Greenhouses are awesome for harvesting fresh fruits and vegetables for your family year round. However, winter weather can damage veggies & plants in your greenhouse--unless you take steps to ensure the temperate doesn't get too warm or cold.
Typically, a greenhouse can maintain temperatures 30 degrees Fahrenheit higher than outdoor temperatures. However, there's a lot of variability in how warm a winter greenhouse can get, depending on your climate, the outdoor temperature, greenhouse glazing, and other factors.
What things influence how warm a greenhouse gets in winter?
Probably the biggest factors that influence how warm a winter greenhouse gets are:
- your climate
- amount of snow cover
- greenhouse design & materials
Effects of your region & climate (growing zone)
Your climate--especially your minimum nighttime temperature and amount of sunshine per day--has the biggest affect on how warm your winter greenhouse can get.
For example, a greenhouse in the southwest Texas will get warmer than a similar greenhouse in Georgia, simply because southwest Texas tends to get more sunshine than Georgia.
Likewise, a greenhouse in arid Wyoming will be colder than southwest Texas, even though both get lots of sunshine, simply because Wyoming winters get pretty cold.
Amount of snow cover
It might seem counter-intuitive, but having more snow blanketing your greenhouse can often make your greenhouse warmer.
That's because snow can act as an insulator, with all the tiny air spaces between snowflakes that prevent heat transfer--and heat loss--from your greenhouse to the outdoors. If you ever built a snow fort as a kid, you'll remember how warm & cozy your fort stayed, even though the icy winds howled outside.
So, instead of removing snow from your greenhouse roof (as long as your greenhouse is sturdy), some of greenhouse growers leave the snow where it is so it can keep the heat inside--so long as the snow is dry.
However, heavy wet snow can risk collapsing your greenhouse, so it's best to remove it.
Greenhouse design & materials
If you've already built your greenhouse, there's not much you can change with its basic design. Unless you can move it to a sunnier spot or change its orientation so it has more of a south-facing exposure, there's not much you can do.
Likewise, what your greenhouse is made of will also affect how warm it'll get. For example, plastic won't retain as much heat as glass or polycarbonate glazing.
Prepping your greenhouse for winter
Just like a raised bed garden, greenhouses require seasonal preparation year round. You'll want to consider things like:
- cold-hardy plants
Your warm, humid, greenhouse will be very attractive to unwanted pests in the winter time. Before it gets too cold, make sure to clean out your greenhouse of any fallen dead leaves or plants, piles of stone or brick, unattended pots, and anything that would make a warm home for pests.
Also, be realistic with the growth of the plants in your greenhouse.
If you know the air around you won’t reach above 20 degrees Fahrenheit in the deep of winter, maybe change out your current plants for cold-hardy plants.
Plants friendly to cold temperatures are onions, shallots, garlic, scallions, beans, pears, chard, bok choy, spinach, herbs, and more.
And don't forget insulation--you need to hang on to the heat you're getting. See the Insulation section below for more details & recommendations.
Manage your soil temperature for winter plant growth
Soil temperatures rarely rise above 50 degrees Fahrenheit in the winter months, and most plants grow between 65-75 degrees Fahrenheit, so you’ll need to take some proactive steps to keep your plants happy and healthy through the colder weather.
Generally in winter, there are 5-6 solid hours of sunlight a day.
With proper insulation, greenhouses can remain 30 degrees Fahrenheit warmer than the air outside. This means that you can have a productive greenhouse in the winter down to -17 degrees Fahrenheit before the air in your greenhouse reaches the frost zone.
Hardy winter plants are happy at 35 degrees Fahrenheit, but can still grow all the way down to the teens.
As long as you have ample sunshine (5-6 hours a day), the air remains above 0 degrees Fahrenheit, and you have proper winter insulation, you more than likely can avoid any artificial room heater.
The sun’s energy will still warm the greenhouse up throughout the day, but at night it will cool off too quickly and damage your plants. This is why it’s so important to put some good insulators into your greenhouse that will absorb extra energy throughout the day and slowly release it overnight.
Methods to heat your greenhouse during the winter
There are many options to heat your greenhouse, from composting all the way to electric room heaters. Just as your greenhouse needs special customization based around the environment you live in, so will your heating/insulation system.
Also, be wise about the plants you choose to grow in the winter. Most will be okay with the proper heat/humidity ratio, but be mindful that there will be slight, inevitable decreases in sunlight and temperature and your plant variety should adjust accordingly.
The winter temperature of your greenhouse will vary based on where you live. If you live in a colder area, an insulation or heating system is a necessary investment. Temperate areas will also need a heating system, but in mild growing zones, passive heating options can often be sufficient.
Even if your greenhouse is attached to your home or garage, there will be a change in the daylight and environment, so you’ll need to help keep your greenhouse warm.
Next, some good insulation will work wonders. It could be as home-grown as bubble wrap all the way up to some frost fabric.
Bubble wrap is a great insulator because the clear plastic allows for sunlight to shine through on your plants, but the air bubbles trap warm air to be later released at night into the greenhouse.
Frost fabric is another great form of insulation, especially if you live in an area without such harsh winters. Frost fabric typically allows water to flow through while keeping in precious warmth. If your plants thrive in warm humid environments, frost fabric could be a good option.
Retain the heat you already get
If you live in a more mild climate, you’ll be able to get by with a natural heating method. The benefit to these methods is they are low on your carbon footprint and your electric bill. The con though is these methods will only warm your greenhouse a few degrees.
In some cases, a few degrees is all you need to continue germination, but these methods won’t cut it for deep winter areas.
2 ways to keep the heat you already have:
- add thermal mass with water barrels
- create a heat sink
The first sustainable, passive heat method is to use barrel(s) of water in your greenhouse.
Water is a thermal mass material: basically, it absorbs a lot of heat energy throughout the day, and then will slowly release it through the night back into the environment. Kind of like a heat battery, that gets recharged during the day, and releases the stored heat at night.
Whether you need just a bucket in a smaller greenhouse or a few 55-gallon barrels in a larger structure, make sure your containers are painted black in order to maximize the heat energy absorbed.
Second, and similar to the water barrels, you can create something called a heat sink in your greenhouse.
There are lots of ways to create heat sinks in a greenhouse, but a common way is a ditch dug in the center (or near center) of your greenhouse. There’s a perforated cover over the sink to allow mobility, but with that there should also be a pipe sticking out of the sink to allow warm air to move up and out into the greenhouse. Heat sinks will vary in warmth based on what you fill them with. For a warmer heat sink, you’ll fill it with compost. For a less dramatic temperature change, bricks or concrete will do.
Throughout the day, the contents of your sink will absorb energy and warmth, and will slowly release them through the pipe throughout the night. Along with the barrel method, heat sinks don't require fossil fuels or electricity, but will only warm your greenhouse up by a few degrees Fahrenheit.
Adding even more heat to your greenhouse
In harsher winter areas, more drastic measures must be taken.
If you have a larger greenhouse, a wood or pellet stove can be used to really fight those cold winter nights. However, it's crucial that you ventilate your stove properly--otherwise, deadly carbon monoxide & carbon dioxide can build up.
If your greenhouse is too small for a stove, a smaller electric room heater designed for greenhouses specifically are a great investment.
If a greenhouse heater isn’t for you, heat lamps combined with heated soil cables will keep any plant toasty.
In the northern areas that get below the teens (Fahrenheit) it's often best to bury the bottom-most portion of your greenhouse, leaving the roof exposed. The earth is heated from the center out, and you can see these heating effects with just digging four feet under. At four feet, the soil stays from 50 degrees to 60 degrees Fahrenheit, even if the temperature of the air gets as low as the single digits.
Burying your greenhouse will allow you and your family to enjoy fresh fruits and vegetables year round.
When building an underground greenhouse, it all depends on your position relative to the sun. The roof of your greenhouse should face the winter solstice sun at directly 90 degrees. The angle of your greenhouse roof is also dependent on your latitude. If you find your latitude, most in the United States are between 50 and 70 degrees, then add 23, will give you the ideal slant decline for your roof.
Proper ventilation, even in winter
With every type of winter greenhouse, it might seem promising to seal up the greenhouse in order to keep the heat you’re paying for inside the house.
But sealing your greenhouse creates problems with humidity.
So, it’s important to ventilate your greenhouse to prevent unwanted humidity, your electric heater shutting down, and many more issues.
If you’re in a very cold area, a circulation fan will be beneficial in keeping the air temperature consistent throughout the structure.
Without a fan, the hot air in your greenhouse might rise and the cold air could settle.
If you currently have a ventilation system, don’t fret. You don’t have to ditch it, but you do need to adjust it to ventilate only once a day or even longer. That way, your plants can gain some fresh air but also stay nice and warm.
When it comes to winter greenhouses, there’s so many adjustments you can make. However, that doesn’t mean you should make them all. Most temperate areas might just need a simple insulation adjustment. It just depends on where you are. No matter what winter your home experiences, it’s very important to customize your set up to ensure growth year round.
Ideal greenhouse temperature and humidity
The ideal temperature & humidity for most greenhouse plants is typically 70-80 degrees Fahrenheit and roughly 90% relative humidity. However, it'll depend on the plant--warm-climate plants will be able to tolerate higher humidity.
What to grow in a greenhouse for beginners
Some of the most common plants for beginners to grow in a greenhouse are varieties that are easy to grow. For example: lettuce & greens, herbs like basil, tomatoes, radishes, or ornamental plants like African violets. Starting seeds indoors in a greenhouse, weeks before the last frost date can give a head-start on the growing season.
Growing vegetables in a greenhouse year round
The easiest vegetables to grow year-round in a greenhouse are greens like kale, lettuce, spinach, and microgreens. The exception is during the hottest summer months, when greens need cooler temperatures to prevent bolting.