When pulling radishes in warm weather, cool them right away by dropping them into a pail of cold water. Use a sharp knife or kitchen shears to remove the leaves, then store in the refrigerator for up to three weeks. Large oriental varieties can be left in the ground well into fall and dug just before the soil freezes.
Harvest salad radishes once they are bigger than grapes. Radishes left in the ground too long develop a pithy texture and often crack following heavy rains.
Hot weather and drought encourage the development of spicy flavor compounds, which are similar to those found in horseradish.


Read more: http://www.motherearthnews.com/Organic-Gardening/2008-02-01/Growing-Radishes.aspx#ixzz20kuM1pHk