How to Grow Radicchio
Cichorium intybus L Full Sun Soil pH: 5.5 to 6.8
Space the plants 8 to 12 inches apart. Mix an abundance of organic matter into soil prior to planting . Radicchio is shallow-rooted and needs consistent moisture. Organic matter, as well as a layer of organic mulch will help to retain soil moisture as well as suppress insects and weeds.
Water the plants deeply, about 1 to 1 1/2 inches of water weekly. Water more heavily for about a week prior to harvesting.
After planting, fertilize with a low salt fertilizer, don't use the high nitrogen fertilizer, although nitrogen fertilizers are good for growth of foliage in the case of radicchio's excessive nitrogen can cause plants to bolt and become bitter. Soil pH should be 5.5 to 6.8
Heads form best in the cool weather. Cooler temperatures are known to sweeten leaf flavor. Excluding the the coldest of regions, allowing the crowns to remain in soil after harvest may produce a second harvest in spring.
To pick, cut the entire plant at ground level ,just above the soil line. Harvest heads when they’re young. Like people the older heads become more bitter and tough. They will generally be harvestable at varying intervals regardless of whether they were planted at the same time or not. Once radicchio heads reach full maturity, they won’t continue to visibly grow but will instead develop a core, flowering stem. When this core forms, the flavor becomes extremely bitter, and the radicchio useless for anything other than seed. Heads typically last 3 to 4 weeks when refrigerated.
As stated mature radicchio plants handle fall frosts very well. In cold climates, frozen heads can still be harvested and eaten. After thawing slowly remove and discard the outer, damaged leaves.
Radicchio: Tasty but So Misunderstood - NY Times December 21, 1988