Building and using a cold frame
A cold frame is one of several types of constructions that gardeners have used for centuries to extend the growing season (conditions) of a specific crop - and this has almost always meant a food crop. Many contemporary gardeners use variations of the familiar greenhouse - often outfitted with electricity, fans, heat, water and different methods to manage temperature.
Others use a combination greenhouse/potting shed primarily for hardening off seedlings before planting them in the spring. Still others are experimenting with hoop houses and cold frames.
(See photo at right. That cold frame probably is built on a bed of manure for heat and utilizes banked earth for insulation.)
A bit of history
The desire to enjoy foods "out of season" is nothing new. The Roman Emperor, Tiberius, is reputed to have had a greenhouse of sorts, a specularium, constructed at his villa on the Italian island of Capri so that he could enjoy cucumbers, his favorite food.
But it was the passion for citrus - lemons and oranges - that turned the controlled environment of greenhouses from practical to fashionable. Several centuries ago, the ingenuity of estate gardeners and nurserymen was stretched to develop a method for keeping large potted citrus trees alive through dark, cold northern winters.
The solution, of course, was the development of glasshouses, constructions which were close to what we now know as greenhouses. The precious citrus trees gave their name to these constructions - orangeries. Some of these give new meaning to the word, 'ostentatious'. Louis XIV, King of France had a greenhouse that, according to historical sources, had over twenty thousand square feet of floor space. Its ceiling rose over 700 feet. Czar Alexander the First of Russia maintained three greenhouses.
This desire for seasonally 'forbidden fruit' was not a European indulgence alone. A greenhouse, constructed in Boston, Massachusetts in the early 1700s was owned by Andrew Faneuil, for whom Faneuil Hall is named.
But while we here in the Black Hills are neither Kings nor Emperors, we do enjoy extending our already short growing season as much as possible (for at least two weeks in the spring and again in the fall)and of all of the above-mentioned strategies for extending the growing season, one of the easiest and least expensive to establish, use and maintain is the cold frame.
The Cold Frame
At its most simple, a cold frame is a box with openable or removeable lids and the body of the box either set totally or partially below the soil level. It is outfitted with translucent lids that open to admit sunlight (the heat source) and moving air (the ventilation source) and close to retain soil passive heat. (See photo below where the cold frame utilizes hay bales for insulation. Its 'floor' is built below ground level and the plastic 'lid' simply rolls up and down.)
Various factors will determine the effectiveness of one's cold frame. The first consideration is soil surface and subsurface. For those fortunate to have rock-free, loamy soil there is every advantage to having 6-8 inches of the box below the surface. For those who have almost no soil or an abundance of rocks, the answer is to construct the cold frame on the soil surface.
Another factor is positioning the cold frame to receive maximum sunlight. That means that the inside of the frame (where the plants are) should receive maximum sunlight. For most of us that means siting the cold frame so that the lids open on the south or southwest sides. Care and thought should be spent to be certain that the side of the cold frame that receives the prevailing winds be protected.
Draine's Cold Frame in the hills around Black Hawk
This is how we built our cold frame. The dimensions are roughly 4' deep long x 2' high at the front and 4' high at the back. We used 1" x 6" greenboard. The vertical braces (on the outside of the cold frame) were placed at each end and at half and a quarter of the length both front and back. This was to support the sides of the cold frame against the pressure of the soil. Because we built into a hill, we had to be aware of the force of gravity and the pressure that could put on the back wall.
When we sited our cold frame at our home on the east side of the hogback in Black Hawk (see photo below, right), we were able to place the back of the frame into a soil bank that would protect it from the north wind. Thus, the back of the cold frame was at the north east. The opening at west south west gives maximum sun hours and maximum protection from the north wind. We had to build on the surface of our rocky, thin soil.
Once the frame was built, (see photo below, left) the top of the cold frame was fitted with three equally sized lids of greenhouse translucent panels which we purchased at Hobby Greenhouse in Rapid City.
The lids are heavy but not unmanageable. We screwed some uprights into the back of the cold frame and hooked up a rope and pully system to raise, lower and secure the lids. In the front of the cold frame, we experimented with various pieces of board to also hold the lids up.
We built the cold frame in the fall and filled it with layers of old hay, horse and chicken manure, compost from our compost pile, and an assortment of egg shells, coffee grounds and fruit and vegetable peelings.
(This is the same soil-building principle advocated by other no-till gardeners like author Ruth Stout (See: The Ruth Stout No-Work Garden Book: Secrets of the Famous Year-Round Mulch Method and more recently Pat Lanza in Lasagna Gardening. See: http://www.lasagnagardening.com/.)
We watered the contents of our new cold frame it as though it were a garden and as long into the winter as the material would take water. We did not attach the lids that first winter so that the open cold frame would catch the snow and rain.
In the spring we discovered a glorious, rich, sweet, crumbly loam just perfect for planting. Depending on the spring weather, we can usually plant cool crops - a variety of lettuces, radishes, onions and spinach in the cold frame the first week in May. I usually put in the garlic around the middle of May. By the third week in May I set in young plants of sweet alyssum, stocks, petunias and short marigolds to attract pollinators. By the time we ate the first crop of lettuce and spinach, I slipped in some started herbs or beet seed for summer production.
We discovered that we also had to add an 'inner lid' of chicken wire to keep the family chicken flock from consuming the tender greens in the cold frame. As the seeds sprouted, we put down a mulch of straw between the rows to keep the soil cool and moist.
We have to pay attention to the weather and close the lids in the event of hard winds. Beyond that, once the cold frame was built and planted, it has required almost no maintenance or repair. We add manure to it in the fall and keep about 2" of mulch on the surface of the soil.
A cold frame is also a fine place to hold a flat of seedlings or other young plants as they harden off prior to planting. In the illustration below, the cold frame is planted and fully functioning. The short planks visible on the right side are 'steps' to avoid compressing the soil. The yellow plastic tent stakes visible in the middle section are covered with petroleum jelly to trap small flying insects that were eating radish tops.
We have grown a variety of lettuces, herbs, onions, radishes, spinach, garlic, carrots, beets, bok choi, mizuna, chives, miscellaneous salad and soup greens and low-growing flowers to attract pollinators. The cold frame is within easy distance of both water and the compost pile. It provides greens for salads and soups almost all summer. The chickens or the compost pile get the trimmings and the cold frame gets the compost made from the garden and animal waste.
We estimated that the initial cost of the cold frame was about $300 which included the use of a neighbor's Bobcat to help make the cut into the hillside. We've incurred no additional expenses in the four years we have used it. We look forward to more years of pleasure and salads from our family cold frame.
If one is considering, but not yet certain about the effectiveness of a permanent, constructed cold frame, try this (as in the photo right).It is cheap and easy - with straw bales and scrap lumber and some heavy plastic stretched over a frame. One can always use the straw for mulch!
In Black Hawk we normally have about 55-60 days of 55 degree temps at night in the summer (criteria for the germination of most seed). By utilizing the cold frame we are able to extend the growing season by 2 weeks in the spring and another two weeks in the fall. Czar Alexander and Loius XIV might not have been impressed by that, but we surely are.
Cathie Draine, Black Hawk