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Coldfames (not mine)

A COLDFRAME IS WARM, UNLESS IT’S…

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A Fourth Dimension for the Garden

A “coldframe” is one of a few ways to add a new dimension — time — to gardening. Especially, for me, vegetable gardening. It inserts time where time does not exist. Instead of my gardening season screeching to a stop with a hard freeze sometime in late November or early December, a coldframe extends it a few weeks, possibly even more. And it can do the same thing at the beginning of the season, getting plants going and harvested sooner. (Multidimensional vegetable gardening is covered more thoroughly in my book Weedless Gardening.)

At its most primitive, a coldframe is nothing more than a clear plastic or glass topped box set directly on the ground, functioning in the garden as a miniature greenhouse. My simplest coldframe was made from four pieces of scrap pine boards notched together into a three-foot by six-foot rectangle. The covering was quarter-inch plexiglass whose previous incarnation was that of a floor runner beneath office chairs.Primitive coldframe

My most elaborate coldframe was a purchased structure, looking much like a miniature barn with a double-wall, polycarbonate plastic roof that folded open or closed along tracks in the eaves. Read more

FRESH-PICKED SALADS ALL WINTER

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Greenhouse, Coldframe, Hotbed = Fresh Lettuce, Mâche, & other Salad Fixings 

You can’t beat the luxuriousness of entering a greenhouse on a sunny, cold winter day, and hitting that welcome wall of moist, warm air. Once you get through that soft wall, you drink in the redolence and visual vibrancy of green, growing plants. All this is possible even with a relatively inexpensive greenhouse, such as mine, which is, basically, 2 layers of plastic film supported by sturdy, steel hoops (plus thermostats, a propane heater, a cooling fan, and a “squirrel cage” fan to inflate the space between the layers of plastic for better insulating value).

Fresh greens in the greenhouse

Fresh greens in the greenhouse

Still, a greenhouse — my greenhouse, at least — isn’t for virtual trips to Puerto Rico; It’s for growing edible plants, mostly fresh lettuce, celery, mâche, claytonia, and arugula to fill the salad bowl every day from late fall through early spring.

This same benefit, though, can almost be achieved much more simply and much more cheaply with a coldframe. A coldframe, in its most basic incarnation, is a box frame covered with glass or some other transparent material and within which plants are grown or protected through the coldest months.

I have both a greenhouse and a coldframe. The coldframe came first. With a greenhouse full of greenery, I admit to usually neglecting the coldframe, which is unheated and only 25 square feet in contrast to the greenhouse’s heated 750 square feet. Still, last fall I had an extra 25 lettuce transplants and lacked even a square foot of extra space in which to plant them in the greenhouse. So I spread an inch of compost in the coldframe, firmed the transplants into holes there, and then gave the whole bed a good watering before replacing the cover.

Cold frame with cover closed, and Sammy

Cold frame with cover closed, and Sammy

Except for one subsequent watering, I’ve pretty much ignored the coldframe. The polycarbonate (‘Exolite’) cover is translucent so the coldframe’s innards are hidden from view unless the cover is lifted. Today, mostly out of curiosity, I lifted the cover to see what was going on inside. The lettuce was alive and had grown. Fall weather was admittedly relatively mild although temperatures here did drop below 10°F at least one night — pretty good survival for plants growing in little more than a covered box outdoors.

With short days and cold weather, lettuce is growing very slowly. Individual leaves, perhaps whole heads, probably will not ready for harvest until towards the end of February. That’s if the lettuce survives that long, which it might not even in the coldframe, depending on the length and depth of winter cold. Spinach would more likely make it through winter; mâche and claytonia would definitely weather the coldest days and nights.

Beefing Up the Coldframe

In the absence of a greenhouse, I would beef up coldframe salad production to yield more  and for more of the winter. I would, first of all, construct more coldframes because, with slow growth inside in winter, one harvest could decimate the crop, for a period at least, from just one coldframe.

Anything that helps the coldframe fend off cold and retain heat would also help increase the amount and duration of harvest. Insulating the sides helps, and said insulation need be nothing more than wood chips or straw piled up against the sides. Or make the box itself out of whole bales of straw!

Cover open, Sammy on guard

Cover open, Sammy on guard

Heat rises and glass and plastic are poor insulators, so most of the heat is lost through the translucent or clear covering. The Exolite cover on my coldframe is actually 2 layers of polycarbonate plastic sandwiching polycarbonate ribs that hold the layers 1/4” apart, proving better insulating value than a single layer. On really cold nights, an insulating mat, even just a blanket could be thrown over the cover. In Europe a hundred years ago, cold frames were used extensively for commercial vegetable production, and mats of woven straw covered the glass to keep out the coldest weather.

A few feet down into the earth, temperatures remain constantly in the 50s. That earth is a source of heat, tapped into by sinking the floor of the coldframe a couple of feet deeper, which also allows head space for taller vegetables, such as Romaine lettuce. Too deep, of course, makes harvesting very inconvenient.

And finally, winter production can be beefed up by actually heating a coldframe, in which case it’s no longer a “coldframe” but what’s called a “hotbed.” The traditional heat source for a hotbed is horse manure, layered into the bottom of the bed after a foot or so of soil has been dug out. A certain art is involved in getting the right amount and moisture content of manure, packed in just right and then covered with soil for a steady even heat. I made my coldframe into a hot bed a couple of years ago, or, rather, tried to. I didn’t get it just right; will have to try again.

True, you can’t climb into a coldframe for a tropical junket, but the coldframe, with care, can become a hotbed of tasty, fresh, organic, very locally-grown vegetables from fall to spring.

It Ain’t Over — The Fall Garden Begins

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If I wasn’t a gardener, I’d look upon the late summer and fall weather as a glorious succession of warm sunny days and crisp nights with intermittent periods of mostly gentle rains. As a gardener, the crisp nights make me a little nervous.
Temperatures so far have only dipped into the low 40s; any night, though, that low could plummet below freezing. Below freezing temperatures would ring the death-knell for okra, peppers, tomatoes, and the any other summer vegetables still braving cooling temperatures. 
Actually, there’s not much oomph left in the okra and tomatoes. Okra doesn’t bear pods except when temperatures are downright hot. And disease, mostly early blight (not the dreaded late blight of a few years ago) has reduced tomatoes to nothing more than bare stems capped by a few green leaves and fruits. So loss of tomatoes or okra would hardly be noticed except to indicate it was finally time to clear those beds and ready them for next spring.
Peppers are another story. The leaves are still green and spry, and cool weather hurries the fruits along on their way to full red ripeness. 
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With cooler and cooler weather descending on the garden, the race is on with cool-weather vegetables — cauliflower, broccoli, cabbage, turnips, and winter radishes. These vegetables tolerate temperatures well below freezing. But will they do what they have to do, whether it’s making flower buds, heads, or roots, before their growth slows almost to a standstill? Further south, they could wait out winter and finish growing as weather warmed. This far north, they eventually succumb to cold, perhaps headless, flower bud-less, or fat root-less.
I seeded cauliflower, broccoli, and cabbage in early June and transplanted them a month later. Turnips and winter radish seeds went directly into the ground in early August. According to my records, they should all be on time. But roots are only just beginning to swell in the ground beneath the turnips and radishes, and there’s no sign yet of flower buds on broccoli and cauliflower or developing heads on cabbage. Which makes me nervous.
Kale, of course, is yielding abundantly. It enjoys both warm and cool weather. All it needs to do is keep growing leaves, which it does.
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No need to leave everything to Mother Nature; I’m going to change the weather. On a very small scale, within my cold frame. For years, I’ve used my cold frame — essentially a 5 foot by 5 foot wooden box with a hinged, clear top sloping southward — to extend the season for fresh salad fixings. This year it has become a hotbed, which is a cold frame with warmed soil.
Soil can be warmed in a number of ways. Most modern is with an electric soil heating cable made for this purpose and woven back and forth beneath the ground. Most old-fashioned — and the method I’m using — is with manure, utilizing the heat of its fermentation.
Successfully growing plants in a manure-heated hotbed demands a mix of art and science. Too much heat, too fast is to be avoided as is, of course, too little heat. To start, I and my able assistant David dug almost 2 feet of soil out of the cold frame. I had on hand horse manure mixed with wood shavings and a little straw; the mix was fairly fresh and moist. As David forked the manure mix into the bed, I watered it and occasionally got right into the frame and stomped it down — not too hard, which would drive out all the air, but enough to pack everything together for a critical mass to start heating.
Once the bed was filled, the mix was topped with a 4 inch layer of ripe compost. Those few inches are needed to keep young, tender roots above the hot layer which, as it eventually cools, can increasingly accommodate roots. A few days later I scratched out furrows into which I sprinkled lettuce, spinach, and arugula seeds. Spinach usually survives winter here in a cold frame. Lettuce and arugula do not, so will provide a good measure, along with speed of growth, of the efficacy of the heating. 
One week after planting, temperatures about a foot down into the bed are around 80°F.
I’m lucky in not having to leave things to Mother Nature even if the hotbed’s performance proves less than stellar. A few steps away is my greenhouse, now sprouting a panoply of cool weather greens, as well as ripening cucumbers and figs.