Cold frames for the Front Range and mountain towns
A cold frame adds a few degrees and a venting chore. Siting, build, and the crops that belong inside it at altitude.
A cold frame is a low box with a transparent lid, set on the soil, used to start hardy crops earlier and to hold them later. It is not a greenhouse. On the Front Range it is the difference between a lettuce sowing in March and one that waits until the wind settles in May.
In a mountain town it is how you get brassicas and greens through a June frost without pretending you can ripen melons. The Colorado Vegetable Guide lists covers, including rigid ones, among the tools that can hold air and soil up to about 10°F warmer than the open garden.
University of Idaho’s season-extension bulletin treats the whole category as a response to a season whose start cannot be trusted, because first and last frost dates are estimates. Spend the money and the Saturday on a frame you will open. A frame you leave shut on a sunny day is a cooker.
Where to put it
Face the slope of the lid toward the south. Set the frame where it gets sun in the shoulder seasons, out of the worst wind, and close enough to a door that opening the lid is a habit rather than an expedition. A south wall behind the frame adds thermal mass.
Idaho’s woody-plant bulletin notes that south- and west-facing masonry can provide on the order of five degrees of frost protection for nearby plantings, and that the same exposure can be a problem for woody evergreens. The frame is for vegetables and hardy annuals. Do not use it as the winter home of a shrub that needs to stay dormant.
Cold air still pools. A frame in a hollow will be the coldest frame on the property once the lid is open, and it can be surprisingly cold inside on a clear night if the soil under it is isolated from the earth’s deeper warmth.
Set the box on loosened ground, not on a deck and not on a pallet. The soil is the heat reservoir. A frame with a floor is a cold box.
Wind will take a light lid. Hinges on the north edge, a prop stick or an automatic vent opener, and a latch for chinook nights are part of the build.
Automatic openers, the wax-cylinder kind sold for this purpose, solve the “nobody is home at noon” problem in March. They do not replace a person during a multi-day storm. Have a plan for a lid that can be tied down.
A plain build
The classic proportion is a box roughly three by six feet, tall enough at the back for a heading lettuce or a young broccoli plant, lower at the front so the lid sheds snow and meets the sun at a better winter angle. Insulated sides help in a mountain town.
Straw bales as a temporary wall with old windows on top are an honest version of the same idea, and they come apart in summer when you need the bed. Recycled windows are fine if the glass is not loose and the frame is not painted with something you would not want flaking into the soil. Polycarbonate is lighter and safer in a hail climate.
Glass and hail are a bad pair along the Front Range, where the hail season cited by CSU’s CO-Horts blog runs from mid-April through mid-September. If hail is the dominant risk, a hail cloth frame may deserve the budget before a glass lid does. You can put a cold frame inside a larger hail structure if you have both problems.
Do not seal the frame to the ground with plastic that prevents drainage. Water that runs off the lid should be directed away from the base so the sill does not rot and the interior does not flood. A drip line inside is better than watering with a hose that forces you to open the lid and chill the plants on a marginal morning.
What to grow in it
Use the frame for hardy crops first. Lettuce, spinach, kale, radishes, Asian greens, and hardened brassica transplants are the plants that pay for the lumber. CSU’s mountain guidance allows frost-tolerant crops about four weeks before the last frost in the open garden, or at a 40°F soil temperature.
A frame can move that sowing earlier still, because the soil inside warms on sunny days.
On the Front Range, a March spinach sowing in a frame is ordinary.
In a town with a median last frost in mid-June, the frame is what makes an April or early May green sowing plausible. Check the soil with a thermometer. Sun on the lid does not mean the root zone is ready.
Tomatoes and peppers can start their outdoor life in a frame and then outgrow it. The frame is a transition, not their summer home, unless you are growing a very small determinate and you will remove the lid for the season.
Leaving a tomato under a low lid in July is how you cook a crop that survived May. The warm-season page covers that handoff. Cucumbers and melons in a frame are a Front Range experiment with the lid off by early summer. They are not a mountain staple.
In autumn the frame reverses. A September sowing of spinach and lettuce on the Front Range, covered as nights fall, will often carry salad into November.
Gilpin County’s average first frost of September 15, at 9,300 feet, means the fall frame there is a short extension, not a winter kitchen. Know which of those gardens you have. The calendars are the place to put the month.
Venting is the crop protection
The failure mode is heat, not cold. A March afternoon at 5,280 feet can push a closed frame past 90°F while the shade temperature outside is 50. Greens bolt or cook. Brassica transplants wilt and then never quite recover.
Open the lid when the interior is much above 60°F for cool-season crops, and prop it fully open, or take it off, once days are reliably mild. Close it in midafternoon so the soil can store something for the night.
On a night forecast to drop well below the crop’s limit, add a blanket over the lid and take it off in the morning. A blanket left on through a sunny day undoes the point of a transparent lid.
Water in the morning, so leaves dry. A frame is a humid little room, and damping-off and gray mold like that humidity.
Thin the seedlings. A frame tempts people to sow thickly because the space is small and precious. Crowding makes weak plants and disease.
What a frame will not do
It will not turn Climax, with a NOAA median last 32°F date of July 7, into a pepper farm. It will not protect a plant you forgot to water. It will not survive a lid that becomes a sail. It will not replace hardening.
Plants raised in a closed frame are as soft as plants raised on a windowsill until you vent them over several days. Move them to the open garden in the window CSU suggests for that crop, not on the first day you are tired of the lid.
A manure-heated hotbed, the older cousin of the cold frame, can add bottom heat. It is also a compost pile you are gardening on, and it is easy to run too hot or too ammonia-rich. For most home gardens a cold frame plus a row cover is the controllable version. If you want the deeper background on short-season covers, University of Idaho’s bulletin 936 is the regional extension piece written for exactly this climate.
Checklist
- Site the frame in winter sun, out of the worst wind, on soil rather than on a deck.
- Hinge the lid and give it a vent that works when you are gone.
- Prefer polycarbonate where hail is common.
- Sow hardy greens and brassicas, not a permanent tomato.
- Vent at about 60°F inside for cool crops. Close before dusk.
- Blanket the lid on the coldest nights and remove it after sunrise.
- Water in the morning and thin the seedlings.
- Use the frame as a bridge to the open bed, timed to your station’s median and late frost dates.
Pair it with row covers, which cover a larger area for less money, and with raised beds if you want the hoop sockets built into the lumber from the start.
Sources
- CSU Extension, Colorado Vegetable Guide, on covers and season extension.
- CSU Extension, Vegetable Gardening in the Mountains.
- University of Idaho Extension, BUL 936, An Introduction to Season Extension for High-Altitude, Short-Season Gardens.
- University of Idaho Extension, BUL 859, on south- and west-facing masonry and frost protection.
- CSU CO-Horts blog on hail season, citing the Colorado Climate Center.
- Gilpin County Extension climate summary, for the 9,300-foot frost averages.
- NOAA NCEI Colorado freeze-occurrence table, Climax median.