A cold frame is a container like a bottomless box with a clear cover that allows you to extend your growing season, harden off plants, protect tender ones, and many other uses.
You can build your own in a few hours for less than $50, or purchase excellent ones from garden centers or online for a few hundred dollars. Their versatility is one of the reasons they've been used for centuries: They have enough different kinds of uses that you are likely to make your small investment worth it.
When you use a cold frame, it's like moving one or two USDA hardiness zone further south. So whatever you do with your cold frame, you'll want to adjust your use to a warmer climate than you normally would. With that in mind, here are some of the possibilities.
Cold Frame vs. Greenhouse
The main difference between a cold frame and a greenhouse is the size. Both create heat by, well, the greenhouse effect, by trapping the sun's heat in but not letting it out. But unlike cold frames, greenhouses can be heated artificially, making it possible to grow (not just preserve) plants in them year-round.
1. Soil Warmer
You don't need to actually grow anything in a cold frame. You can just use it to warm up your garden's soil to get it ready for the growing season. If the cold frame is portable enough, you can move it from section to section of your garden to warm up the soil underneath. If you have a raised bed and the width is right, you can lay an old window frame over sections of it to get it ready. Just remember that once you remove the window, ambient temperatures will resort to normal, even if the temperature of the soil underneath stays relatively warm. Stick to cold-hardy plants until the last frost has passed.
2. Staggered Starts
You can use a cold frame as a seedling relocation center if you've run out of room for seedlings under your grow lights. If you're planning on starting seeds indoors, you'll need to start them grouped into various start dates: Some need to be started indoors 6-8 weeks before the last frost, others 4-6 weeks, and so on. You can transfer some of your mature seedlings to a cold frame to make room for another round of seedlings.
3. Direct Sow
You can also start your seeds early directly into the cold frame. Some seed packets recommend sowing seeds directly into the ground, especially if they do not transplant well. In a cold frame, your average last frost date might be a month earlier than in your garden. Just keep the seedbed moist and vent it frequently to avoid damping-off, where seeds or seedlings rot before they get a chance to mature.
4. Hardening Off
If you've started seeds indoors under grow lights, allow your seedlings to adjust to outdoor temperatures and light variations by introducing them first in a cold frame before you plant them into the ground.
Before you bring your seedlings outdoors and into the cold frame, wait until the seedlings develop two or more sets of leaves beyond the first set, called cotyledons, that appear soon after germination.
What Are Cotyledons?
Cotyledons are part of the embryo of seed-bearing plants and only last a few days before true leaves begin to grow on the plant.
The appearance of true leaves means the plant is producing its own food rather than relying on the food reserves in the seed. It has established roots able to extract nutrients from the soil and is photosynthesizing carbon dioxide from the atmosphere. It's ready to stand on its own and can be hardened off in the cold frame.
As with starting seeds in the previous option, vent the cold frame frequently, especially on warm sunny spring days. Allow your seedlings to harden off for two weeks in the cold frame before transplanting them into your garden.
5. Frost Protection
Early in the season, you may have put out potted plants, hoping that you've judged it correctly that there will be no more frosts. But if a frost is in the forecast, you can move your pots into a cold frame to protect them. A cold frame will be 5 degrees F warmer, or more, than the outside temperatures, so this won't protect impatiens when it's 20 degrees below outside, but it may protect them when it's 36 degrees F.
6. Propagation Center
Use your cold frame any time of year to propagate new plants. Snip off runners or suckers such as those on a mint or tomato plant, pot them up, and keep them in your cold frame (covered or not) to give them a good start before transplanting.
7. Artificial Tropics
Give your tropical plants a taste of home. You can convince tomatoes and chili peppers that they live in their native tropics by growing them in a cold frame all summer. Just make sure that they are adequately watered and vented regularly, especially in extreme heat.
8. Detention Center
If your cold frame has a bottom (most don't), you can use it to grow plants outdoors that might otherwise take over your garden. Mint is a notoriously vigorous colonizer, sending rhizomes in every direction. With a garden box filled with at least 18 inches of soil, you can grow mint and other aggressive colonizers outdoors without fear. Just don't fill your cold frame with so much soil that the plant can send runners over the top of the frame and out into your garden.
9. Season Extender
Perhaps the most enticing reason for a cold frame is the possibility of eating fresh vegetables throughout the winter.
Grow cold-tolerant herbs and vegetables directly in your cold frame by starting seeds in late summer or early fall. Your plants will grow until the days get shorter and winter approaches. After their growing season ends, keep your plants moist over the winter, though do not over-water. Keep the plants out of the wind and away from direct sun (to prevent premature growth). Vent the cold frame periodically. Protect the plants with a layer of leaves or mulch. Over the winter, your veggies and herbs will stay in semi-dormancy, ready for you to harvest them until spring comes.
10. Mini Shed
When you're not growing anything in your cold frame, you can use it as a convenient place to store your gardening tools, especially those that you use all the time during the growing season.