If you have the space, greenhouses are one of the greatest resources any home gardener can have at their disposal. They expand how much you can grow during the season, and they allow you to have crops that are a bit more sensitive to weather/temperature fluctuations because you can house them in a controlled environment. But how do unheated greenhouses fit in? If one of the benefits is controlling how warm or how cool the space is, what’s the point?
An unheated greenhouse has its benefits, not the least of which is saving on heating costs. Because they’re outdoors, the sun will naturally heat the space during the day, even in the winter! So although there are concerns about inconsistency, you’ll still be able to save money and lower your bills during the colder season.
Unfortunately, you can’t grow warm-weather crops in an unheated greenhouse; however, that doesn’t mean you’re out of options! From root veggies to cold-hardy greens to starting seeds, there are plenty of things you can grow. Aside from that, the pros line up with having any other greenhouse. You get to grow longer than the outdoor season, protect dormant plants, and bring your container plants into an indoor environment without cluttering up your home.
How to set one up at home
Setting up an unheated greenhouse is as simple as setting up a standard greenhouse, just without the heating system. You’ll want all the basics: a thermostat, a water system (if it’s large enough), and a few grow lights. If you’re the average home gardener, it’s likely that your greenhouse set-up isn’t that extravagant — likely a cold frame or hoop structure — so it shouldn’t be too difficult to get ready for cold weather growing.
Make sure that where you set up your greenhouse is in an area with lots of natural lighting where it won’t get too rattled by the wind. If you can, you’ll want to keep it close to a water source (like a hose) to make things simpler.
The thermometer will be one of the most important parts of your unheated greenhouse, as it will help you keep an eye on how cold or how warm it is. Although you can’t heat the space yourself, you can provide your crops a little extra protection at night where needed and provide proper ventilation as temperatures start to warm up.
Protecting your crops without heat
An unheated greenhouse doesn’t mean your plants will go without protection — it just takes a little more work on your end. (Note: Unheated greenhouses may not be the best option for home gardeners that don’t have a lot of time to dedicate to manually caring for the space.)
Although the greenhouses trap heat during the day and keep the cold hardy crops relatively comfortable, nighttime winter temperatures can get a bit dicey. Frost can still damage crops in an unheated greenhouse, so you’ll have to give them extra protection and keep an eye on the forecast for when temperatures may drop.
Protection for unheated greenhouses and their crops can be as simple as putting a layer or two of horticultural fleece over the plants and wrapping containers with bubble wrap. Be mindful to remove the fleece during the day, as you don’t want to overheat the plants. You can also layer the inside of your greenhouse with horticultural bubble wrap, since the sunlight can still come through but it will help insulate and keep the cold out at night.
When you have a greenhouse, the onset of winter doesn’t have to mean the end of fresh veggies and greens — and that’s true with unheated greenhouses, too! There are plenty of crops that are cold hardy and able to withstand frosts and freezing temperatures. Plus, due to the nature of how greenhouses are built, sunlight will be able to naturally heat it more than you might think.
Starting seeds in unheated greenhouses is pretty easy, too, and can help you get your seasonal annuals ready in time for planting. Veggies like broccoli, carrots, and turnips are relatively cold-hardy and will grow well in an unheated greenhouse. You can also grow greens like spinach and kale, which means that you don’t have to forgo fresh side salads just because it’s winter! Their care requirements are relatively similar to the ones you follow during the growing season, so long as you’re mindful of overwatering and adding extra protection at night.
Although it may be tempting to keep your plants closer together in an unheated greenhouse as an attempt to increase humidity and warmth, you want to do just the opposite. Plants in an unheated greenhouse — similar to plants in an outdoor garden bed or controlled greenhouse — need room to grow correctly. If they’re too cluttered, they won’t grow as full as you’d like.
Some home gardeners prefer container plants for their greenhouse for this very reason. You can manage and allocate the space better, and you don’t have to worry about overcrowding. If you choose to use raised garden beds, make sure you go through the same thinning out the weaker plants, so the stronger ones have space to thrive.
Make sure you check in on your unheated greenhouse every day! Aside from needing to add protection at night and remove it in the morning, you’ll want to inspect for any signs of disease or pests. Damaged foliage should be removed as soon as possible to keep issues from spreading and allow the plant to direct energy to healthier parts. With luck, your winter harvest will be a bountiful one.
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