How to Cover Plants for Frost Protection
In areas where a late spring frost or even an early fall hard frost can threaten garden and landscaping plants, there are ways plant lovers can protect them. Weather is unpredictable, even in those areas where plants are suitable for their growing zone. You can protect plants from frost if you are both prepared—and aware of the pending weather.
Soft woods, actively blooming plants, and potted plants are the most susceptible to frost damage. The greatest threat of frost usually occurs overnight when the temperature drops enough to freeze the moisture on plant leaves and buds. The signs of frost damage are usually visible within two to three days and include browned and mushy leaves and buds. To protect plants from frost, you will need to cover them to keep the moisture from freezing.
Plastic can be used to protect plants from frost, but it’s not the best or most effective material, and some expert gardeners warn against it. Plastic or vinyl materials do not breathe, causing moisture to get trapped inside. If the temperature drops low enough, the increase in moisture presents a greater threat to the plants. Instead of plastic, try using natural fabrics like cotton or linen, an opened burlap bag, or newspaper, as a covering to protect plants from frost.
A fabric covering will allow moisture to escape but will still protect plants from frost by preventing the freezing air from coming into direct contact with the moisture. Bed sheets work well for covering large plants and shrubs, as well as young sprouts. Newspaper can be used on low-growing foliage, but won’t stay on top of larger plants well.
In a pinch, you can use plastic sheets, but be sure to remove the plastic covers early in the morning to let the increasingly warmer daytime air reach the plants. If the threat of frost is prolonged and temperatures remain low during the day, be sure to use a fabric covering. When there is a threat of frost, cover your plants before sunset.
You can also purchase commercial coverings designed to protect plants from frost. These may be more attractive than other methods, but usually bed sheets or burlap work just as well. If your efforts to protect plants from frost fail, you will have to allow nature to take its course. Early spring perennial flowers like the daffodil, tulip, and crocus may be damaged for the current season, but they should return in good health the next spring.
Depending on the weather, some plants may peek out earlier than normal, only to be threatened by a late frost. In some cases, they will bud again but often they will die and you’ll have to start over. Typically, the more established a plant is, the better it will fare. If you have vulnerable plants that would be expensive to replace, it’s best to try to protect them.
The cold, cloudless evenings in the fall, winter and spring may be harmful to your plants. During the day, your plants and the soil absorb and store heat from the sun. As the day turns into night, your plants quickly begin to lose all of their stored heat. Clouds will help to insulate and slow the loss of the heat, but a cloudless, wind-free night will afford no protection from frost. The temperature within the soil and in the plant’s cells may even drop to a few degrees colder than the air.
As the temperature decreases, the moisture in the air condenses into dew, which then freezes when the temperature reaches 32 degrees F. on the plant surfaces. At 32 degrees, damage to most plants may be minimal and only affect a small amount of leaves. However, if the temperature drops far enough for the plant cells to freeze, non-hardy plants will die.
Frost can occur even in supposedly frost-free areas. It is important to heed the weatherman’s warnings of ‘a chance of frost,’ and take precautions to protect your garden. It is possible to extend your growing season by several weeks if you are able to keep your plants alive through a single early frost!
September 19th, 2008 at 4:49 am
Other ways to help your garden survive a frost.
The best way to avoid frost damage to your plants is to grow plants that can withstand the frost. The term ‘frost hardy’ is often misleading because of the degrees of frost (i.e., light frost vs. hard, killing frost). It is a good idea to ask a qualified local nurseryman what is suitable to grow in your area. Even better, look around your own neighborhood, and see what survives and thrives in other yards and gardens.
Choose varieties of plants that flower late, in areas where late spring frosts may occur. Some annual and perennial plants will survive frost on the foliage, but the same frost might kill any flower buds that have emerged.
Cold air is denser than warm air, so it sinks to the lowest point. Low-lying areas of the garden can be several degrees colder. Consequently, frost may occur in these areas when there is no frost evident anywhere else in the garden. Plant tender species on higher ground or on slopes where the cold air will flow past the plants as it moves to the low point. Any sloping area is less prone to frost, because the cold air won’t settle there as readily.
Precondition your plants to withstand cold temperatures by discontinuing fertilizing in early fall, so that no new foliage is on the plant when cold temperatures arrive. Older leaves are much tougher and more able to withstand frost.
When frost is predicted, there are several things you can do to protect your plants:
Water your plants thoroughly before nightfall. The soil will release moisture into the air around your plants during the night, keeping the air somewhat warmer.
Even a slight breeze will prevent cold air from settling near the ground during the night. You can help keep frost from forming by providing this breeze with an electric fan. Be sure to protect the fan and all electrical connections from moisture and the elements.
Cover up before dusk! By the time it gets dark much of the stored heat in the garden has already been lost. If you have time, build a simple frame around the plant, or row of plants. (Even a single stake can be used in many cases.) Then drape a cover of newspaper, cardboard, plastic tarps, bed sheeting or any other lightweight material over the frame to create a tent. If you don’t have time to create a frame, lay the protective cover directly onto the plant. This will help to slow the loss of heat rising from the foliage and the ground. Remove the covers in the morning, once the frost has disappeared, to let the light and fresh air back in, and to prevent overheating by the sun.
For smaller, individual plants you can use glass jars, milk jugs with the bottom removed, paper cups turned upside down, or flower pots as heat traps. Don’t forget to remove these covers in the morning.
You can collect heat during the day by painting plastic milk jugs black and filling them with water. Place them around your plants where they will collect heat during the day. Water loses heat more slowly than soil or air. This collected heat will radiate out throughout the night.
Potted plants are particularly susceptible to frosts because the roots are less insulated. If you are unable to move your container plants indoors or under cover, remember to also wrap the pot in burlap or bubble wrap, or simply bury the pot in the ground, in addition to protecting the foliage.
If your efforts were too late, or too little to protect your plants from a frost, resist the urge to cut off the damaged parts of the plants. To a certain extent, these dead leaves and stems will provide limited insulation from further frost damage. Your plants will still need to be pruned in spring.
USDA Zone Last Frost Date
1 July 15
2 August 15
3 September 15
4 September 15
5 October 15
6 October 15
7 October 15
8 November 15
9 December 15
10 December 15
11 No frost
November 3rd, 2008 at 8:16 am
When covering plants for frost, do I leave the frost protection bag on my plants all day or do I take it off in the morning?
November 3rd, 2008 at 8:30 am
Dear Ann, covering plants for frost is very simple. The night before the frost is expected, cover your plants. The next morning after the sun has risen, you can remove your plant covers. You continue this until the weather turns cold enough that the plant dies down for the year season.
January 22nd, 2009 at 8:41 pm
I have a friend when it gets 32 degrees and below, run his yard sprinklers to put a coating of ice on the plants. What does this ice coating on the plants do to protect the plants? Is it better than covering them up?
January 26th, 2009 at 5:16 am
I gotta think it’s harder on the sprinkler system than the plants!
According to the US Dept. of Energy, depending on how long the temps maintain freezing and below, this is not harmful. Why? Well, the water in a liquid state has to undergo a physical change to transform to a solid. When it freezes it gives off heat energy during that transformation. Once the freezing process stops, the ice will fall to ambient temperatures, so an extended period of freezing would require a continued spraying. The extended spraying, however, would create an increased amount of weight from the ice load, risking damage to the plant from breaking branches. But in addition to the energy released during freezing, two other reasons apply. Ice has a low thermal conductivity, especially if sprayed on in the form of a “snow” and the ice/snow helps to protect the plant from wind, which will wick moisture and heat away (that dreaded “wind-chill” factor). It’s why igloos work or why you burrow into the snow when caught in a snow storm. If you don’t want to water your plants in the freezing temps, make sure that the ground has sufficient moisture by watering the ground around the plants. If you have had a period of drought, your plants may already be stressed and less able to endure a sudden freeze, so in general keeping moisture consistent at the root level is also important.
Because of watering limitations in commercial use, new sprays are being developed to help plants endure sub-freezing temps. Covering
plants with cotton fabric or plastic to protect against cold and frost is a simple physical barrier against the elements, but be sure to remove in the mornings so sunlight can warm the soil and air can circulate around the plants.