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How Do Cabbage Plants Make Seed?


We often get questions here at GardenJoy.com and we try to answer them all. Here is one from Ken C.

“Can you tell me how cabbage plants make seed?”

Answer: Cabbage, broccoli, brussels sprouts, cauliflower, kale, and kohlrabi are all the same species, Brassica oleracea. They all have the same seeding and pollination habits, each producing a flower stalk that needs to be cross-pollinated by insects (which means that they won’t accept their own pollen).

The cabbage plant sends this flower/seed stalk directly out of the cabbage core. This process is called “bolting“. Cabbage will normally bolt when the temperatures are too high, usually above 80°F; however, bolting can also be caused by the cold. If you have planted your cabbage when you thought the cold temperatures were past, but then get a cold snap, your cabbage may bolt.

You can also force your cabbage to bolt, though this is a much more involved process. Firstly, you should know that cabbage plants may easily cross-pollinate with broccoli, brussels sprouts, cauliflower, etc.; with any brassica, in fact. Basically, if more than one of your brassica varieties bolt, you will probably not be able to separate the cabbage seeds from the broccoli seeds, for instance.  Cabbage plants are also biennial, meaning they will flower and set seeds in their second year of life.

In regions that experience 10-12 weeks of weather below 50°F and that does not go below 35°F on a regular basis, you can overwinter your cabbage in your garden. However, if you live in a colder climate, you will need to dig up a few choice specimens (full healthy heads), complete with the roots, put them in damp potting soil, and then store them in a cool, dark area at between 34° and 40°F. You should trim off any brown or wilted leaves before storing. A cool, dark place may be your unheated basement, garage, or shed.

Then, you will replant your cabbage plants in early spring, leaving two to three feet in between them. The seed stalk will grow directly from the middle of your cabbage plant. Cabbage seeds ripen slowly and fall off immediately when they are ripe, so you may want to either harvest the whole plant as the pods turn yellow or pick the dry pods when they turn brown. Some gardeners find it easier to just remove entire branches.

Regardless of how you harvest, you need to be sure that the seeds inside the pods are dry and brown. Opening one first will indicate if the others are ready or need a day or two more to mature. Mature cabbage seeds should separate easily from the pod. You can use your hands or a thick stone or branch to break the pods open, but you may want to open the pods on a white sheet or pillow case to be able to easily see the seeds; they are quite small. Store your cabbage seeds, once fully dry, in a cool and dark space in airtight containers. Cabbage seeds can remain viable for several years if stored properly.

Ken C., your question helped other gardeners who may have been wondering the same thing. We always appreciate any comments, questions, or suggestions from our customers.