Vail Daily garden column: Cold frames are an easy way to extend garden season |

Vail Daily garden column: Cold frames are an easy way to extend garden season

What a lovely spring in the Rockies we’re having. One day it’s gorgeous sun and 70 degrees; the next day it snows and rains. But it’s fine weather for garden projects, like building cold frames to get an early start on the growing season.

Cold frames are an easy, cheap way to start growing vegetable seedlings when the weather is still too frosty at night or snowy some days.

Many people like to start their early gardening by planting seeds indoors in seed pots and trays, where they can protect them from the cold, then transplant the seedlings out to the garden when the weather warms. But starting seedling trays indoors can be tricky, and labor intensive. My results are often discouraging. And when you move the trays outdoors (if they survive the first few weeks) to harden them, they are often so fragile they break in the wind. It’s really all too labor intensive for my busy schedule. Once you plant the little guys in garden, the plants can take another week or two to recover from “transplant shock,” where they just go on strike and refuse to grow for a couple weeks. Seedlings are just too fussy for me.

I find it easier and more dependable to start early plants in cold frames, right in the dirt outside. There are elaborate kits online or at, but I just build my own frame box, screwed together with lumber, sized to fit an old, single pane window I get at the recycle store. The box frame, about 3 to 4 feet big, is placed over the topsoil in my garden bed, and the window sets snugly on top, creating a mini-greenhouse. After amending the soil with compost, I let the little greenhouse sit for a few days in the sun to pre-warm the soil. Seeds won’t sprout until the soil is consistently above about 65 degrees. Then I plant my seeds of choice.

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For early season planting, I must stay with cold-tolerant veggies such as bok choy, spinach, kale and any leafy greens except basil. Nights will still get to 20 degrees even in the cold frame, but usually it won’t hurt these veggies. All root veggies do well in a cold frame, too. These can all be planted, starting in mid-April at mid-valley, giving me a two- to four-week start on the growing season. (In Vail, depending on the snow cover it could be mid-May to June.)

Cold hardy veggies will continue growing happily in the protection of the cold frame right through late season snow storms. During cold spells, they will simply slow down, but bounce back as soon as warm spring sun shines again. Once these cold-hardys have a good start, (they should be 2 to 4 inches by mid-May), then you can move the cold frame to the next spot and start warmer veggies such as zucchini or tomatoes around mid to late May. By the first weeks of June, your warm-veggies will be off to a good start. Remove the cold frames, and store them for the rest of the season. The main advantage this has over transplanted seedlings is that these young plants are already adapted to the garden environment. When the warm days finally arrive in June, they are ready to jump into the growing season. Whereas transplanted seedlings may still take another week or two to adjust before they start growing. I usually have my first harvest of bok choy and radishes by June 1.


But cold frames take a lot of monitoring, too. You must put a thermometer inside the box so you can watch the temperature. Keep the boxes covered at night to maintain warmth and open the box during the day or it will quickly get above 90 degrees and cook the seedlings. I use a convenient hydraulic lift arm for this purpose, called a “window vent opener,” found at greenhouse suppliers like The folding hydraulic arm attaches to the inside of the box and onto the window frame. When the temperature inside the box gets above 70 degrees, the hydraulic arm expands, opening the box. At night, as soon as it cools, the arm collapses, closing the box for the night automatically.

The other issue is watering. You must devise a way to water every few days without the water lines freezing over night. You can use a watering can, but if that is too cumbersome, I use a hose to water in the morning, then disconnect it and let it drain downhill so it won’t freeze.

All this for a few fresh veggies, and it’s worth it. If you need more advice on starting cold frames, you may contact me at

Lori Russell is a professional landscaper. She teaches high altitude garden techniques at CMC in Edwards. Email comments to

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