White cabbage and the red, savoy and pointed white varieties are biannual and cross-fertilize with all other Brassica Oleracea varieties.
Two different methods are used to handle the wintering of the plants. Storing whole trimmed plants in earth cellars, clamps or greenhouses. Root-stems, too, can be stored in the same way. Half-grown plants are wintered in the plot earthed over or covered with plant materials.
The chief advantage with the first method is to be able to select the healthiest plants, the ones most typical of the sort and a safer wintering, away from hungry animals, hard frosts and long-lasting rains.
The second method is used chiefly by market gardens in milder climates. The plants should have made small heads (8-10 cm) first. This is the only way to grow early summer sorts and pointed white cabbage. They rarely cope with the whole winter in a cellar or clamp.
In spring, after possible setting out with a mutual distance of 60-70 cm,  earth up and cut a 2-3 cm deep cross in the tops of the heads to enable the flower-stems to come through. It might be difficult otherwise for the flower-stems to break through the cover leaves.
The flower-stems should be supported and the seedpods harvested in turn as they grow light brown and the seeds dry and dark. Read more under "Growing cabbage plants".