and will bolt when it has built up enough food reserves to produce seed. It may also be stimulated to flower by the long days of summer (another reason they are easier to grow in spring and fall). Crowding and other types of stress may also hasten bolting.

Root is woody

Insufficient water, or high temperatures (above 70˚ F) can cause the roots to be woody or pithy.

Cabbage Root Maggot

Cabbage Root Maggot is the big pest of the Radish. In fact they are sometimes planted to lure the little worms away from more valuable Brassicas. This pest is most problematical in spring and late summer.

No root

If a Radish doesn’t produce a swollen root, it is usually because the growing conditions weren’t good enough. It simply wasn’t producing enough food to have a surplus to store in the root and make it swell. This may occur because of competition from neighboring crop plants or weeds, low light levels, growing temperatures that are too high or insufficient water or nutrients.