Cabbages are classified as early, mid-season, late-season, and storage types.  The early varieties are set out in spring;  they work well for short growing seasons  Mid-season cabbages are sown after the last expected frost.  Late-season cabbages are intended for fall harvest, and moderately cool temperatures allow you to keep them in the garden until needed;  a chill in the air brings out the flavor of cabbages.
Cabbages are ready to roll when firm and fully formed.  Early varieties are quick to mature, and should be havested promptly before they can crack open.  They can  be stored for just a month or two, given near-freezing temperatures and high humidiy.  Late varieties can be allowed to stay out in the garden; a light frost or two will sweeten their flavor, but they should be brought in for storage in a cool place before hard frosts.  Rather than yanking on the plants, roots and all, try cutting the heads so that as much of the stem as possible is left intact;  you may see small heads forming for a second, especially tender harvest.  here's a time-honored trick for slowing the development of mature heads you don't want to harvest right away.  Grasp the head and twist a quarter of the way around, so that some roots are severed.  Younger heads store best..
This is a two-year project, and also the chance that your prize cabbage variety will swap pollen with both other cabbages and cabbage family relatives.  Keep plant intended for seed at least 300 feet from them.  Use a loose mulch to help plant overwinter, or in colder zones, unearth the plant, roots and all, and keep them indoors on a cool, humid spot for setting out in the spring.  In the second year, you can help the flower stalk to come forth by slashing an X in the top of each cabbage's head.  Wait until the seed heads turn brown before collecting the seeds.
The Gardener's Guide to Growing and Storing Vegetable and Flower Seeds
by Marc Rogers

Learn how to select, harvest, and store seeds from more that 100 vegetables and flowers commonly grown in home gardens.
192 pages, 6 x 9, paperback,

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Growing ~ Harvesting ~ and ~Saving Cabbage Seeds

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