Most people buying their first greenhouse have never used one. They know it extends the season. Beyond that, the honest answer is usually a shrug. So here is what a greenhouse actually does in this climate — including the parts that don't work.
You get about a month at each end
Utah State Extension research on unheated structures puts it at roughly a month of extra growing in early spring and again in late autumn. Cornell's work on unheated high tunnels found up to ten extra weeks across both ends combined.
In practical terms: transplants go out in April instead of late May, and you are still picking into November.
The zones here have already shifted
Every town we deliver to has warmed at least half a plant hardiness zone since the 1961–1990 climate normals. Kamloops, Vernon and Kelowna have moved a full zone.
What has not moved with it is frost timing. Vernon still averages its last spring frost on May 5. Kelowna — the warmest zone rating in our area — has the shortest frost-free window of all, about 130 days, because cold air settles on the valley floor. A warmer zone does not mean a longer season.
Yes, you can grow through winter — some things
Spinach, kale and collards planted in late summer will grow through winter with no heat at all. University of New Hampshire trials found winter-grown spinach is sweeter than the summer crop, because cold stress raises the sugar content.
Tomatoes and peppers will not. Extension guidance is explicit that these need supplemental heat and risk chilling injury even in April. An unheated greenhouse does not make them a winter crop, and anyone who tells you otherwise is selling something.
Frost protection is real, but smaller than you'd think
During the day an unheated greenhouse runs 10–15°F warmer than outside. But a small structure falls back to roughly outdoor temperature within an hour or two of sunset. It softens a light frost. It will not carry you through a hard freeze.
The trick that does work is a second layer inside — row cover draped over the plants within the greenhouse. Trials measured that inner zone at 18°F warmer than outside overnight, and 26°F warmer on the coldest nights. Two cheap layers beat one expensive one.
Better yields, and the gap widens in bad years
A peer-reviewed trial found high tunnel tomatoes yielded 33% more per area than the same variety in the field. In a wet season one farm recorded nearly three times the field yield. In a good summer the difference narrows — the greenhouse earns its keep in the years the weather doesn't cooperate.
More food in the same footprint comes from growing upward. The highest-yielding tunnels in on-farm research ran about four square feet per tomato plant, which only works when they are trellised to a single stem.
Rain off the leaves matters more than you'd expect
Early blight, Septoria leaf spot and bacterial spot all spread by rain splashing soil onto wet foliage. Keeping rain off is one of the best-evidenced benefits there is — North Carolina State Extension confirms these are markedly easier to manage under cover.
What it does not do
We would rather tell you this now than have you find out in July.
- It is not a pest barrier. Unless the vents are screened, small flying insects get in. And the warmth that helps your plants helps theirs — spider mites go from egg to adult in as little as seven days inside, against three weeks outdoors.
- Humidity becomes your problem instead of rain. Botrytis, powdery mildew and leaf mould all thrive in still, humid air. Venting is not optional, and cutting humidity matters more than trapping every last degree of heat.
- It uses more water, not less. No rain reaches the soil, so irrigation becomes the only source. What you gain is control, not savings.
- It will cook on a hot day if you leave it shut. Small structures swing fast in both directions.
- Hail, wind and deer. A solid structure obviously helps. But no research quantifies any of it, so we are not going to put a number on it.
The snow question
Snow on the outside insulates — low tunnels buried for a month have come through fine. The risk is to the structure, not the plants. Each inch of water-equivalent snow puts about 5.2 lbs per square foot on the roof, and heavy wet snow reaches that in three or four inches.
What reduces the risk is a steep peaked roof rather than a rounded one, single-piece frame members rather than bolted sections — every joint is a failure point — and proper bracing. Both our models are built that way, on galvanized steel rated for Canadian snow loads.
Where the numbers come from
Hardiness zones: Natural Resources Canada. Frost dates: Environment and Climate Change Canada Climate Normals 1991–2020. Season extension and yield: Utah State University Extension, Cornell University, and O'Connell et al. 2012 in HortScience. Frost and winter growing: University of New Hampshire Extension. Disease and humidity: NC State Extension. Pests: Colorado State and University of Wisconsin Extension. Snow loading: University of Vermont and UConn Extension.
Still deciding which size? Read our sizing guide, or ask us — we grow in the same weather you do.
