Finocchio di Sarno is a fennel built around a round heart that is fleshy all the way through, not a thin or hollow core. Very thick white sheaths close over that heart, and the head stays compact at a medium size, so the vegetable looks dense, pale and ready to take centre stage rather than sit as a garnish.
The calendar is equally definite. Seed goes in between 10 and 20 July; gathering then rides the turn of November into December. That midsummer sowing and late-year cut keep the crop tied to cooler months, when a compact white fennel feels especially welcome.
Taken together, the round fleshy heart, the very thick white sheaths and the compact medium size, plus that July sowing and November–December harvest, give a clear picture of how this named fennel is grown and what it looks like when it is ready. That shape and calendar are enough to keep curiosity on this fennel rather than on a generic shopping list.