Although most instructional manuals will recommend planning several seeds per cube of rockwool, we advise you to plant only one seed per cube.

In fact, when planting more than one seed per cube, you will have several fennel bulbs/plants growing out of one hole of the planting section. Having 2 to 3 bulbs of fennel growing together out of one hole looks amazing, however, when several bulbs of fennel grow out of the same hole/cube of rockwool, they do not develop as well as when there is only one bulb per hole.  Single bulbs do not need to compete for space and therefore will grow bigger and deliver a better texture and a sweeter flavor.