As the growing season shifts to winter mode, your attention turns to protecting your food plants and flower beds and baskets for as long as possible. You watch for the first signs frost, which sneaks in on a clear windless autumn night. Fog or cloud cover tends to prevent a frost by trapping the heat that would otherwise escape skyward. Frost occurs when temperatures drop to just below freezing. Why all this frost watch diligence? Frost causes ice crystals to form in plant cells and thereby damages the cell walls. Leaves then shrivel and turn brown or black and the plant is doomed.
Hopefully you have prepared for the onset of frost by planting frost-sensitive plants which will survive a light frost, or by planting tender species on a mound so the cold air, which sinks because it is denser than warm air, will flow past the plants as it settles in low lying areas of your garden. You also discontinued fertilizing by mid-September to avoid any new growth, which makes a plant susceptible to frost. Older leaves are tough enough to weather a frost.
When the weather report suggests frost, protect your plants by watering them thoroughly before sunset. (Once the sun sets, much of the stored heat in the ground has dissipated.) The soil will release moisture into the air around your plants throughout the night, keeping the air warmer.
Another way to collect heat in the garden is to paint plastic milk jugs black and fill them with water. Place them around your plants. The idea is that water loses heat at a slower rate than soil or air, so the collected heat will radiate outwards throughout the night, protecting the plants from frost.
To help slow the loss of heat rising from the plants and the ground, and to protect the moisture in the plant cells from freezing, build a simple frame around the plants and cover them before sunset using breathable material such as burlap, newspaper, cardboard, or even old bed sheets (avoid plastic which doesn’t allow moisture to escape), and secure with clothespins or giant binder clips. Uncover the plants in the morning to release any moisture and allow them to warm up. If the temperature stays low, leave the plant covered.
You can also use commercial coverings designed to protect plants. These units may look more attractive than your old bed sheet, but the sheet does work just as well.
For smaller plants, make a mini greenhouse for each plant by cutting the bottoms off milk or soda jugs, removing the cap, and placing them over the plant. The plants stay warm and any condensation escapes through the top. Remove these in the morning. Alternatively, dig up your plants and move them indoors.
If you can, move container plants inside or into a greenhouse. They can also be covered and wrapped, with a layer of insulation such as burlap or bubble wrap around the pot to protect the roots. Or, bury the pot in the ground and cover the plant with newspaper or old sheets. At the very least, provide protection by moving them against the side of the house or shed.
Most plants respond well to being covered, but delicate foliage may not survive, even with the extra insulation. If your plants succumb to a frost, leave the damaged parts intact as they will provide some insulation from further frost damage. The key is to be prepared. Plan ahead and plant species that are recommended for your climate zone. Check your neighbours’ yards to see which plants survive the frost. Some vegetables such as leeks, beets, carrots, cabbage, kale, and broccoli are in no danger of frost. Even parsley, sage, and chives will survive. Alas, Mother Nature may not be so kind even when you’ve gone to all the effort. Accept the fact and plan for next season.