Raab is only available in the spring, when over-wintered plants in the brassica family start to flower and send out their seed shoots. Raab is most tender before its florets burst into yellow or white flowers, and are a fantastic spring treat in stirfrys, raw in salads, added to soups, or on the grill. Some farmers plant specific varieties of broccoli raab, for example, that have been breed specifically for floret production. Most producers, however, take advantage of the natural life cycle of the plant by harvesting cabbage, collard and kale raab only in the spring.
(Thank you, Kia Armstrong of Nash's Organic Produce, for providing this description of raab. Nash's offers three kinds of cabbage raab, 2 kinds of kale raab and collard raab. As different varieties come on at different speeds, Kia explains, not all the raab are available at once, so look for a progressive wave of raabs in the first half of spring.)