Broccoli, Brussels sprouts, cabbages, cauliflowers, kale, kohlrabi, mustard and turnips are all part of the brassicas, sometimes considered the cabbage family, or "greens".
Many brassicas can be grown as spring or fall crops, preferring the cooler weather. Here in Texas, it's a bit tricky trying to raise brassicas, since we have such hot summers. There are varieties that can withstand our heat a little better, though.
Broccoli, for example, planted in spring may stand in the ground well into the following year. Then flowering and going to seed.
And all brassicas will cross with each other, with the help of honeybees, of course. Cabbage, cauliflower, broccoli, and Brussels sprouts cross readily with each other, as do radish and mustard.
Honeybees pollinating broccoli flowers
If you allow your brassicas to go to seed, remember that brassica seed shatters easily. Harvesting is best done when 60% to 70% of the pods have turned brown and most of their inner seeds are light brown and firm.
Once ready, cut seed stalks into a paper bag, or other container and let set in a dark place, like the garage for about a week, or whenever you remember them. Pods will easily break open by shaking or slightly rubbing and fall into the container.
For more info, see growing kale here: http://cubits.org/CP101/thread...