Turn your raised bed into an instant cold frame by adding a lid.

Gardeners with raised beds in which to grow vegetables seem to have all the luck. After all, raised beds are easier to tend, drain more quickly, and offer many other benefits. One additional benefit you may not have thought of is to turn your raised bed into a cold frame.

What is a cold frame? A cold frame is a removable, glass or plastic-covered frame that fits over the ground or over a raised bed. It protects plants from the elements and keeps them warmer inside the frame than in the surrounding areas using only natural heat sources.

By turning your raised bed vegetable garden into a cold frame, you can extend the growing season well into the fall and depending on your gardening zone, even into the winter months. Many gardeners find that with a cold frame they can continue growing lettuce, radishes, spinach and other cool-season crops throughout the winter.

Glass Tops for Cold Frames

The easiest way to turn a raised bed into a cold frame is by using recycled storm windows, doors or other glass windows placed on a frame over the cold frame bed to create a miniature greenhouse. You can find old windows at salvage yards or junk shops. If you see someone having new windows put onto their home, stop by and ask if you can have the homeowner’s permission to take one of the old ones. You never know when they’ll say “yes” and then you have an abundance of recycled storm windows to use for your project! (But always ask for permission first).

It’s simple to add a glass topped cold frame lid to a raised bed.

• Find a salvaged window that is the same width as your current cold frame. If you can’t find one exactly the same size, one that’s slightly larger works better than one slightly smaller. The larger size keeps the air inside and prevents drafts from reaching plants.
• Wood-framed glass windows or doors make the easiest cold frame lid to work with. The wooden frame provides ample surface on which to affix hinges.
• Simply drill a hole into the window frame and the edge of your raised bed to match the holes on a hinge. Add a large hinge or two smaller ones to the edge. If it’s easier, you may also want to add a handle, similar to the kind sold in the kitchen design department at your local hardware store, to the opposite end to easily raise and lower the lid.
• Once the hinges are affixed and the window is down, voila - instant cold frame.

When choosing salvaged windows or doors for your cold frame lid, make sure that the surface area has small panes rather than one large sheet of glass. If you do have a large sheet of glass set in a door frame, make sure it is the kind that’s shatterproof, or reinforce it underneath. Snow and ice are heavy, and if they sit on top of a flat frame, can crack the glass easily.

Clear Acrylic Panels

Clear acrylic panels can also be used to form mini greenhouses over raised beds, but the same caveat about support applies. Acrylic sheets that are too large easily bow or bend in the middle. It’s better to make several smaller panels and frame them with wood, screwing them via hinges to the raised bed sides rather than trying to make one big panel. Not only does this give them better support, it also enables you greater control and customization over the temperature inside the cold frame because you can raise or lower specific windows.

• If you don’t have a wooden frame, have the hardware store cut a sheet of clear acrylic to the size you need. You can add a wooden frame around the acrylic, and then use the wood to secure hinges to the frame and the raised bed frame.
• Bricks or rocks can also be used for clear sheets of acrylic placed over the top of the raised bed frame without a hinged lid. This keeps them in place. Acrylic is so lightweight that a strong window billows it up, acting like a sail and sending the lid soaring over the yard. Bricks hold it in place.

Depending on the depth of your raised bed, you may find that just placing a window over the top of the bed is enough. Others may find that the plants growing inside the raised bed need more room to grow. In that case, you’ll need to build out a temporary frame to prop the window up and away from the raised bed. It’s like placing a cold frame over an in-ground garden bed.

Tips for Using Cold Frames

Cold frames can be a blessing or a curse, depending on the winter weather and what you’re trying to get out of them. It may sound like a great idea to grow vegetables during the winter, but trudging through a heavy crust of snow and ice to check and harvest your vegetables isn’t much fun. Think about plowing or shoveling a walkway to your raised bed so that you can tend it easily during the worst winter weather.

Be sure to plan ahead for how you will manage snow and ice accumulating on the top of your cold frame, too. The warmer temperatures inside the cold frame should help snow and ice to melt off the glass or acrylic surface, but you may need to help Mother Nature along by brushing or removing snow from the lid by hand. If you don’t, several problems can arise. Plants may not get adequate light. The glass or acrylic can crack or bow under the weight of the snow. Make sure you check on your cold frames after a really heavy snowfall.

Keep a brick or stick handy to prop open the window on hot days. If you don’t prop the lid open on your cold frame, the interior may get so hot it actually ‘cooks’ the plants - even on cold days! The angle of the sun’s rays can be surprisingly strong even in the depths of winter.

Cold frames keep the air and soil underneath free from snow and ice - but that also means little water gets to the plants. You’ll still need to water your plants, especially if you’re growing vegetables and other edibles during the winter months.

A cold frame can extend the growing season and keep gardeners from going “snow crazy” during the long winter months when there’s nothing but houseplants to tend to. Although they are a bit of work, if you love growing vegetables at home, adding a lid to your raised bed can turn it into a cold frame quickly and easily.