In the middle of winter, the selection of “fresh” vegetables can be quite meager.
Sure, the produce on the store’s shelves could be considered fresh, but it’s highly unlikely it was harvested from anywhere close. Winter production in the northern hemisphere, at least on our continent, is relegated to southern California, southern Arizona, southern Florida and Mexico. The vast amount of produce we see in the grocery stores comes from one of those areas, the closest of which is probably 10 hours away.
When I lived in southern Florida, wintertime brought (in addition to snowbirds from New York, New Jersey, Pennsylvania, etc.) beans, tomatoes, peppers, lettuce and more sweet corn than you could ever possibly consume. Believe me, I tried. There are about 30,000 acres of sweet corn in Palm Beach County alone, and, in contrast to our typical growing season of June through September, almost all vegetable crops are grown from October to April. Summers in south Florida are just too hot, humid and wet.
For those of us who want to have some semblance of freshness during the dregs of winter and don’t want to move to California or Florida, there are few options. Not all of us have the ability to build a heated greenhouse. But if you have limited construction skills, a couple of tools and even some extra building materials, you can build a cold frame relatively easily and cheaply.
A cold frame is nothing more than a bottomless box with an affixed lid that transmits light. The theory is that sunlight enters during the day and heats the soil and air inside. The benefit for us is that we tend to have a plethora of sunny winter days – at least that’s what the tourism office keeps telling us.
At night, the soil radiates the heat it absorbed during the day. Without the cold frame, this heat would just be lost to the atmosphere. But we are trapping it, with the hopes we can trap enough for long enough to keep temperatures elevated.
No, you will not be able to grow warm-season crops. So no tomatoes, peppers, potatoes, beans, melons or corn. But cold-season crops – lettuce, spinach, salad greens – have the ability to grow, albeit slower than normal, even during the dead of winter. And during the shoulder months of March and April in the spring and October through December in the fall and winter, you also can try to grow other crops such as peas, carrots, radishes and leeks.
Construction of the cold frame is relatively simple. The sides of the box can be built with bricks or masonry blocks, which are good insulators but make the structure essentially permanent, or you can use wood. A wooden cold frame is most common, because it is cheaper and lighter and still can provide decent insulation, especially if you use two-inch thick lumber. The back, or north, wall should be about 18 inches tall, while the south wall should be shorter, about 12 inches high. This angle will allow for better radiation gain.
The sash, or lid, ideally is attached with hinges, as cold frames actually can overheat during the day and need to be opened to let out some of the heat. Just remember to close the lid before the sun goes down. Materials for the lid could be fiberglass panels, old windows (double-paned work great) or polyethylene film.
For more information about extending the growing season and using frost protection, visit the Colorado Master Gardener GardenNotes at www.ext.colostate.edu/mg/Gardennotes/722.html.
firstname.lastname@example.org or 382-6464. Darrin Parmenter is director and horticulture agent of the La Plata County Extension Office.