Last autumn I attended a dinner where the host used a huge garden carrot as the centerpiece. The giant carrot was a bit far gone for eating, but it was exactly the kind of carrot you might store through winter, plant out in early spring, and grow for its flowers, seeds or both.
Why bother? Like the other biennial vegetables listed below, carrots bloom at a time in late spring when nectar plants are in short supply, usually after spring-flowering trees and shrubs, but before summer annuals. Over 300 insect species visit carrot flowers, including beneficial hoverflies and numerous small wasps. Onion flowers are equally popular among bees and wasps.
Once they have filled this flowering gap, biennial vegetables like celery, parsley and hardy greens can produce huge amounts of seeds! One year I grew beetroot seeds from overwintered roots, and the seed spikes got so heavy they required multiple stakes. The spring after my cutting celery made seeds, I had hundreds of volunteer celery seedlings around the compost pile. It was a good problem to have.
List of Biennial Vegetables
Botanically speaking, biennials are plants that grow one season and bloom the following spring after a period of winter chilling. Among vegetables, popular biennials include beetroot, Brussels sprouts, cabbage, carrots, cauliflower, celery, chard, collards, kale, kohlrabi, leek, onion, parsley, parsnip, salsify, swede, and turnip.
That’s a hefty list to choose from, and I like to work with a couple of biennial vegetables every year. The hardiest of them, notably Brussels sprouts, kale, and many onions, will survive winter in the garden, while those that turn to mush in frozen ground are best handled as “stecklings” – roots or other plant parts that are stored through winter and replanted in early spring.
How to Overwinter Biennial Vegetables
There are several ways to help biennial vegetables make it through winter so they can come into their blooming glory in spring.
Deep mulch that reduces repeated freezing and thawing of the soil is a good way to enhance the cold tolerance of parsley, celery, other biennial vegetables with fibrous roots. Once winter gets going, I like to add a tunnel cloche to provide a barrier to ice, snow and howling winter winds.
Buckets filled with compost or damp, well-rotted sawdust make fine winter homes for cabbage, Brussels sprouts, and other cabbage cousins that would never survive winter in the open garden. Leeks can be handled this way, too, and it’s worth a try with celery. Only a small mass of roots is required to keep the plants alive, but it’s important to keep garden soil packed around the primary roots as the plants are dug and moved to buckets. To reduce moisture loss, trim off all leaves. Keep the rooted plants in a cold place like an unheated garage or outbuilding. If you replant them in spring, most of the new growth will emerge from secondary buds along the main stem.
Damp sawdust or sand are good packing materials for biennial root crops including beetroots, carrots, parsnips and turnips because they keep the vegetables from drying out. The challenge for many gardeners is finding a place with consistently cool temperatures, between 32° and 45°F (0°-7°C). Most modern homes do not come equipped with root cellars.
Which brings us to the refrigerator, the ideal storage space for biennial root vegetables you want to grow for flowers or seeds in spring. Stored in separate plastic bags, sound specimens will keep in great condition until late winter or early spring. Refrigeration of roots at just above freezing is method used by commercial seed growers of biennial root crops, because it preserves their quality and subjects them to an extended cold period. After a few months in the fridge, the replanted roots are ready to bloom like crazy and produce an abundance of seeds.