Answer: Once the radicchio has gone to seed, the plant is spent. There won't be any more heads. All you can really do now is save the seed for future radicchio.
Radicchio, which is a type of chicory, enjoys cooler temperatures, even soil moisture levels and rich soil, just like lettuce. Heat waves, drought or other poor growing conditions can cause them to "bolt" (send up a blossom stalk). Try again in spring - plant seed about a month prior to your last spring frost date, or start seedlings indoors two months before your last frost. This will give you a "head" start on the process (sorry for the pun - I couldn't resist!).
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