I arrived at around 10:30 a.m. on a day in late February. The previous night's snow had already been swept off the top, and the sashes were still closed. Even though the temperature was 28 degrees, the morning sun was causing condensation to collect on the underside of the glass-covered sashes.
The owner was a professional organ player named Paul Ladd. He and his wife had raised 12 children on veggies grown in a French-intensive, raised-bed garden the size of a soccer field, located on the other side of the house. As we walked out onto the patio containing the cold frame, Paul picked up several short pieces of 2-by-4 framing lumber and propped up two of the six sashes so hot air could escape. He did this not because I was there but because his cold frame needed to be vented. Hot air was accumulating inside this season-extension contraption, and he knew exactly what he needed to do to release it without losing too much heat.
Have you ever gone through a winter heating with a wood stove? When the cold weather first arrives, you don't remember precisely what it takes to make that stove burn through the night, but by midwinter you've figured out how to pack the firebox and what you need to do to provide enough air so it will burn smoothly while you sleep. By late December, you've developed an intimate relationship with your wood-burning stove.
That's exactly the sort of relationship Paul Ladd had with his cold frame. For the previous 35 or so winters, Paul and his wife had grown Burpee's 'Green Ice' lettuce through the coldest months. He didn't grow anything else in it, and he'd figured out exactly what to do because he was working in close partnership with an inanimate garden appliance that he'd come to know so well.
Recently, a gardening pal and I installed a cold frame in her garden. It's a modest one, made from two 28-by-28-inch double-hung-window sashes hinged at the top of a bottomless box that sits on the ground. (For construction details, see below.) We amended the soil heavily with good stuff from the worm bin and a bag of mushroom compost. Although the growing surface is only 28 by 56 inches, that's plenty of room for growing lettuce when temperatures fall.
It won't supply all of these folks' salad needs, but it will provide nice flushes of lettuce as seedlings and seeds planted in succession mature. That lettuce will be an extra treat, because the varieties have been chosen for their exceptional beauty and flavor as well as their ability to thrive in cold weather. And come next March, the cold frame will hold five professional-size seedling flats to get things started for the spring garden.
Growing food in a cold frame in winter is not for someone with absolutely no gardening skills. But if you have a modest amount of experience in growing food, learning to use a cold frame is like learning to ride a bike -- you get the basics figured out early and then just sort out the details over time. If you're checking on your cold frame every day, it won't take you long to discover how to help it consistently produce what you want.
Provide outstanding soil in your frame, and don't compact the surface by stepping on it. Plant it the way you would any garden space, and just pay attention to your plants. Hey, you're a gardener -- and you'll know what your plants need by watching them.
How to make a simple cold frame
A cold frame can be an effective and versatile tool for the home gardener, and a simple one is not hard to build.
There are different ways to make the sashes (i.e., the translucent top that's hinged to the bottomless frame "box" below). Old wooden double-hung windows (which can be found in many sizes) are great. And those old-fashioned wood-frame storm windows are awesome. They're generally about 32 by 60 inches -- the perfect size (if you can score three of them) to make a 5-foot-by-8-foot covered growing surface.
Old window sashes are pretty easy to find. I reinforce the corners by wrapping an inexpensive framing plate around each one and screwing it to the frame. Once that's done, I break out the glass and replace it with plastic, stapled to the frame. Glass has a long tradition in season extension, but here's the rub -- it's not a matter of IF a pet, or a child, or a slightly inebriated adult will sit or step on the sloping top of a cold frame ... it's simply a matter of when.
For that reason, it's just a bad idea to use glass, especially when there are other materials -- 6-mil construction-grade plastic, corrugated roofing panels, hard plastic sheets salvaged from elsewhere, to name a few -- that are safe and provide equal protection for your plants. These other materials also make for a more moderate climate inside the frame than glass does. Glass frames overheat more quickly and thus require babysitting the vent system all day, a luxury most folks can't afford. The sashes on the frame we just made were covered with UV-resistant greenhouse plastic, with a strip of floating row cover at the top to allow excess heat to escape from the frame while the family is away each day.
You can size a bottomless box made of 2-by-10 or 2-by-12 framing lumber by cutting the front and back pieces to the width of however many sashes you're using plus a half-inch between sashes to allow for clearance. Cut the sides to the length of the sashes minus 3 inches (to allow for the thickness of the front and back). Don't worry about cutting the side pieces on an angle to achieve the characteristic slope for your cold frame. Just nail the sides, sandwiched between the front and back pieces. Attach a pair of strap hinges to the top of each sash, then connect them to the back of the frame and you're ready to go. When you put the box in the ground, slope it to the south a bit (burying the front edge a couple of inches deep) to get more of that low winter sun on the growing surface.
It's also really easy to make custom sashes to accommodate any size frame you wish. If you possess a modest amount of carpentry skill, you can find directions for making custom sashes in my out-of-print book, The 12-Month Gardener, which can generally be found for less than five bucks at www.abe.com.
Don't use toxic, pressure-treated lumber for the frame: Remember, you're growing food here. For your first frame, you should use spruce, hemlock or whatever is cheap and untreated. If you're just setting the frame on the ground, you can place a sacrificial strip of cedar along the bottom edge to increase its life span.
We made our frame out of hemlock, and it was set in the ground because that's what worked for this particular site adjacent to the house. Because it's buried, this frame will rot past redemption in three or four years. But by then the owners will probably want a bigger one anyway, and they can make it out of a rot-resistant wood (such as cedar or cypress), guided by a deeper understanding of how they want their cold frame to function.
Studies show that your plants will do better if you paint the inside of the frame white (I use white exterior primer), because it reflects more light onto the plants.
[Jeff Ashton lives in Weaverville.]