Cold frames, low tunnels, and a small greenhouse
Which structure is worth building on the Front Range or in a mountain town, what it protects, and how to compare the cost.
A cold frame buys you a salad bed you can vent. A plastic low tunnel buys a longer row if you will open it. A hobby greenhouse buys a place to stand up, and it buys a heating bill, a snow load, and a wind problem the catalog photograph left out.
Pick the structure for the crop and the nights you actually have, then price that structure. Do not price a greenhouse because the frame at the store looked like a photograph of June.
Colorado State University’s mountain fact sheet puts the gains in weeks, not in wishes.
Floating row covers add a couple of weeks at each end. Frost blankets add a week or two more. Plastic low tunnels can add about a month at each end, with the warning that wind and snow damage them if you use them too early or too late.
A cold frame is the small rigid version of that tunnel: a box, a transparent lid, soil as the heat sink. The cold frame page is how to site and vent one. This page is which box to build, and what a greenhouse does that a frame does not.
Three structures, three jobs
A cold frame, roughly 3 by 6 feet. One or two old windows, or a polycarbonate lid, on a wood box. It holds lettuce, spinach, and hardened brassicas.
On the Front Range it makes a March sowing plausible. In a town whose median last 32°F date is late June, it makes an earlier green sowing plausible. It does not hold a July tomato. The lid has to open.
An automatic vent, the wax-cylinder kind, is worth more than a prettier hinge if you leave for work.
Glass and Front Range hail are a bad pair. Polycarbonate costs more than a recycled window and survives the storm the window will not.
A low tunnel on a 4-by-12 bed. Hoops and UV film, weighted, as the mountain sheet describes: half-inch PVC, conduit, or heavy wire, plastic pulled tight, sandbags, ends staked. This is the month-at-each-end tool.
It is also the tool that cooks a crop on the first bright day you forget. If you will not vent, buy row cover instead and accept the smaller gain. The fabric choices are on the fabrics page.
A freestanding greenhouse, 8 by 10 feet or larger. This is a building. It needs a site out of the worst wind, a foundation or anchors that will hold a chinook, and a plan for snow sliding off a polycarbonate roof onto the path you walk. It can start transplants and hold pots.
It does not repeal a July 7 median last freeze at Dillon unless you heat it, and heating a leaky hobby house at 9,000 feet is a fuel cost, not a gardening technique. Unheated, it still freezes on a clear night. The soil and the thermal mass inside are what keep it above the outside air, and only by some degrees, not by a season.
What to count before you buy a kit
| Question | Cold frame | Low tunnel | Greenhouse |
|---|---|---|---|
| Square feet you can actually plant | About 15 to 18 | The bed, often 48 | The floor minus the path, often half the footprint |
| Venting | Lid, every sunny day | Ends or sides, every sunny day | Door, ridge vent, or you boil the plants |
| Hail | Prefer polycarbonate | Plastic punctures. Have a patch. | Panels rated for your snow and hail, or do not bother |
| Wind | Hinge on the north edge, latch | Weight the sides or it is a kite | Anchor to the ground, not to hope |
| Crop it is for | Winter and shoulder greens | A row of hardy crops, or a warm start | Transplants, pots, and greens if you will heat or accept freezes |
| What dominates the cost | The lid | The UV film, replaced when it clouds | The frame, the panels, the anchors, and any heater |
Cost per planted square foot is the comparison that matters. A frame that protects 18 square feet of lettuce can be the better buy than a greenhouse that protects 40 square feet and then freezes the same night the garden does, because you spent the money on headroom. Measure the planted area, not the brochure.
A frame you can build from lumber you can name
A 3-by-6 box, 12 inches at the back and 8 inches at the front, is a modest slope for a lid. The lumber is similar to a short run of raised-bed stock: 2x12 or stacked 2x8, with a hinged lid.
Set it on loosened soil, not on a deck and not on a pallet. The soil is the battery. A frame with a floor is a cold box.
Recycled windows are acceptable if the glaze is tight and the paint is not flaking into the bed. If hail is the local sport, buy the polycarbonate sheet and skip the nostalgia.
Do not seal the sill in a way that traps water against the wood. The lid sheds onto the ground beside the frame, not into the lettuce. A drip line inside beats opening the lid to water on a marginal morning.
The same vent rule as every other cover on this site: if the interior is much above 60°F and the crop is a cool-season green, open the lid. Close it in the afternoon so the soil can hold something for the night. A blanket over the lid on the coldest night helps. A blanket left on through the next sunny day undoes the transparent lid.
When a greenhouse is the wrong purchase
If your median last frost is in June or July, an unheated greenhouse will not give you peppers in July. It will give you a place to harden plants and a place to lose them if you trust it on a clear night. Spend the same money on a well-built frame, a low tunnel you will vent, and the 8-foot deer fence if deer are the thing actually eating the garden. A greenhouse with deer browsing the beds around it is a glass shed.
If you are on the Front Range and you already grow greens in the ground by April, a greenhouse is a convenience for seedlings, not a requirement. Start the tomatoes under lights in the house, six to eight weeks before you set them out, and use the frame for the two weeks of nights you still do not trust. That is a smaller structure and a smaller regret.
Snow load is the mountain-town line on the kit. Ask what the roof is rated for, in pounds, and compare it with a winter you have actually shoveled.
A kit rated for a light southeastern snow is a kite and a collapse in a Gunnison April. Anchors matter as much as the rating. A chinook will test the cheap ones.
Checklist
- Buy a cold frame for greens you will vent. Buy plastic tunnels only if you will open them.
- Prefer polycarbonate where hail is regular.
- Set the frame on soil, lid sloping south, hinge on the north side.
- Compare cost per planted square foot, not cost per pretty photograph.
- Do not heat a hobby greenhouse at high elevation until you have priced the fuel against the crop.
- Skip the greenhouse if an 8-foot fence or a hail frame would save more food.
- Use CSU’s week-and-month gains as the expectation. A structure that is shut on a sunny day misses them.
Sources
- CSU Extension, Vegetable Gardening in the Mountains, on row covers, frost blankets, and plastic low tunnels.
- CSU Extension, Frost Protection and Extending the Growing Season.
- University of Idaho Extension, BUL 936, season extension where frost dates are estimates and a midsummer frost is possible.