Start indoors in the early spring, four to six weeks prior to your last expected frost date. The optimal temperature range for germination is from 60ºF to 70°F. Transplant into the garden, four to six inches apart, in early spring after all danger of frost has passed. Fennel prefers locations with rich, well-drained soil that receives full sun. Give your plants plenty of room to develop properly. Final spacing after thinning should be eight to twelve inches apart. For the sweetest, crispiest, largest bulbs, maintain consistent soil moisture throughout the season.
Informational Reference:- "Fennel in the Garden," by Ken Adams and Dan Drost, Utah State University, 2012. [PDF]