Harvest radicchio almost any time during growth—when leaves are just 2 to 3 inches (5-7 cm) long or after a head forms and is firm to the touch.
Radicchio matures 60 to 64 days after sowing.
Radicchio will grow green or reddish-green until cold weather arrives then leaves will change to a range of reds, from pink to dark burgundy.
Grow radicchio to mature in spring or fall; the optimal growing temperatures for radicchio are in the 60°sF (15°-20°C).
Sustained temperatures in the mid to high 70°sF (21°+C) will cause radicchio to send up a seed stalk (called bolting) and become bitter. If hot weather is predicted it is better to harvest the crop and store it in the refrigerator than watch it go to seed and become inedible.
Cut radicchio leaf-by-leaf if you don’t want to pull or cut the whole plant at once. Snip or break away older leaves—whatever size—from the outside allowing younger leaves at the center (called the central bud) to continue growing for later use. This type of harvest is called “cut-and-come-again.”
Radicchio also can be harvested whole. Cut the head away from the roots a little above the ground with a serrated knife. If the weather is cool, but not cold, leaves will re-sprout from the crown of the plant and be ready for picking in about two weeks.
Store radicchio in a cold and moist place (32°-40°F /0°-5°C and 95 percent relative humidity). Place radicchio in the refrigerator in a perforated plastic bag in the vegetable crisper section.
Radicchio will store in the refrigerator for 3 to 4 weeks.
Radicchio that matures in cool or cold weather will hold in the garden for several weeks and even under snow if protected by a thick layer of straw mulch or a plastic tunnel or cold frame.