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Unheated Winter Greenhouses: What to Grow and How to Grow It

Lynn has been a professional organic farmer for the last 35 years and runs a 210-acre farm in Western Colorado with her husband.

You'll be surprised by how many veggies you can grow in an unheated greenhouse!

You'll be surprised by how many veggies you can grow in an unheated greenhouse!

Would you like to have fresh, homegrown food all year round? Are you dreaming about building a greenhouse in your backyard but you're not sure where to begin? Or do you already have a greenhouse and you don't know what to do with it come winter?

I've been a professional organic farmer for 35 years, and my husband and I have 9,000 square feet of greenhouses on our farm that we operate year-round in the Rocky Mountains.

What Can You Grow in an Unheated Greenhouse?

A lot! Most people think that in order to grow food in a greenhouse, it must be heated. But you will be amazed at what you can grow in an unheated greenhouse. That said, there are two main ideas that you need to consider when you're deciding what vegetables to grow in your unheated greenhouse: crop selection and nighttime temperatures.

Carrots are a great winter-weather veggie to grow in the colder months.

Carrots are a great winter-weather veggie to grow in the colder months.

Pick Cold-Hardy Vegetables

For crop selection, you want to pick the right type of vegetables that will lead to your success. So you'll want to grow cool-weather crops such as:

  • Leafy Greens: lettuce, spinach, bok-choy, kale, Swiss chard
  • Root Veggies: carrots, radishes, turnips, onions, leeks, celery
  • Cruciferous Veggies: broccoli, cabbage

All these plants can take a frost and will grow in the lower light of the winter days. Warm-weather crops such as tomatoes, cucumbers, green beans, squash, and corn will not take a frost, and they require long day lengths to produce food, therefore they do not work well in an unheated winter greenhouse.

November 15th through February 1st is considered the dark period for the northern hemisphere and is not conducive to getting warm-weather plants to flower and produce food. So for success, you'll want to select from the cool-weather plants list for your unheated greenhouse and your winter growing.

Know the Nighttime Temps in Your Zone

Once you have your crops selected, then you need to consider the night temperatures in your planting zone. Winters can vary from location to location—some of us live in areas where it rarely freezes in the winter and others of us live in areas that are below freezing for days on end. To figure out what you can grow (or if it's even worth the effort) you will need to know the planting zone right in your region.

How to Find Your Planting Zone

If you don't know your planting zone, simply do a quick internet search on planting zones and your town's name or zip code (e.g., "Telluride hardiness zone" or "81435 hardiness zone"). The search will bring up your zone along with other helpful information such as the estimated first frost and last frost.

All zones, no matter what they are, will require you to grow cool weather plans for the darker days of winter: November 15th through February 1st.

All temperature averages pulled from the USDA Plant Hardiness Zone Map

ZoneAvg. Winter LowsRecommendations

1–3

-60˚F to -30˚F

An unheated greenhouse isn't really going to work for you. Avoid planting for 8 weeks between Dec. 1 and Feb 1., then plant your cool-weather veggies in February.

4–7

-30˚F to 10˚F

Use low tunnels and frost blankets to protect the plants in your unheated greenhouse.

8–10

10˚F to 40˚F

You'll rarely see a frost in these zones, so you should be able to grow all winter long in an unheated greenhouse without needing low tunnels or frost blankets as long as you grow cold-hardy plants. You can start your warm-weather plants by Feb. 1.

Read More From Dengarden

Low tunnels allow you to trap heat in a much smaller area than trying to keep your entire greenhouse warm.

Low tunnels allow you to trap heat in a much smaller area than trying to keep your entire greenhouse warm.

How to Keep Plants Warm in an Unheated Greenhouse

Use low tunnels and frost blankets to cover the plants at night and then uncover them during the day. My husband and I are in zone 5 here, and we use this easy system to cover our plants every night in winter when it's below freezing.

Build Low Tunnels for Your Plants

Buy cement mesh to cover your rows. We use pieces that are cut six feet wide and then we leave little "feet" on it (we cut the wire and leave a little piece sticking out). You can tailor the size based on your own garden. We then bend the mesh into a half-circle and place it over the plants, with the little feed going into the soil to hold the wire covers on. This wire cover will then remain in place all season.

Covering your low tunnels with frost blankets at night helps keep them insulated against dropping temperatures.

Covering your low tunnels with frost blankets at night helps keep them insulated against dropping temperatures.

Cover Your Low Tunnels with Frost Blankets

At night, we put on these heavy frost blankets to keep our plants insulated. If you can't find a heavy frost blanket, you can order the lighter frost blankets and just use two of them. In a pinch, you can also use towels or blankets—anything that's got heaviness to it (sheets are too light).

Cover your tunnels a little bit before sundown and then uncover them again in the morning. You can either fold the blanket back halfway and let the sun come in or completely roll it back off of the plants so that they get full sunlight.

We've found that this works really, really well even here in zone 5. We've had temperatures down to -13˚F outside, and our plants still didn't freeze under their covers.

Make Sure Your Greenhouse Is Watertight

Since you're growing in a greenhouse, you don't have to put plastic on top of the covers. A sturdy roof will keep the moisture off your covers and also keep the wind out when you've got your greenhouse sealed up.

Why Does This System Work So Well?

What I love about it is that we're only trying to keep this little bitty dome of a greenhouse warm as opposed to the whole great big greenhouse. The sun radiates heat during the day, and then we can collect that by putting those frost blankets on at night.

It's also a super simple and affordable way to keep you from needing a heater in your greenhouse over the winter months.

Is there anything yummier than homegrown vegetables?!

Is there anything yummier than homegrown vegetables?!

Enjoy Your Homegrown Winter Veggies!

Winter may be a time of darkness and colder weather, but with a little knowledge and a few covers, you'll be surprised by what you can grow in an unheated greenhouse. Growing food all winter long is really fun and a great adventure—and it's tasty, too. As always, may your garden be easy, fun, productive, and always organic!

More Cold-Weather Garden Inspiration

  • 3 Winter Greenhouse Watering Tips for Healthy Plants
    Learn how to grow fresh food year-round in a greenhouse. This article provides three essential watering tips to ensure your plants thrive through the winter months.
  • How to Protect Your Garden From Frost
    If you want to save your plants from frost so that you can harvest at the end of the season, you'll need to follow a protocol. We have perfected our frost protocol with three easy steps.
  • How to Get Rid of Powdery Mildew Safely and Naturally
    A fungus called powdery mildew often preys upon cool, damp gardens in the fall. If this is happening to your garden, you can safely and naturally control the spread. Six homemade sprays can be used as natural remedies to treat powdery mildew.

Sources

  • Plant Hardiness Zone Map
    In the United States, the U.S. Department of Agriculture (USDA) Plant Hardiness Zone Map (PHZM) is the primary reference for de-fining geospatial patterns of extreme winter cold for the horticulture and nursery industries, home gardeners, agrometeorol

© 2021 Lynn Gillespie

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