Greenhouses are a wonderful tool that many gardeners use to extend their growing season. Depending on the style, it’s also possible to garden all winter long, including in areas that experience harsh winter weather. In this article, we’ll look at the considerations in winter greenhouse gardening. We’ll look at what you need to know to keep your crops on track, even when the temperature outside drops to uncomfortable lows.
Selecting a Winter Greenhouse
Greenhouses are great at protecting your crops from harsh winter weather, but they’re not all created equal. There are lots of different greenhouses available, and each one has its own pros and cons.
The biggest consideration is your space and location. You need to make sure that you have a location that is both large enough to accommodate your greenhouse, but also gets enough bright light. Greenhouses only work correctly when placed in a sunny location.
There are numerous sized greenhouses available ranging from small to large. Here’s an example of a small greenhouse that could fit on a deck, and here’s a larger one for those that have more space. Either of these can work, and it is up to you to choose based on the space you have available and how many crops you want to grow.
Another concern during the winter is storms and snow accumulation. You want to make sure that your chosen greenhouse can stand up to storms, as well as handle any snow you might see. This is going to largely depend on your local climate.
For snow, getting a domed-shaped greenhouse is a good option. These will naturally shed their snow as it gives it less flat room to stick. That’s not to say other styles won’t work, but you may need to be more diligent in removing snow buildup. They’re also quite sturdy, and can stand up to most weather when properly secured.
Speaking of securing, that’s a critical step if you live anywhere that experiences inclement weather. For larger greenhouses, they usually come with some sort of securing system. This usually involves you building a simple foundation, then drilling into it to secure the frame.
Smaller greenhouses can often be easily moved and placed in sheltered locations. Up against a house or garage is a good way to minimize the chance that they get blown over by heavy winds. In both cases, you should check the greenhouse frequently and remove any snow buildup to prevent any damage.
Increasing The Temperature In a Winter Greenhouse
Depending on your location, dealing with heat in your greenhouse is going to be a major concern. While greenhouses are good at taking in light and heat, their materials are often poor insulators and lose heat quickly. This means that a greenhouse can rapidly drop temperature at night which can be problematic depending on your plants.
That in mind, here are a couple popular ways to increase and keep the heat in your winter greenhouse longer.
The first step is always to make sure that you’re optimizing the natural heat intake of your greenhouse. This means placing it in a location that gets the most amount of sunlight during the day.
The more sunlight your greenhouse takes in, the warmer it will get. If you can get it to a decently warm temperature then it’s just a matter of keeping that heat in. Greenhouses that don’t get enough light are going to be very difficult to keep at a warm temperature during the coldest nights of winter.
Add Thermal Mass
The thermal mass of an object is its ability to absorb and store heat. The more thermal mass an object has the more heat it will absorb and the longer it will take to radiate that heat away.
Unfortunately, the typical materials of a greenhouse, glass or plastic, have low thermal mass. This means that they will lose any stored heat quickly, and the surrounding temperature will drop quickly after.
To counteract this, you can add material with a high thermal mass into your greenhouse. This is typical water, usually in large drums to maximize how much volume it has. Water has a very high thermal mass, it radiates its heat slowly. This is why on a cold day areas near large bodies of water are typically a bit warmer than those further out.
Adding water, or another high thermal mass material, to your greenhouse produces the same effect. It will naturally keep more heat, and ensure that the temperature of your greenhouse remains higher than outdoors.
For best effect, leave your thermal mass in the sunlight during the day. This will ensure that it absorbs enough heat to have a noticeable effect once the sun goes down.
Use a Heater
In a pinch, heaters are another good option. They can quickly increase the temperature in your greenhouse and keep it there as long as you need.
The downside is that running a heater constantly can get very expensive. For that reason, it’s best to use it as sparingly as possible. Try to only use it on particular cold nights where other techniques might not do enough. You can also couple it with thermal mass items; use the heaters to add heat to them then allow them to radiate it away over time.
Two words of caution when using heaters. The first is to avoid placing them too close to your plants. The overly warm air can be harmful to your plants, especially since it’s usually a drastic change from the natural temperature in your greenhouse.
The other is to be mindful of the potential fire hazard a heater can cause. Make sure it’s placed away from any flammable material, and always keep an eye on it while it’s running.
Mulch is naturally a good insulator and has a reasonably high thermal mass. This helps keep your plants and the neighboring soil warm.
Adding a deep layer of mulch to all your plants at the beginning of the winter is a good way to insulate your plants. Mulching can also provide some benefits with watering and feeding, much the same as when planting outdoors. Adding mulch is quick and easy, so there’s really no reason not to do it.
Utilize Existing heat Sources
Lastly, you can utilize existing heat and insulation from other structures like your home or garage. Placing your greenhouse up against another building can help both insulate against heat loss, but also potentially take some heat from the structure.
This is the reason many smaller greenhouses are lean-to style. By utilizing one wall from an existing structure it both cuts down on construction costs, but also provides a noticeable heat and insulation bonus. Consider this type of construction if you have particularly harsh winters, or move smaller greenhouses up against an already heated structure. Even a little heat can go a long way during the cold winter nights.
Crop Selection For a Winter Greenhouse
Another important part of winter greenhouse gardening is selecting the right crops. While greenhouses can do a lot in terms of environment, depending on your climate they can still get quite cold during the winter. This can be problematic for warmer weather crops, even in the greenhouse they might not fare well.
The first step is to take an honest assessment of your climate and determine what’s feasible. For those in climate zone 7, for example, should stick to cold hardy veggies like kale or lettuce. Those in more temperate, like zone 5, have a wider range and can get away with herbs like chives or thyme that are fairly hardy.
For first timers, your best bet is to choose cold hardy crops that have a good chance of survival. This gives you the chance to see exactly what types of temperatures your greenhouse gets to and the care involved. Then, the following winter you can experiment based on your previous knowledge.
Good Cold Weather Crops For a Greenhouse
For those looking for some good cool weather crops to try here are a couple of my favorites.
Any of these are generally good choices that can survive a bit of a temperature drop. Bear in mind that there are sub-categories to each of these that can be more cold resistant. For example, Siberian Kale is specifically bred to grow in low temperatures. Choosing a cold hardy variety will make your winter greenhouse gardening that much easier.
When To Plant For a Winter Garden?
The time you plant will largely depend on the type of crop you’re growing. You usually want your plants to nearly mature just as the cold weather begins to roll around. This usually involves planting sometime in early to mid fall. Their growth will slow in winter, so this will help ensure that they have enough time to be harvest ready sometime in the winter.
You can also start new crops in the winter, but should take care with temperature depending on the variety. You also probably won’t be able to harvest them during the winter as they will likely not mature quick enough. What this will do though is give you a head-start come spring. Lots of gardeners start their plants in mid to late winter, and then have them ready for their first harvest come spring.
Caring For Your Winter Greenhouse Crops
With your crops selected it’s time to make sure that you’re providing proper care for them. Winter greenhouse gardening is a little different than normal summer gardening, even when done indoors or in a greenhouse. In this section we’ll look at some of the specific care tips that will make your winter garden a success.
You generally won’t need to water much if at all during the winter months.You can generally water about once a week, being fairly conservative with the amount. Plants don’t need much water during the winter, so doing too much can cause more harm than good. Plants are generally slow growers during the winter, and therefore don’t need much water.
Many gardeners are a bit hesitant to water when the temperature drops below freezing. The thought is that water in the soil will freeze, and this will damage the plant’s roots.
The reverse is actually true, water present in the soil can actually protect your plants. This is due to water’s high thermal mass, it will generally retain some heat which it can give off to your plant’s roots. Note that you should still be careful watering in very low temperatures, this protective effect won’t last if the temperature gets and stays below freezing for long periods of time.
Generally, you won’t need to fertilize much during the winter. Plant growth is slowed down, and therefore plants aren’t taking in as much nutrients as they normally would.
You can generally add some slow-release fertilizer at the beginning of the growing season, or add some organic compost. This is generally enough to provide enough nutrients over the winter without overdoing it. Then, once spring arrives, you can go back to your normal feeding schedule.
Winter Greenhouse Gardening
Growing your crops in a winter greenhouse can be a fun and challenging way to spend the winter. Heat is always a concern, especially for those living in areas that experience harsh winters. However, following some heat saving tips and choosing the right plants to grow goes a long way. For those that put in the time, having a thriving winter greenhouse is well worth the effort.