After an elongated spring due to cold weather stretching almost until June (Ice fishing in May anyone?) i thought it might be good to go over the issues of frost on gardens – specifically vegetable gardens here in the north.
What is frost?
Frost simply is the deposition/formation of ice crystals from water vapour when the air temperature falls below the freezing point of water. As the air cools at night, the dew point is reached, which causes water to condense on plants, then if the temperature falls below freezing the condenses water vapour will crystallize and grow ice crystals with more and more humid air near the ground solidifying and adding as the temperature falls.
Light Frost: Temperatures -2C to 0C
Hard Frost: Temperatures below -2C
How plants are affected by frost?
Ice is just water in its solid state, however unlike pretty much all other liquids, when water is in its solid state it expands. Expansion of ice is an incredibly strong and damaging force – ask anyone who has had frozen/burst pipes. The expansion of the water in a plant cell (mostly water) usually results in the rupturing and killing of the cell. The deeper the freezing penetrates into the plant, the more damage it causes.
The following is a list of some common vegetables and their frost tolerances.
Likely damaged by light frost: Beans, Cucumbers, Spinach, Peppers, Pumpkins, Summer Squash, Corn, Tomatoes.
Can withstand light frost: Beets, Carrots, Chinese cabbage, Lettuce, Parsnips, Peas, Swiss Chard, Bok choi.
Can withstand hard frost: Broccoli, Brussels sprouts, Cabbage, Collards, Kale, Kohlrabi, Peas, Pumpkins, Radishes, Spinach, Winter Squash, Turnips, Rutabaga.
Some vegetables actually benefit from a frost or two. Rutabaga, turnip, parsnips, carrots will actually get sweeter if exposed to frost.
Frost mitigation (protection and prevention)
Typically with a light frost, out come the plastic sheets and blankets. A light blanket or plastic sheet is put out to cover the fragile plants. This serves two purposes; one the coverings will trap heat stored in the soils from the day keeping the ground warmer for longer, two, the condensing dew tends to accumulate on top of the coverings and do not reach the plants.
Prevention of frost requires a little more hardware. Cold frames and hoop houses are two semi-permanent methods whereby you actually build small greenhouses, with hoop houses right over the crop in the garden.
Cold frames are wooden boxes with windows on the top, can be open on the bottom. When the windows are closed the cold frame provides a good seal against the outside. During the day the windows heat up the earth beneath considerably faster and hotter than outside. This heat is then dissipated overnight and with good seals will last the night.
Hoop houses are half hoops of thin pvc pipe or electrical conduit formed into an arch over your garden box. The “hoops” then support a sheet of plastic which is then sealed all around and to the box. The effect is the same as the cold frame.
With both of these methods you can add a heat source (old incandescent Xmas lights, roof de-icing cable, etc.) and actually extend the growing season considerably – usually on the spring end for cold frames for transplants, however hoop houses can be erected quickly over existing crops. (you are using square foot gardening aren’t you?)