Brassica spp.are members of the family Brassicaceae. This family consists of over 350 genera. The Brassica genus consists of many species including napus (rutabaga and rape), oleracea (broccoli, cabbage, kale, etc.), and rapa (turnip and Chinese cabbage) to name a few.
Brassicas have perfect flowers but require insect pollinators. Seed producers must keep in mind that varieties of the same species will cross-pollinate with other varieties of the same species. That means that broccoli, Brussels sprouts, cabbage, kohlrabi, collards, cauliflower and kale can all cross-pollinate each other (all are B. oleracea). Isolation distances of at least 1/2 - 1 mile should be sufficient between varieties. Minimum population sizes for all brassicas should be at least 100 plants.
Most brassicas are biennials (taking 2 seasons to produce seed), and require a cold treatment (vernalization) in order to flower. After first year production, cabbage plants should be dug up and overwintered in a root cellar with the roots buried in sawdust or sand. Before planting for seed the second year, trim the heads (see picture to right).
Harvest seed pods when they are fully mature, they will not mature any further after harvest. As they start to mature, the pods will start to dry and turn brown. If the seed pods are allowed to dry completely in the field, you run the risk of them shattering. It is best to harvest the mature plants and allow them to dry completely in mesh bags or on tarps.
Brassica pods need to be threshed first (we use a belt thresher but this can be done by hand threshing) and then the smaller chaff can be separated by using screens or an air column or simply by general winnowing.