Where I live we received our first frost about a week ago. Our climate is warming and this year our first frost of the season came about 1 month later than normal. While it is great that I am still harvesting tomatoes well into winter, it is also a little unsettling not knowing from one year to the next what the seasons will bring.
Frost forms when the air gets so cold that the water vapor in the atmosphere turns into liquid and then freezes. Cool air sinks and then rolls slowly downhill, often collecting in low-lying areas. Soil that has been heated by the sun during the day radiates warmth into the atmosphere overnight and this can be used to protect plants which can be damaged by frost.
Now let’s look at some ideas to help your more tender plants survive the freeze.
CHOOSE SUITABLE PLANTS
The top of the list for plants surviving your winters would have to be that you should grow plants that are suitable for your climate. While you can grow tropicals in a cool climate (or vice versa) if you create the right microclimate, it will be far better and much less work if you grow something more suited to where you live. Plants grown too far removed from their preferred conditions will often be weak and unproductive – if they survive at all.
Seaweed based solutions help plants develop an internal resistance to frost. Water it in a few weeks before your expected first frost date and continue every two weeks throughout winter.
You can buy rolls of frost cloth and cut them to size. Frost cloth is a very light weight material that works like a blanket, trapping under it the warm air that is radiating up from the soil. It is better if the cloth doesn’t touch the leaves of your plant if possible so I stick small branches into my shrubs that I want to protect. This creates a “framework” that the frost cloth drapes over rather than being directly on my plant. For extra insulation you can scatter some straw on top of the cloth or do a double layer of frost cloth. You can also use sheets, curtains, shade cloth, bubble wrap, or whatever you have on hand at a pinch but remember to remove these the next day.
Compost piles create their own heat while decomposing. Build a compost pile next to those plants which are frost sensitive to radiate extra warmth into the air around them. In the same way that a compost pile works you could also put straw bales around your plants sprinkled with Blood and Bone, chicken manure, or some other high nitrogen material. The straw bales will create an insulation barrier and will also radiate heat as they break down. Using compost (or straw) in this way also has the added advantage of providing nutrients to your plants.
Black boulders, bricks, concrete, a pond, a black barrel filled with water… all of these things have a high thermal mass which warms up during the day and slowly radiates out warmth during the night, helping keep the air around your plants mild and reducing frost. The soil also has good thermal mass – pull back your mulch and leave the soil exposed to make best use of the thermal mass in the ground to keep your plants warm.
SHADE FROM EARLY MORNING SUN
While it seems counter-intuitive, don’t be tempted to try to warm your plants too quickly in the morning. Gentle thawing will reduce cell damage so if you can shade your plants from the early morning sun you can lessen the damage caused by defrosting too fast.
WHAT ABOUT SPRINKLERS?
You may have heard of the method of using a sprinkler or hosing your plants to protect them from frost damage. My advice regarding this is – handle with care! You can actually cause MORE damage under certain circumstances. I won’t go into the science regarding using a sprinkler on nights where you expect a frost except to say that sprinklers only protect down to -5°C and this is only if there is no wind. This protection lessens if there is a breeze. This means that; if your overnight temperature drops below -5°C, or if you stop watering too soon, or if you start watering too late, or if a breeze picks up, you may actually cause more damage to your plants than if you had done nothing. An area that has been watered and is wet will be colder than other areas that have not been irrigated. This means that you cannot stop the sprinklers until after the temperature climbs above 0°C and the morning sun has started to warm the soil. Protecting your plants with one of the other methods listed above is a much safer option – and does not waste water.