Warm Up a Greenhouse for Winter
Add heat and light to keep plants growing through the winter
Greenhouses extend the gardening season in spring and fall, giving you a warm and sheltered place to start seeds, nurture seedlings and maintain comfortable growing conditions when the weather starts to cool down. Summer’s heat and bright sun make greenhouses an inviting spot for heat-loving plants, especially vegetables. When it comes to the longer and colder nights of winter, however, the temperature drop at night can make it hard to keep plants and seedlings alive, even when days are sunny. As a result, many home greenhouses go unused during the winter months.
Below are some suggestions for how you can make your greenhouse just a bit more hospitable throughout the winter. Some tips are fairly simple; others require a bit more work and a source of electricity. Aim to keep the daytime temperatures inside your greenhouse above freezing, ideally with lows of about 36 to 45 degrees Fahrenheit, or 2 to 7 degrees Celsius.
Below are some suggestions for how you can make your greenhouse just a bit more hospitable throughout the winter. Some tips are fairly simple; others require a bit more work and a source of electricity. Aim to keep the daytime temperatures inside your greenhouse above freezing, ideally with lows of about 36 to 45 degrees Fahrenheit, or 2 to 7 degrees Celsius.
Keep it sunny. The No. 1 tip is to make sure your greenhouse will get sunlight for most or all of the day. A greenhouse that might be in full sun in summer may be shadowed by neighboring buildings and other obstacles during winter, when the sun’s trajectory is lower in the sky. If you want to use your greenhouse year-round, make sure it will be in a sunny spot in the winter months.
Add thermal mass. One of the simplest and most low-tech ways to add heat is by using closed containers of water to create what’s known as thermal mass or a heat sink — a source that releases heat during the nighttime hours. The easiest way to do this is to put closed containers of water in your greenhouse. The water will absorb the heat of the sun during the day, then slowly release it at night. It won’t raise the temperature by much, but depending on your location, it can be enough to keep plants and seeds happy.
If you have a smaller hobby greenhouse where large barrels will take up too much of the existing space, or if you are simply starting several flats, fill one-gallon plastic jugs about three-quarters full with water, add a cap and place them in your greenhouse among the plants. Paint the outside of the jugs black, or add black food coloring or paint to the water to increase heat absorption. A small amount of bleach will deter algae and mold. Don’t use the water for drinking or watering plants.
Take the same approach with larger containers, from five-gallon buckets to 55-gallon plastic, aluminum or steel barrels or drums, which will release even more heat during the night. Paint the outsides black, place them in the corners of your greenhouse, then fill them about three-quarters full with water and close off the tops.
Add thermal mass. One of the simplest and most low-tech ways to add heat is by using closed containers of water to create what’s known as thermal mass or a heat sink — a source that releases heat during the nighttime hours. The easiest way to do this is to put closed containers of water in your greenhouse. The water will absorb the heat of the sun during the day, then slowly release it at night. It won’t raise the temperature by much, but depending on your location, it can be enough to keep plants and seeds happy.
If you have a smaller hobby greenhouse where large barrels will take up too much of the existing space, or if you are simply starting several flats, fill one-gallon plastic jugs about three-quarters full with water, add a cap and place them in your greenhouse among the plants. Paint the outside of the jugs black, or add black food coloring or paint to the water to increase heat absorption. A small amount of bleach will deter algae and mold. Don’t use the water for drinking or watering plants.
Take the same approach with larger containers, from five-gallon buckets to 55-gallon plastic, aluminum or steel barrels or drums, which will release even more heat during the night. Paint the outsides black, place them in the corners of your greenhouse, then fill them about three-quarters full with water and close off the tops.
Light things up. If you have easy access to an electrical source and your electricity costs aren’t too high, put some lights in your greenhouse. Old-fashioned Christmas lights, the ones that actually produce some heat, can be strung along your greenhouse walls or across the roof.
A grow light or radiant lights, like those used for aquariums, can also add some light and heat to the space. Just think how delightful your greenhouse will look on a winter night when the lights are glowing.
A grow light or radiant lights, like those used for aquariums, can also add some light and heat to the space. Just think how delightful your greenhouse will look on a winter night when the lights are glowing.
Provide cover. Adding cloth or plastic covers over your plants will also help keep them warmer. You can use row covers or cloches, or simply improvise with sheets. Just be careful to keep the covers away from the plants themselves. You’ll also need to remove them during the day if your greenhouse temperature or the humidity level gets too high.
Use a germination mat. Place a germination mat under seeds you’re starting in a greenhouse, just as you would if starting them indoors. Adding a clear cover and a grow light will increase the air temperature around the seeds and soil.
If you simply want to get your seeds started a little earlier without having them indoors, set them on a germination mat inside your greenhouse.
If you simply want to get your seeds started a little earlier without having them indoors, set them on a germination mat inside your greenhouse.
Attach some insulation. Attach bubble wrap to the interior walls of your greenhouse. Hold it in place with double-sided adhesive tape, Velcro tape or specialty clips. Nurseries and online sources have horticultural bubble wrap with UV protection and larger bubbles, but you can also find lighter-weight versions at home improvement centers, office supply stores and shipping centers. These probably won’t last as long as the sturdier horticultural versions, but they are a good start.
A similar option is horticultural fleece. Use it as a liner inside your greenhouse or as a row cover. Greenhouse suppliers also have covers for the outside of your greenhouse.
A similar option is horticultural fleece. Use it as a liner inside your greenhouse or as a row cover. Greenhouse suppliers also have covers for the outside of your greenhouse.
Create a compost pile. Why not move your compost pile to your greenhouse for the winter, especially if you live in a cold-winter climate? In terms of heat, it’s a win-win. Your compost pile will be protected from the elements and have a warmer spot to speed up the composting process, which in turn will help raise the temperature inside the greenhouse itself.
The ideal spot would be dead-center in the greenhouse, but practicality will probably dictate a different location. No matter where you put it, you’ll have the benefits of added heat and readily available compost in spring.
Learn more about winter composting
The ideal spot would be dead-center in the greenhouse, but practicality will probably dictate a different location. No matter where you put it, you’ll have the benefits of added heat and readily available compost in spring.
Learn more about winter composting
Include a heater. A small electric space heater can also warm the ambient air in your greenhouse. You can find ones specifically designed for greenhouses at greenhouse suppliers, or check home improvement stores. Look for one with a fan as well to help move the air. A disadvantage of heaters is that the heat can dissipate quickly once the heater is turned off. They also can be expensive to run.
You’ll also find propane heaters designed for greenhouse use. These have the advantage of being self-contained, so you don’t have to run extension cables to the greenhouse. However, you will need to be careful about both venting the carbon monoxide (plants love it, but it’s not as good for people) and not overheating the space.
You’ll also find propane heaters designed for greenhouse use. These have the advantage of being self-contained, so you don’t have to run extension cables to the greenhouse. However, you will need to be careful about both venting the carbon monoxide (plants love it, but it’s not as good for people) and not overheating the space.
Other heat sources include wood or pellet stoves and even solar or geothermal systems. They’re worth checking into if you’re planning a serious winter garden or have a large greenhouse, but do investigate the pros and cons before choosing a system. There are safety concerns with stoves of any type, and a geothermal or solar system can be expensive.
More
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10 Things to Include in Your Greenhouse
Tips for Decluttering Your Garden Workspace
Planting Time: Get Your Garden Started With Seeds
Ventilation is just as important in winter as it is the rest of the year. This is particularly true if your greenhouse heats up to above 80 to 85 degrees Fahrenheit, or 26.7 to 29.4 degrees Celsius, a real possibility in warm-winter climates. And while plants love humidity, preferring a level above 50 percent, you’ll need to keep an eye on the humidity levels, especially in warm-winter or rainy climates, to be sure it doesn’t get too high for the plants you’re growing, especially if they prefer a drier or cooler climate.