If you follow conventional gardening wisdom, Edmonton appears to have only a four-month growing season, from the guaranteed frost-free days between mid-May and mid-September. Of course, many vegetable crops extend into the shoulder seasons on either end: you can start sowing many types of cool-weather seeds (spinach, lettuce, peas) right into the ground as soon as it can be worked in the spring, while other crops are tolerant of a few light frosts in the fall (kale, broccoli, cabbage). With a cold frame or hoop house, however, you can extend that shoulder season even farther. Coupled with climate change and this year’s mild, El Niño-driven fall, Edmonton gardeners can’t complain about too short of a growing season.
Think of cold frames and hoop houses like a mini microclimate that you’re building into your garden. A cold frame is a piece of glass (or transparent heavy-duty plastic) that is usually enclosed in a wood frame and placed on top of a garden bed. Hoop houses are a series of erect semi-circles covered in heavy-duty plastic sheeting, which can be either low (about a foot or two tall) or high (tall enough to stand in). Both cold frames and hoop houses create a greenhouse effect that raises the temperature of the soil underneath by several degrees, as well as protects the plants from frost and deluges of rain or snow. Many cold frames are angled to capture sunlight in the darkening days of fall, as well as to slough off rain and snow. Horizontal cold frames work too, however, and they are certainly better than just bare soil.
While commercial cold frames and hoop houses are available, they tend to be rather expensive. Making your own is fairly easy, though it does require a bit of DIY know-how to accomplish; this is your excuse to either learn a new skill or call in a favour from one of your handier friends/family members. The Internet abounds with dozens of free plans and video tutorials for homemade cold frames and hoop houses, so there are a lot of free resources available.
Sourcing out the materials will be the biggest challenge, though with a bit of scrounging you should be able to find everything you need for minimal cost. Any sturdy scrap wood can be used for a cold frame, but it’s worthwhile to use pressure-treated wood or stain it first to make it last a lot longer. Many cold frames are made with recycled windows, though you can really use anything so long as it’s transparent and durable: old shower doors or plastic, for example. The easiest way to make hoop houses is with PVC pipe and heavy-duty plastic sheeting, which any hardware store will carry and are relatively inexpensive. Check Kijiji and keep an eye out for listings of people or contractors doing home renovations—many of them are willing to part with materials for free or cheap.
In the fall, don’t expect the plants in your cold frame or hoop house to be as lush and abundant as they were in the middle of the summer: even more than the cold, reduced sunlight hours serve to stunt plant growth. You’ll probably end up with baby greens only, which is why you should plant vegetables with a shorter duration to maturity: radishes, leaf lettuces, spinach, arugula, peas (for the shoots, not the pods) and beets (for the greens, not the root). In the spring, vegetables may come in a bit spindly but will quickly start to fill out as the hours of daylight increase.
Whether you’re using a cold frame or hoop house, you must remember to vent. This is pretty much what it sounds like: on warm days, you have to lift the lid of a cold frame (either by propping it open with a stick or lifting it off completely), or rolling up the sides of a hoop house. If you don’t, your plants will cook. The temperature differential between the outside and inside of a cold frame can be quite significant, so on any sunny day above five to 10C, you should uncover your plants during the day. Keeping a thermometer inside the cold frame or hoop house is a good idea: if it starts inching towards the high twenties, you definitely need to vent. This is more of a concern in the spring, though Edmonton has proven that our fall season can also have several days of unexpected, blissful warmth.