8 Tips For Winterizing Your Greenhouse

Greenhouses are a great way for many small growers to extend the season, giving several extra months of fresh vegetables. But that still leaves many gardens bare during the coldest months of the year. Here are tips on how to grow vegetables in the winter in a greenhouse.

Heated greenhouses are generally not a reasonable option for self-sufficient growers, but unheated greenhouses offer lots of possibilities. To grow all winter in a greenhouse choose cold-hardy vegetables. Try combining your compost bin, chicken house, and greenhouse together, but be careful of too much humidity. Insulating your greenhouse and your pots is also a good idea. Winter greenhouse vegetable will probably need supplemental lighting and there are many options available. Or you can build a deep winter greenhouse and use the earth as your heat source.

Greenhouses can either be heated or unheated (also called a cold house). For most small growers, unheated greenhouses are the most practical and beneficial for the garden. Below, we will look at whether heating a greenhouse is worth it, and ideas on how to grow in an unheated greenhouse.

READ NEXT: 8 Common questions about growing vegetables indoors over winter.

Heated Greenhouse

Depending on where you live the heat of the sun alone won’t be enough to keep your greenhouse warm. In this case, you have to decide if heating your greenhouse over winter is worth the cost (both financial and environmental), and if it is even possible on a small scale.

We live in Zone 2b, which has the minimum temperature averaging -46 °C to -40 °C (-50 to -40 °F), with a low rising, weak sun in the middle of winter. Our frost-free growing season is from the end of May to the middle of September, and we often have snow 10 months of the year. However, commercial greenhouses run all year round in our area even though it takes a large amount of energy to keep a greenhouse at optimal temperatures in this frigid climate. The solution is, unfortunately, coal. Coal is locally and readily available in our area yet its environmental impact is very intense and negative. Running a heated greenhouse on coal is not an environmentally responsible solution for small growers (nor is it very self-sufficient), but what about other sources of energy?

On the whole, heated greenhouses are not very environmentally friendly, though in some situations it can be the right choice. An acquaintance of mine was building a mudroom onto their farmhouse and turned it into a greenhouse heated when necessary with a small wood stove. Instead of building a single-purpose structure that needed to be heated anyway, they turned it into a multi-purpose growing opportunity in our cold area. There are always situations where a heated greenhouse is the right idea, but you have to decide if the benefits offset the potential negative impacts.

Unheated Greenhouse

The vast majority of small-scale greenhouses are unheated, or cold greenhouses. Most growers use them for season extension, such as starting seedlings early in the spring or protecting frost-sensitive plants in the fall. But can a greenhouse grow vegetables all winter long? The answer depends on where you live. Winters can be extremely cold where we live, and it makes me laugh and cry a little when people say you can grow all vegetables all year round, even in a greenhouse. But in some more moderate climates, greenhouses can supply you with fresh vegetables all year round.

Because they rely exclusively on the heat of the sun, it takes a bit of time and effort to really make your plants grow inside your greenhouse all winter long. Here are 8 tips to help your winter greenhouse thrive.

  1. Choose cold-hardy vegetables
  2. Watch the humidity
  3. Compost
  4. Insulate
  5. Keep it black
  6. Turn on the light
  7. Bring in the chickens
  8. Deep winter greenhouse

1. Choose Cold-Hardy Vegetables

The first place to start is to choose cold-hardy vegetables. Many vegetables do well in cold weather, and some of the most popular are spinach, lettuce, carrots, cabbage, broccoli, cauliflower, peas, parsley, and chives. Talk to your local garden centre about finding varieties that are especially cold-hardy and that work best in your local climate.

2. Watch The Humidity

As the temperature rises inside and falls outside, humidity can become a big problem. Too much humidity can lead to disease, so make sure you have a good ventilation system that doesn’t lose precious heat at the same time. Excess humidity can also lead to frost damage if your temperature drops drastically in the night.

3. Compost

As organic matter generates heat as it decomposes. Adding compost to your soil can help raise the temperature, but why not build your compost piles or bins right in the greenhouse? While your greenhouse would have to be bigger to accommodate the compost, it might be well worth it considering how much heat rotting compost can generate.

4. Insulate

You can either insulate the entire greenhouse or the plant themselves. To insulate the greenhouse, add 1 or 2 layers of bubble wrap to the inside walls and ceiling of your greenhouse. This transparent material will still allow light through but it will help retain more heat inside.

To insulate the plants, horticultural fleece works very well. Also known as garden fleece, you can wrap it around your pots to insulate the soil or around the plants to keep the frost off and to insulate the leaves and stems. Horticultural fleece can be left on the pots all the time, but make sure to take it off the plants every morning.

5. Keep It Black

The colour black absorbs sunlight and converts it to heat. To utilize this scientific phenomenon, put down black mulch on the floor of your greenhouse to trap as much heat as possible.

6. Turn On The Light

Besides the temperature, winter greenhouses also have to contend with the lack of sunlight. In most cases, the winter sun is not sufficient for optimum plant growth so you are better off supplementing. In another article, we discuss growing vegetables under artificial lights in the winter.

Image by Carmen Edenhofer
Image by Carmen Edenhofer

7. Bring In The Chickens

Chickens produce a lot of heat (and so does their poop), and they can often be beneficial to your growing plants. Consider combining your chicken house with your greenhouse to have the best of both worlds. If you do this, you will really have to monitor the humidity as we talked about above.

8. Deep Winter Greenhouse

A fairly new, and revolutionary design is the deep winter greenhouse. This is a carefully constructed greenhouse that maximizes light penetration from the south combined with an insulated north wall. The ground inside the greenhouse is used as an ‘earth battery’ where the heat received during the day is stored in the earth and slowly released during the night. These greenhouses are attempting to replace unsustainable resources to heat commercial greenhouses, and this design is also practical on a small scale for the self-sufficient winter garden.