Grow with Harvester: Winter Gardening with Cold Frames

If you’ve ever wished your garden could produce into the frost, a cold frame might be the ticket. A cold frame is essentially a mini-greenhouse – a low, unheated structure (often built over a raised bed or planter box) with a clear lid that traps solar heat. In Colorado, where winters are cold but sunny, cold frames let hardy greens thrive under glass or plastic. In fact, one Colorado gardening expert calls them “a much less expensive alternative to greenhouses for growing in winter months,” since they “trap solar heat to keep the bed warm”. We’ve seen our clients pick fresh kale and lettuce under a thin layer of snow.

Placing and building your cold frame right is key. For best results, we like to set ours against a south- or southwest-facing wall – a brick or concrete wall absorbs heat by day and radiates it at night. The lid should slope toward the sun (never face the higher side to the south) so incoming light isn’t blocked. Use strong, clear panels (we recommend 6mm polycarbonate) for the cover – it won’t shatter in hail like glass can. Build the frame out of rot-resistant wood (cedar is a classic choice) and, if you like, paint or oil it for extra longevity. We also add an automatic vent opener: on sunny winter days the frame can hit 100°F inside, so a self-opening vent prevents “cooking” our spinach and chard.

Here are some practical tips we follow in Colorado:

• Materials: Use a sloped lid of sturdy clear material. Old windows can work, but thicker polycarbonate panels (like the sheets sold locally) survive hail much better.

• Ventilation: Install an automatic wax-vent or removable prop. Even on cold days, sun can heat the frame fast. We open the lid on warmer winter afternoons so our cool-season crops don’t overheat.

• Insulation: Drop a layer of mulch or straw between plants in deep winter – it protects roots and adds nutrients as it breaks down. Don’t let snow pile up overnight; sweep off fresh snow by midday so light can flood in and warm the soil.

• Crops: Focus on cold-hardy veggies. Think lettuce, kale, spinach, Swiss chard, arugula, beets, radishes and other greens. You can also plant scallions or even carrots – these will grow slowly, but stay alive. (Later in spring, use the same frame to start tender seedlings early.)

Every farm-to-table enthusiast knows there’s something magical about cutting garden greens on a winter day. Beyond fresh produce, cold frames help cut food miles and plastic use: you’ll never need bagged lettuce again. At Harvester, we believe in practical sustainability. If the idea of “year-round salad” appeals to you, we’d love to show you how a custom cold frame can fit your yard’s sun and style. Connect with us at Harvester – we can design and build a winter-proof bed that keeps you growing through Colorado’s chilly months.