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How to Overwinter Dormant Biennials in a Cold Frame

Bob Wildfong

Saving seeds is pretty easy for most plants: you wait for the flowers or fruit to ripen fully and then you collect the mature seeds. But what if the plant doesn't grow flowers?

Almost all plants grow flowers, and almost all flowers make seeds, but there are a lot of plants whose flowers you might never have seen. What about cabbage? Where are the flowers on beets? Why don't parsnips and celery have any flowers? The reason is that they're biennials — they make flowers in the second year of their lives.

Biennials are familiar to most gardeners who grow flowers from seed. Columbine, sweet william, lunaria, and foxglove are some of my favourite flowers, but they don't bloom in their first year. They make leaves and stems in year one, then survive the winter to produce beautiful flowers in year two. Most people don't think of cabbages and celery as biennials, but that is the simple reason why you might never have seen their flowers.

The main challenge of growing seeds for biennial vegetables is storing the plants over winter. Our vegetables mostly come from warm parts of the world, or they've been "domesticated" to be more tender than their wild ancestors. For both of those reasons, few vegetables can survive in the ground during a typical Canadian winter. Many seed growers harvest their beets, carrots, cabbage, and other biennials, and store them in a cold cellar all winter. The plants remain alive if they are cold and humid, but not frozen, and they sprout back quickly when they are replanted in the spring to grow for another year - complete and mature with flowers and seeds.

Wish you had a cold cellar? Me too. A good cold cellar is really a kind of glorified hole in the ground, cold enough to prevent plants from sprouting, but not so cold that they freeze. Between 1 and 4 degrees Celsius is ideal. Also, the air should be fresh and well ventilated, and humid to prevent the dormant roots from drying out. It turns out that a well drained cavity in the ground often provides those conditions during winter, making it easy to store roots and vegetables if you have a big shovel and lots of space to dig.

For those of us who have small yards, or just don't feel like digging caverns underneath them, there are other choices. One of my favourites is to use cold frames.

A cold frame is sort of like a mini-greenhouse that keeps your plants slightly warmer during the cold season. It can be any structure that's tall enough to contain the plants, with a transparent or semi-transparent top. You can find hundreds of designs on the internet, but that's because there's no single design that actually works best. I've used cold frames that were specially built from old windows, some made with just a few boards nailed together with a piece of translucent awning lying on top (with a rock to hold it down), and I've even seen someone use the cab of a derelict pickup truck as a cold frame. 

The idea is that the cold frame doesn't generate its own heat, but it allows sunlight in during the day and holds heat during the night, so the net temperature will tend to be about 10 degrees warmer than the outside air. Cold frames are popular for growing lettuce and spinach late into the fall, and also for starting transplants early in the spring. They can also give you a good place to overwinter your dormant biennials.

Take cabbage as an example. Cabbage and related crops such as kale and kohlrabi are quite tolerant to frost, and if they are gradually and naturally introduced to cold weather they can survive as low as -10 degrees Celsius. In the garden, most areas of Canada see colder winter nights than that, so cabbage usually doesn't make it through the winter on its own. But with a little protection, even the coldest night can be made safe.

Soil and mulch are good insulators, and protection from wind can made a real difference to actual temperature. That means a well insulated cold frame, with good protection from drafts, and exposure to the daylight sun can remain above -10 degrees on even the coldest nights (depending where you live).

Either dig up your cabbages and transplant them - roots and all - in your cold frame for the winter, or just build a cold frame right in your cabbage patch around your best plants. The transplanting method is actually easier, because the dormant plants can be spaced close together in the cold frame to maximize room, and planted out in spring with lots of extra space to grow.

During the winter, check your cold frame often to see how the plants are doing. Don't worry if snow piles up on top, because it's a great insulator and sunlight goes through the snow just fine. Look out for damage at the ground level, and don't worry if the tips of leaves are damaged. Leaves will grow back, but stems won't.

Here are some tips for cold frame success:

• Mulch around the plants with loose material like straw that prevents drafts inside the cold frame but still allows slow ventilation. Leaves are okay if they're dry and crinkly, but wet leaves just tend to hold water and can cause plants to rot.
• Locate your cold frame close to a building, preferably on the south side and adjacent to bricks or concrete.
• Mound soil up on the outside of the frame. It's a great insulator, and the sides of the cold frame are one of the places where heat can escape easily. Soil also helps plug any gaps in the frame that would let cold air inside.
• Dig out a few inches of soil inside the cold frame before you plant in it. If the plants are a little lower in the ground, they will have more protection from the soil, which is much warmer than the air at night.
• If a really cold night comes along, you can put a few buckets of water in the cold frame. Water will absorb a lot of cold from the air as it gradually freezes overnight, and it will store a lot of warmth from the sun during the day.
• You don't have to use glass for the lid of the cold frame, or even a transparent material. Even a semi-transparent cover will let good light through, which warms up the cold frame's soil during the day.

The bottom line with cold frames is that they aren't complicated, but they take a little experimentation to get right. Think about insulating the sides, think about keeping the lid tight to prevent drafts, think about how to locate it in the sun and out of the wind at the same time, and think about nearby thermal masses like south-facing brick walls. Make a little cold frame out of spare materials, and try it out this fall and winter. By next year you'll know how to confidently keep biennials alive over winter, in your own back yard.

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Bob Wildfong is Seeds of Diversity's executive director

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