Spring has arrived in all its temptation for vegetable gardeners eager to get started, but for some, this is harvest time. The dead of winter was the height of the growing season.
These are the cold-frame gardeners, who create miniature climates to reap arctic rewards. It`s not easy, but with a combination of hotbeds and cold frames, winter vegetable gardening becomes a challenging possibility.
With warmer weather comes the time to build those two key components, which can be made with simple materials, or they can be a bit more elaborate in design and setup.
A cold frame is designed to shield plant material from the cold, especially the wind. Usually it is made of wood, though stone, concrete and bricks can be used. A minimum size is 3 by 6 feet, but if you`re an avid gardener, you may want to go larger. Anything smaller isn`t practical.
Select a site, preferably on a well-drained slope, that would provide protection from wind. To maximize the sun, try building the frame so it has a southern exposure. A free-standing cold frame can be used, but the ideal location is against a building, especially near electrical and water sources. Plan to have the frame at least 15 inches high in back, sloping to 9 inches high in front. With these proportions, ice and snow will be less likely to build up and cut off solar heating.
If wood is being used for the frame, make sure the boards are 1 to 2 inches thick. To provide stability, nail the box to 2-by-4s driven deep into the ground. For a brick, stone or concrete cold frame, make sure the footings or foundation are several inches below the frost line, which runs about 3 feet in the Chicago area.
Once you`ve selected a location and materials, the next decision is how to build a cover, also known as a sash. It doesn`t have to be fancy, but it should provide protection so water doesn`t drip into the frame. If you don`t feel like building a sash, old windows, storm doors or storm windows can be substituted.
If you make your own cover, a thin layer of clear polyethylene film
(about 6 to 8 mils thick) attached to a light wooden frame is easy to build and generally effective. A little wire mesh can be used to support the film. This can be especially important to prevent the cover from collapsing under the weight of snow. No matter what kind of cover you make, be sure it is hinged so it can be opened for ventilating.
Heating the frame
Cold frames commonly are built over a hotbed, which is a naturally or electrically heated garden plot.
If you are a traditional gardener, you may want build the cold frame over a 6- to 8-inch pit of fresh manure. The effect is similar to that found in a compost pile because as the manure decomposes, heat is released. Horse manure mixed with one-third straw is the best method, though any type of manure will work.
Tradition is hard work. If you want a manure-heated hotbed, you will need to dig a hole 2 1/2 feet deep and at least a food wider and longer than the frame. Before placing the manure in the ground, you should place at least a 4- inch layer of gravel at the bottom to promote drainage. The next layer is the manure, topped by a 3- to 4-inch layer of soil.
A manure-heated hotbed has the distinct advantage of being low in cost, assuming you can find a source of manure, but there are several drawbacks to this method. A chief disadvantage is that it is messy. Also, as the season progresses, it becomes more and more difficult to maintain a constant temperature, which ultimately results in the manure-heated bed degenerating into little more than a cold frame.
The easiest method of heating a hotbed is with electricity. As is the case with a manure-heated hotbed, start with a fairly substantial pit. Unlike the 30 inches needed for the manure-heated hotbed, you need only go to a depth of 12 to 18 inches.
The first layer is an inch of carefully spread sand. Make sure it is level before putting the electrical cables in place.
Several types of cables can be used, such as the plastic-covered kind. Cables are sold at garden centers and come with instructions. But a word of caution: For safety`s sake, never splice cables together.
Selection of cables should take into consideration the length and wattage per square foot. Some cables are 60 feet long and rated at 6.7 watts per foot, and others may be 120 feet long and rated at 800 watts, which also calculates to 6.7 watts per foot. As a point of reference, a 6-by-6-foot hotbed generally requires 60 feet of cable.
The cable should be laid in serpentine fashion, creating U shapes back and forth with 8 inches of space between the lines in each loop. Note, however, that the outer cable should be 4 inches away from the walls of the frame.
Once the cable is in place, cover it with a 2-inch layer of sand and place a half-inch mesh hardware cloth over the sand to prevent damage to the cable when you work in the bed. If you are planting seeds or simply transplanting directly into the hotbed, place about 6 inches of soil on top of the mesh cloth. This will greatly reduce the possibility of cable damage.
Temperature control
Controlling the temperature in a cold-frame-hotbed combination is essential. The thermostat should have an operating range of 45 to 85 degrees. Put the sensor above the cable but not in contact with it. You may want to install two thermostats: one for air temperature, a second for soil temperature.
In a manure-heated bed, several materials can be used to control temperature. In extreme cold, crumbled newspaper, straw or hay helps insulate the frame. Tarps and blankets also work.
An ideal soil temperature is 60 to 70 degrees when seeds are germinating. Once seedlings emerge, the temperatures can be adjusted, depending on the crop planted. It`s best to use cool-season root or leafy crops. They require less light and can withstand cooler temperatures. Cold-season crops, which include cabbage, lettuce and cauliflower, can tolerate temperatures of 60 to 65 degrees.
The air temperature in a frame fluctuates, and it`s not unusual for it to reach 85 degrees. Monitor the temperature as closely as possible, and if the weather is unusually mild, as it has been this year, prop the frame cover open to avoid overheating.
Because the bed is enclosed, it will receive no rain. Like any garden, it needs water, sometimes daily. Watering should be done early in the day. If done too late, plant leaves may not dry off before nightfall, when
temperatures usually are at their coldest.
Vegetables should be planted before mid-December, or the harvest will run too far into warmer weather, when the cold frame would get too hot.
It might seem like a major effort, but using a cold frame or hotbed not only extends the growing season, but it offers the home gardener a new perspective to gardening.
