Lessons Learned From a Clunky Cold Frame

Spring can be frustrating in Missouri. We are far enough south that we regularly enjoy warm, southerly breezes in January, February and March. By early March these balmy episodes begin to awaken the urge in many of us to plant things in the garden. However, since we know that there is a about a 1% probability that we will get through March without our Alabama-like weather turning Alberta-like, we tend to stifle our gardening instincts. Those with particularly irresistible gardening urges and some fiscal resources may build a greenhouse or sun porch. A grow light in the basement is a simpler, but still effective tool to allow one to gain the therapeutic benefits of working with plants well before the soil outside has thawed for good. However, for some, there is simply no effective substitute for getting one’s fingers in the garden soil. For these, a cold frame may be the best treatment to relieve stress caused by gardening-urge suppression.

There is a wealth of information available on the construction and operation of cold frames and hotbeds. An excellent example is MU guide G6965 entitled "Building and Using Hotbeds and Coldframes" which can be downloaded at http://muextension.missouri.edu/xplor/agguides/hort/g06965.htm. While the information in this and other guides is excellent, it is somewhat intimidating. Faced with the logistics of building a 6-foot-wide and 12-foot long concrete block structure with heavy window sashes and equipped with heating cables, most of us would quickly put the project in the "maybe someday" file folder. However, this author is living proof that it is possible for a klutz to build a structure in three hours, for under $50, from which lettuce can be harvested in March. The beauty of such a structure is that it allows one to get fingers in the soil in February and to learn how to use a cold frame. This experience, as well as the joy of picking garden fresh lettuce while standing in snow, may provide the motivation for building a more sophisticated version. Following is a brief description of how I built my first cold frame and what I learned from it the first year.

Having thought for about two years that the south side of my garden shed looked like a perfect place for a cold frame, I finally managed to overcome the inertia preventing construction of my first simple frame in October of 2000. After scanning through G6965, the first step in creation of my masterpiece was to go to the local salvage retailer and find a window sash that struck my fancy. I purchased a wood-framed, single pane sash 32 inches wide and 67 inches long, costing $15. On the way home, I stopped by the lumber store and picked up a sheet of exterior, one-half inch plywood, a six foot length of 2x2" and some 1- inch wood screws. Referring to G6965, I decided that the front wall of my cold frame should be 12 inches tall and the back (south facing) wall about 15 inches tall (about one inch drop per foot of cold frame width). After an hour of head scratching, measuring, re-measuring, marking and sawing, I emerged from the garage with the four sides of the frame and four 2x2" corner posts, all fairly close to the desired dimensions. Using my low-end, rechargeable drill, I drove about one-half of the screws required to attach the sides to the corner posts before the drill battery ran down. This was a blessing in disguise, since driving the screws without first drilling holes was causing a fair amount of splitting. Three hours after leaving for the window purchase, I carried my crude frame and window sash to the garden.

Being a composter, I had ready access to high quality organic matter to improve the soil beneath my frame. I spaded a 4-inch layer of compost about 12 inches deep into my clay loam soil, ending up with a slightly raised "bed" on which I set the frame. During the spading process, some pencil-sized tree roots reminded me that my frame was not far from a hickory tree. With a good seedbed prepared, I could not resist planting my fall lettuce crop, so it would get off to a good start before really cold weather set in. I attached a remote sensing max/min thermometer to the shed with the bulb placed at plant level in the frame. With the window sash handy, I watered the bed with a watering can and went into the house with a sense of accomplishment and visions of picking lettuce in December.

During October and November I learned a fair amount about cold frames. First, I discovered that there was considerably more shade above my frame than I had noticed previously. Leaves on the aforementioned hickory and on trees to the south in a neighbor’s yard stayed on the trees into early November, reducing light intensity most of the day to about 1/3 that required to grow lettuce. The seedlings that germinated were weak, spindly and disappointing. When the tree leaves finally dropped, I decided to pull up the first crop and reseed.

The second main lesson that I learned was that, while I could keep the air temperature in my little frame well above freezing during the day, the temperature at night was only a few degrees F warmer than outside. This was true even after I lined the inside with foam insulation (not well sealed) and put a sheet of insulation under the sash. Since it turned cold and cloudy in November of 2000, my second crop of lettuce appeared doomed. However, just as I was considering replanting in February, I noticed that many of the seedlings were emerging and growing well. As February progressed, I maintained the temperature within a reasonable range by propping up the sash on warm days with blocks of wood and using the foam cap on particularly cold nights. To my astonishment, by early March 2001, I was harvesting amazing quantities of the tastiest lettuce that ever graced my table.

Since the serendipitous lettuce crop, I continued to learn from my little frame. For example, it soon became apparent that pencil-sized tree roots discovered during spading are an omen indicating that your frame will eventually need to be watered every day after the tree roots really get going. Another lesson was that, one inch of drop per foot of frame width may not be sufficient slope for winter cold frames. I found that there was a band of shade about eight inches wide along the north wall of my frame. However, since I lined my frame with white foam insulation, plants just south of the shade band dried out quickly due to intense reflected light. I have decided to reinforce the sides of the frame to keep the plywood from bowing and make the sash fit more tightly.

The moral of this story is that gardening cannot be learned from books or guide sheets. Too often, we avoid trying things because they sound too complicated or because we feel we know too little to be successful. To be sure, gardening mistakes can be expensive. However, at least half of the enjoyment of gardening is derived from the mental exercise associated with learning from one’s mistakes. By all means, read and consult with gardening “experts,” but no education is complete without personal experience. Gardening pleasure is proportional to your involvement with your plants.

Chris Starbuck, Woody Ornamental Horticulture, UMC