Unheated Greenhouses: What to Expect (& Surprising Uses)

If a greenhouse is on your garden wish list, it’s helpful to know what to expect. In cold climates, unheated greenhouses have different uses at each time of year. Find out what’s possible along with some surprising, alternate uses.

If you are interested in building your own, this shows how I built my lean-to greenhouse step-by-step.

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Using Unheated Greenhouses in Cold Climates

Is it worth it?

It is several years now since I built my lean-to greenhouse next to our house. And, after publishing the article about the build, I’ve received these questions:

How useful is an unheated greenhouse in a cold climate?

Can you use it year-round?

Would you do anything differently if you were building it today?

The quick answers are, yes, I use it year-round and I have made a few changes although they are really cosmetic, not structural. I have added some accessories but the basic design has proven to be a good one.

What I didn’t expect was all the alternate uses.

Yes, an unheated greenhouse is good for growing plants—more at some times of year than others—but that’s just the beginning. It’s also proven handy for everything from food preservation to drying clothes. I’ve given more examples below.

My greenhouse is fairly small by most standards with a 3-foot by 12-foot base (36 square feet). I built it at waist height for easy access. Situated on a south-facing patio wall, it receives maximum sun exposure.

One big benefit to this particular greenhouse design is how I can access the entire space from inside the covered patio without giving up any interior space for a walkway.

I can also 5x the space simply by putting seedling trays on stackable, vertical racks. A small space can go a long way!

Another important point I don’t often see mentioned is to also ensure your greenhouse is protected from strong winds and falling branches. Tucked beside the house, mine has—knock wood—gone through several severe storms with no signs of wear or tear. Lighter, free-standing polycarbonate units out in the open may not fare so well.

Unrealistic Expectations

The biggest misconception I hear about home greenhouses—along with other season extenders like cold frames and polytunnels— is expecting them to maintain consistent growing conditions year-round, even in a cold climate.

Some gardeners envision their new greenhouse will be like those at botanical gardens where they have heating, cooling, humidifiers, and watering systems running year-round.

But no: a basic, unheated greenhouse is really just a shell providing shelter from wind, rain, and snow. They heat up by day and cool down by night. And, despite our wishes, insects, mice, and animals may still find their way in.

Whether polycarbonate, glass, or polyethylene sheeting, any inherent insulating properties will be minimal. There are all sorts of additional things you can try to generate or capture and retain heat, but, as-is, during certain times of year, an unheated greenhouse can get way too hot and cold for most plants.

That’s an eyeopener for new greenhouse owners. Any visions of housing a collection of tropical plants all year-round is neither realistic nor practical.

But, if you’re willing to adapt, there are still plenty of good uses.

Accessories I’ve Added to My Unheated Greenhouse

• A thermometer (with 24-hour memory listing max highs and lows). Get a Wi-fi thermometer if you want notifications when temperatures reach certain levels.
• UV-resistant sun shade cloths (to reduce the intensity of the light). They come in different strengths.
• An air vent (which I open and close manually). You can also use an automated vent arm that opens and closes at certain temperatures.
• Frost cloths to protect tender seedlings.

Nice to Have

• Rubber shelf liners to make it easy to clean up.
• Water outlet. My hose reaches the greenhouse so that’s fine too.

Traditional & Surprising Uses

While I wouldn’t say the cost of buying or building a greenhouse is warranted by any of these uses on their own, with so many options, even a small greenhouse can have lots of uses at any time of year.

Seed Starting

Let’s start with a popular use: seed starting. While I like to start a lot of seeds indoors in my home to get a jump start on the growing season, I’ve found sowing in my greenhouse works nicely too.

Indoors is more reliable but I like the greenhouse option because I don’t worry about any mess or spilled water.

I start sowing trays of seedlings in the greenhouse in late winter. Early on they need some protection like old blankets to insulate the trays and frost cloths over top at night.

As temperatures and light increase, the seedlings sprout and grow, and routine care includes watering and putting up sun shades on bright days.

An added bonus is, for many plants, this sowing process seems to harden off plants naturally in preparation for transplanting outdoors.

Grow Fast-Growing Crops

The fastest growing food crops are cold-tolerant vegetables with edible stems and leaves. Without the need to wait for flowering and fruit production, the quick growers can be harvested any time.

This makes them perfect for greenhouse growing in the moderate seasons.

The ideal soil temperature range (for soil) is 50 to 68 °F (10 to 20° C).

When I do my late winter seed starting, I always get a few containers of salad greens going and sow more every few weeks for a continuous harvest. I like mesclun mix seeds for this.

Other options include microgreens, sunflower sprouts, arugula, pea shoots, and spinach.

I use frost cloths as needed to protect the seedlings as needed.

This process is similar to outdoor winter sowing in milk jugs (or other vented containers)—proving many seedlings to be more resilient than expected.

Overwinter Plants

If you grow tender annuals or perennials in pots, a greenhouse may be just the place to overwinter them. It’s perfect for plants like geraniums (Pelargoniums) and hardy fruit trees including Chicago figs.

If you’re letting the overwintered plant go dormant, you’ll want to cover it to block out light (while allowing air flow) and insulate the pot to prevent roots from freezing. I set a monthly reminder on my phone to check if they need watering.

Ripening Fruit

My little potted lemon tree has produced an abundance of lemons in my greenhouse. With sun shades up, the summer heat is favorable ripening some tree fruits.

Dehydrate Food

From spring to fall, the intense light and heat within a greenhouse can be perfect for dehydrating fruits, vegetables, and herbs.

Thinly slice your items (mango and tomatoes are favorites here) and spread them out on lined trays.

While I wouldn’t want to leave things overnight when mice might be roaming about, a few sunny afternoons may be all it takes.

Dry Laundry

While an outdoor laundry line may be fine, the greenhouse is my go-to drying rack on hot, rainy days. It’s become a sport to see how fast jeans can dry in the intense heat of the greenhouse.

Warm Cats

I’m only half kidding! One of our cats (Clara) loves nothing more than an afternoon nap in the greenhouse. Unfortunately, she also finds laying on sprouting seedlings most comfortable. But any time the space is clear, it’s all hers.

Propagate Plant Cuttings

I mentioned how unheated greenhouses are often expected to work like temperature-controlled greenhouses at botanical gardens, and, of course, they don’t. While I don’t grow tropical plans ongoing in the greenhouse, spring and early fall are good times to propagate plant cuttings.

Use these step-by-step instructions for softwood cuttings in spring and hardwood cuttings in fall.

Other Greenhouse Uses

While I don’t have room in my little greenhouse, here are some other uses I’ve seen on garden tours and online.

Work & Play

It’s not just a greenhouse, it’s a spare room. I’ve seen everything from greenhouses converted into bedrooms listed on Airbnb, for glamping, and use as party rentals. That’s one way to recoup your building costs.

For personal use, add the right furnishings and you’ve got a rec room, office, or art studio.

I’ve also seen greenhouses with hot tubs inside. With that much electricity use, it might be worthwhile to add solar panels to generate some of that power for you.

Aquaponics

It’s fairly common for greenhouses in our area to have space dedicated to raising fish with the waste water used to water plants.

Hatching Chickens

If the conditions are right and you have somewhere to plug in a heat lamp, your greenhouse may be just the place to incubate and hatch chicks.

So many of these ideas are not things you anticipate when getting a greenhouse, but the space can be useful for so many things beyond seed sowing.

Resources

• How to Build a Lean-to Greenhouse (Step-by-Step)
• Greenhouse Kit (6×10 polycarbonate) | Amazon

~Melissa the Empress of Dirt ♛