A cold frame is often employed to protect summer vegetables and extend their harvest season into the fall; however, these mini unheated greenhouses can also be used for an early kick off to the spring garden season. Cold frames warm the soil and air by 7 to 10 degrees, meaning you can start spring vegetables about four weeks earlier.
The key to jumpstarting spring with cold frames is to group crops: Separate heat-loving crops from cool-season ones and directly sown seeds from seeds to be started indoors. Once you’re organized, you can manage your seed-starting needs more effectively. Cold frames allow us to sow cold-hardy crops sooner, and plant both cold-loving and hot-loving seedlings earlier. Here are three ways to hit the ground running this spring.
1. Start Cool-Season Crops
Direct sow cold-hardy vegetable seeds four weeks earlier than their anticipated date inside the cold frame. Some cold-loving vegetables that can be direct sown include:
- beets
- broccoli
- Brussels sprouts
- carrots
- kale
- kohlrabi
- lettuce
- peas
- radishes
- spinach
- turnips
The cold frame will be the permanent home of the crops you plant inside, so choose seeds and placement wisely. For cool-season vegetables, close the cold frame when the temperature dips below 32 degrees F. Two weeks before your last-frost date, your cool-season vegetables will be able to survive on their own. Disassemble your cold frame and store it until fall.
2. Transplant Cold-Hardy Seedlings
Cool-season crops that you start indoors can also benefit from an early start thanks to cold frames. Simply start your plants indoors under grow lights four weeks earlier than you normally would, then move them to the cold frame to be hardened off.
Hardening off is the one-week process by which seedlings started indoors are acclimated to the outdoor elements, including temperature fluctuations, wind, direct sunlight and cold nights. To harden off seedlings in a cold frame, you’ll need to use a thermometer to monitor the temperature inside. Move the seedlings in their trays or pots to a closed cold frame for two days. For the next four days, crack the opening during the day if the inner temperature is in the 60s or above, but close the lid at night. At the end of the week, the seedlings are ready to be planted. After a week inside the cold frame, they’re ready to be planted.
Plant the seedlings inside the cold frame. This will be their permanent bed for the season, so again, choose your plants and cold-frame placement wisely. Keep the cold frame vented most of the time, closing it when temperatures dip below freezing. Two weeks before your last-frost date, your cool-season vegetables will be able to survive on their own outdoors. Disassemble the cold frame and store it until fall.
Some cold-loving vegetables that can be planted as seedlings include:
- broccoli
- Brussels sprouts
- cabbage
- collards
- kale
- lettuce
- onions
3. Transplant Heat-Hardy Seedlings
Heat-loving vegetables, like eggplant, okra, peppers and tomatoes, can be started indoors three weeks earlier thanks to a cold frame.
On the same date that you start your seeds indoors, warm up the garden bed where you will permanently plant the seedlings by covering it with a cold frame, keeping the lid closed and laying an inner protective layer on the bed. Set a thermometer in a shady spot inside the cold frame and monitor the temperature: When it maintains 70 degrees F, it’s time to plant your seedlings. Follow the hardening-off procedure listed above before transplanting the seedlings into the cold-frame bed.
Be aware that the temperature in the cold frame will affect the health of heat-loving seedlings. The ideal temperature for heat-hardy vegetables is around 80 degrees F, but on a sunny day, the temperature inside the cold frame can reach 100 degrees F, so be sure to vent it appropriately. Conversely, keep the cold frame closed when the temperature dips below 65 degrees F: Cold winds will stress or kill the tender vegetables.
Your heat-loving seedlings will be able to survive on their own two weeks after your last-frost date. This is a good time to disassemble your cold frame and store it until the fall.
General Tips for Cold Frame Success
Because you’re altering the garden environment by using a cold frame, it’s important you pay attention to your new starts to ensure they grow healthily. Here are some tips you can keep in mind.
- Water: Soil inside the cold frame should always remain damp. Don’t allow it to get too dry.
- Ventilate: Ventilation is required to remove moisture and prevent disease, as well as regulate temperature. If it isn’t possible to monitor the temperature daily, consider installing an automatic vent opener, which will automatically open the cold frame when temperatures rise to between 55 and 75 degrees F.
- Bundle Up: If the temperature is expected to fall below 33 degrees F, you may need to add blankets on top of and around the cold frame may be needed to protect the seedlings. According to Eliot Coleman’s The Winter Harvest Handbook: “Double coverage moves the covered area about three USDA zones to the ‘south’.”
- Use Additional Season Extenders: Row covers and cloches can warm the soil an additional 2 to 4 degrees F. When used in conjunction with cold frames, vegetables are well protected down to 23 degrees F! Each additional layer will increase frost protection.
