by Bonnie Kirn Donahue, Extension Master Gardener, University of Vermont
NORTHFIELD – Utilizing cold frames in early spring is a great way to jump start the vegetable growing season.
Cold frames are wood boxes with slanted, transparent glass or plastic tops that are placed directly on top of the soil in your garden. These boxes allow the sun to filter in, warming the air in the box. This creates a greenhouse-like environment with moist air and warm soil that encourages plant growth.
As the days get warmer, there is a risk that the cold frames could get too hot and damage your plants. To prevent this, the lid can be propped open to help moderate the air temperature. You can keep a thermometer in the box to track the temperature.
If you are germinating seeds (especially warm season crops like tomatoes, peppers and zucchini), try to keep the air temperature between 70 to 75 degrees Fahrenheit during the day. Cool season crops (like lettuce, spinach and peas) prefer air temperatures to be lower, about 50 to 60 degrees Fahrenheit.
Fresh, leafy vegetables like leaf lettuce, spinach, kale, arugula and Swiss chard can be easily grown in cold frames. Seeds can be planted directly into the ground, or in trays set on the soil surface. If your goal is to grow full heads of lettuce or kale in your cold frame, you may not have as much room as you would if starting seedlings in the cold frame.
For seed and plant spacing, check the seed packets or websites. I like planting lettuce seeds densely in rows so that I can eat what I thin out. This also can be done for other leafy greens such as spinach and kale. While it takes patience to thin out and wash the tiny plants, young greens are a delicacy.
Radishes and scallions also are great vegetables to start in cold frames. Simply follow the spacing instructions in the seed packets or plant them in trays. Thinning the plants over time can provide snacks for you. Radishes may not appreciate being transplanted, so consider this if you plan to grow them in containers.
Just like in summer, make sure to water the plants when the soil is dry. You can test this by checking whether the soil is moist at a depth of one-fourth inch under the surface. If it is moist, you won’t need to water. If it is dry, it is time to water.
While seed packets may say that leaf lettuce or greens take 50 days or more to mature, if you are happy eating smaller plants, you will have greens to eat much sooner. The same goes for scallions. Radishes will be ready to harvest in as soon as three weeks.
Cold frames can be made out of scraps that you may have lying around, including pieces of lumber, old windows (make sure the glass and finishes are lead-free) or a roll of see-through plastic. Check out this resource for tips on how to build your own cold frame: go.uvm.edu/cold-frames.
If you’re excited about getting your garden started this year, give cold frames a try, and soon you’ll be eating fresh vegetables out of your garden.
[Bonnie Kirn Donahue is a UVM Extension Master Gardener and landscape architect from central Vermont.]