If we wish to save seeds from a squash, a pepper, or a zinnia, we simply grow them, and after the fruit or seed head matures in late summer or autumn, we harvest the seeds and store them for next year. But what about a carrot, or a parsnip? Have you ever seen a cabbage flower?

Many of the crops we grow for food in our gardens are not annuals but biennials, requiring two growing seasons to mature and set seed instead of one. However, we often treat them as annuals because we typically grow them for one season and harvest the plant after the first year. Carrots are a prime example. We grow a carrot from seed, planting it in the spring, and in late summer or fall, we dig up the orange root. But if we want carrot seeds, we’ll need to let it overwinter and grow from that root the following spring. 

Sometimes this happens by happy accident. If you missed a carrot or parsnip at harvest time, you may have found a similar but definitely different plant growing up there in spring. Typically, the second-year plant is tall, unlike the low-growing mound of foliage of the first year. The flowers, while not colorful, can be quite complex and beautiful. A carrot in its second year produces a gorgeous, sort of antique-looking display when in bloom. Smaller bees and other pollinators love to visit them, and if you leave a few carrots in the garden to bloom next year, you’ll have a busy plant buzzing with bees.

Whether for curiosity, seed saving, enjoying the flowers, or working with open-pollinated varieties to create your own selection suited to your local climate, there are a few factors to pay attention to when working with these biennial plants. 

Which vegetables are we talking about? 

While many are root vegetables, there are some other types. Commonly grown biennials in our gardens include carrots, turnips, parsnips, beets, rutabagas, onions, leeks, cabbages, Brussels sprouts, kohlrabi, and some kale varieties. This list isn’t exhaustive, so if you are wondering, go ahead and check. If you’ve never seen a particular garden plant in flower, it may be a biennial.

A note on vernalization

The biennial crops we are talking about require a period of cold exposure, called vernalization, in between growing seasons. Vernalization triggers hormonal changes in the plant that cause it to flower in the second year. Plants left out in the garden will likely get their cold exposure naturally, but if you bring them into the house, you’ll need to provide this chilling. Temperatures above freezing, but still refrigerator-like, are ideal, and most do best with 2-3 months of cold exposure. A warm closet in the spare bedroom will likely cause a rotten mess by spring and won't provide enough cold to meet the vernalization requirement.

Dig up, store overwinter, and replant, or leave them in place?

Just like a perennial may or may not be cold hardy in your region, the same goes for biennials. We don’t often think about how cold-hardy a beet or cabbage is, because we grow them as annuals. But if we’d like to keep them around for a second growing season, they’ll have to survive the winter. 

Whether you can leave a carrot in the ground for next year, or a cabbage, depends not only on the specific plant and your weather, but also on your gardening practices. Parsnips, for example, are quite cold-hardy, and can remain in the ground in most climates, even in USDA zone 4 winters. However, most biennials in our gardens will need either a thick mulch blanket, a cold frame, or require digging them up and storing them inside over winter. 

There isn’t a lot of information out there on what hardiness zones are applicable for biennial vegetables. Mulch, soil type, snow depth, and the extent of bitter cold experienced can also affect the issue. If you are unsure whether a particular biennial will overwinter in your location, perform the following experiment.

• Wait for a few consecutive autumn nights with temperatures below freezing, then head out to the garden. The cold nights will help trigger dormancy in the plants.

• Mark a few plants with flags or sticks so you know where to look in spring. Cover these with a thick layer of straw or shredded leaves. You may need to cover the mulch with chicken wire or other material to prevent it from blowing away.

• Maintain the mulch layer as necessary over the winter months. A good blanket of snow also insulates well.

• In spring, remove the mulch and watch for them to reemerge. Be patient, but if nothing has happened by the time you plant your tomatoes, gently dig them up and see if they look alive.

You may find that with a bit of mulch or snow cover, many biennials will survive a surprisingly cold winter and be ready to grow the second year.

For biennial vegetables that need to be brought inside

For plants we need to dig up, timing is not super critical, but make sure you do it before the ground freezes solid.

• After they go dormant, dig up the plants, taking care to avoid cutting the roots.

• Brush off the soil, but don’t wash them.

• For root crops, cut the foliage off just above the crown. For leafy plants like a cabbage, leave the head, crown, and roots intact as a whole plant but trim off loose leaves. 

• Store these plants in a cool, dark location where they won’t freeze. A 32-40℉ spot is ideal. For plants with roots entire root systems still attached like a cabbage, “plant” them in a bucket of damp sand to prevent dehydration. Damp sand works well for root crops as well (except onions which should be stored dry), but it isn’t strictly necessary. 

In spring, after the ground has thawed and is workable, you can replant the stored crops as you would a live transplant. For example, plant the carrot with the crown just at the surface. The cabbage should look like it was never moved, with the soil at the same level as before. 

The second growing season

Whether you’ve let a few plants overwinter in the garden or replanted them in spring after storage, the process for the second growing season is the same. 

During the second summer, these plants will need plenty of room to grow. Their water, sun, and fertility needs are the same as in their first year. You’ll see the plants grow, but their form may be wildly different. Take pictures and enjoy! The pics will make a great entry in your garden journal. 

Allow the flowers to bloom and be pollinated. Leave them intact to fade, and then watch for mature seeds to form. Don’t wait too long, or the seeds may scatter. Collect the seed, ensure it is dry, and then label it and store it in a cool, dry, dark location for future use.

More tips for growing biennial vegetables to seed

• Since biennials won’t flower or set seed until their second year, cross-pollination isn't an issue during year one. You can plant five varieties of cabbage, just decide to save only one type for growing to seed in year two unless you have a truly huge garden with room to keep them separated by hundreds of feet.

• Remember that seeds from hybrid parent plants won’t return true in the next generation. If saving seeds is the goal, choose plants that are open-pollinated.

• Some gardeners like to cut an X into the top of a cabbage head to help the stalk escape during year two, especially for super tight heads.

• Save only those plants that do well if your goal, and replant the best specimens. Over several generations, you can select for traits that perform well for your garden and gardening style.