Cold frames can extend gardening season, start in April

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Lettuce grows best in well-drained, nutrient-rich soil, so soil should be amended accordingly before starting this cool-season vegetable in a cold frame.

By Andrea Knepper, UVM Extension Master Gardener

Get a jump start on the growing season by adding a cold frame to your garden. A cold frame allows you to start some plants two to four weeks before the recommended planting time.

Any cool-season vegetable is a suitable candidate for growing in a cold frame. Popular choices include lettuce, spinach, kale, radishes and scallions.

With the new plant hardiness zones (planthardiness.ars.usda.gov), gardeners in many locations in Vermont will be able to plant in a cold frame beginning in early April. Check the upcoming weather forecast before planting. When outside temperatures are in the 40s, you can expect the average temperature within the cold frame to be comfortably above 50 degrees F.

Prepare the soil in your cold frame with the same care and attention as you would for other garden beds. Lettuce, in particular, prefers well-drained, nutrient-rich soil, so make sure to amend your soil accordingly.

Plant your vegetables according to the spacing and depth recommendations on your seed packets. Leafy greens like lettuce, spinach and kale can be densely seeded and harvested before reaching full maturity. Refer to the seed packet for additional care and growing instructions. Remember to water your plants in the cold frame whenever the soil feels dry.

Monitoring temperature and airflow within the cold frame is essential. Temperatures can quickly rise too high for cool-season vegetables, so close attention is necessary. Prop open the lid of the cold frame to regulate temperature and promote airflow. A small opening of just a couple of inches can be enough. Be sure to close the lid in the evening to protect your vegetables from overnight cold temperatures.

Early spring isn’t the only time a cold frame is useful. Cold frames also can be used for hardening off warm-season vegetables such as tomatoes, eggplants and peppers that you have started indoors. Transplants purchased from a local greenhouse or garden center are typically hardened off before sale although these can be placed in a cold frame for a week or longer before planting in the garden.

While you might need to bring your plants indoors overnight during particularly cold spells, often simply closing the lid on the cold frame is sufficient. Keep an eye on the weather forecast when using your cold frame for hardening off plants.

Fall presents another opportunity to benefit from a cold frame. You can plant cool-season vegetables in the cold frame toward the end of the season.

This can extend your growing season by two weeks or more beyond the first frost. Follow the same process to enjoy a late harvest of leafy vegetables, scallions and radishes well into the fall.