Cold-Frame Savvy

Seasoned Gardener : Episode SGN-303 -- More Projects »
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Portable cold frames can go anywhere. They are lightweight and often less expensive, but they may not insulate as well.
A cold frame is a mini greenhouse that uses the warmth of the sun to create a comfortable growing environment when the outside air temperature is chilly. The moderated temperatures allow plants to grow normally in the soil. Built or placed on top of an existing garden bed, this cozy garden space can extend the growing season by weeks in the colder zones and by months in the warmer zones.

Cold frames come in a variety of sizes and shapes--small enough to fit compactly over individual plants or large enough to accommodate many plants. They also vary in price. Most cold frames can be built from kits that are ordered through catalogs, the Internet or your local garden center. You can easily construct a cold frame using 2x4s and an acrylic or plexi-glass panel or even an old window for the top.

Choose a location that's convenient to your house because you're more likely to maintain the cold frame garden if you don't have to trudge far in the cold weather to get what you've planted. Ideally, you should place the cold frame facing south to catch the most sunlight. Also, be sure the soil has good drainage; plants that sit in water can freeze or rot.

Once you've found an optimum spot, the first thing to do is prepare the soil.

  • Strip all sod in the area. Be sure to remove all grass and root pieces because new grass can grow from these leftovers in the soil. If the sod is difficult to remove, wet the area down and allow it to sit overnight.
  • Break up the soil to a depth of 8 to 10 inches. It's hard work, but it makes all the difference.
  • Amend the soil by adding compost (mushroom, manure or garden) and peat moss. This improves the structure, fertility and drainage of the soil. Add one to two inches of compost evenly forked into the top few inches of the soil to create a great growing medium.

    With the soil prepared, it's time to build the cold frame. In this project, Chris Dawson is using an old window that measures 31 by 32 inches. Dawson suggests that you get your lumber precut at a home improvement center to save time. Materials needed for the cold frame area are:

  • 2x8 red cedar or redwood cut to the dimensions of the window or panel being used
  • 6 feet of 2x2s for the frame supports
  • 2-inch galvanized screws (which won't be harmed by cold weather)
  • wood glue
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    Place 2x2s into the corners of the frame base to stabilize the frame.
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    The top of the box works best when slanted. Cut triangles that slant from front to back to make a frame for the top panel.
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    Create a prop with leftover wood. Screw together various lengths of wood to create several heights at which the top panel can be propped open to allow for air circulation.
    To build the base of the frame:
  • Line up the cut 2x8s at 90-degree angles to create the corners. Use glue between all pieces of wood as you build. Screw the 2x8s to each other and to the 2x2s. The back of the frame will be three 2x8s tall and the front will be two 2x8s tall.
  • Secure the triangle pieces of wood to the sides of base with glue and screws.
  • You can add a chain to the inside of the panel and frame to hold it open while you are working in the cold frame. Or just use a leftover piece of wood or a stick to prop the top panel open.
  • Pack soil against the bottom of the frame, on the inside and outside, to ensure a good insulation seal.

    Although the cold frame creates a warmer habitat for plants, you'll still need to choose hardy cold weather crops. In Zones 6 through 11, you have a longer growing season and a bigger selection of plants that grow successfully. In colder zones, you'll have better success with quick growers like lettuces, baby spinach, arugula and radishes.

    To plants the seeds, simply sprinkle them over an area and then lightly rake soil over the seeds. When the seeds have good seed-to-soil contact, they will germinate. Don't bury them too deeply, or they won't grow. Water them gently with a sweeping motion to avoid flooding the seeds and causing them to flow into a corner of the frame.

    When everything is planted, there are a few things you need to do to maintain your cold frame:

  • Check on the cold frame daily to be sure it's not too hot or too cold inside. On very sunny days, prop the lid of the frame up, but be diligent about closing it at night. Keep an eye on your plants. If they look like they're getting droopy or wilted, the environment is too hot for them. You'd be surprised how hot it can get inside the cold frame on a sunny day.
  • When the outside temperatures start getting really cold at night, wrap burlap around the box to give it extra insulation. Don't cover the top panel.
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    In colder zones, builder's foam adds extra insulation.
  • For extra insulation at night, you can use an old blanket or piece of carpet on top of the frame panel. Be sure to take it off during the day--the plants vitally need the sunlight.
  • Check the soil moisture level daily. Make sure the soil is moist or the seeds won't germinate. Try to water in the morning so the plants have a chance to dry before dark, which will help prevent disease.
    • Also in this Episode
    Seasoned Gardener : Episode SGN-303
    •Assessing the Raised Bed in Fall
    •Getting a Jump on Spring
    •Cold-Frame Savvy