When an unforeseen frost happens, it can turn a meticulously cultivated garden into a plant cemetery. Sam Schmitz, a horticulturist affiliated with the National Garden Bureau, explains that frosts typically happen overnight or when the ground temperatures are warmer than the air.
“If a cold air mass moves into an area and temperatures drop to 32 degrees [Fahrenheit] with enough water vapor in the air, frost will occur,” says Schmitz. “Colder temperatures can affect, alter, and damage a plant’s cell structure, causing blossom and leaf damage and death.”
But remember, frost protection solutions are often only temporary. In the winter, when there’s a consistent hard freeze, most plants won’t survive unless they’re perennial in your specific USDA growing zone.
So if the weather network says there’s an unexpected frost incoming in September, explore the methods below to keep your plants around a little longer. But note that these tricks won’t keep them alive indefinitely — a hard freeze, or killing frost, could eventually do them in.
When a hard freeze sets in and winter is in full swing, more extreme methods like cold frames, greenhouses, mulch, and burlap wraps can keep hardy perennials alive until spring. It can also prevent damage from rapid freezing and thawing that may occur during the winter months.
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Hard frost vs. light frost
When air temperatures reach freezing (or 32 degrees Fahrenheit) plants will experience what’s known as a light frost. With a light frost, there’s a chance that some hardy plants will survive without protection.
When air temperatures are 28 degrees or lower, plants will experience a hard frost. Without proper protection, a hard frost can cause irreparable damage to cold-sensitive plants and plant tissue. Established perennials will still return the following year.
Schmitz states early fall or late spring frosts are most damaging, particularly to annual flowers. Annuals, or plants that complete their life cycle within a year and don’t come back in the spring, tend to be more susceptible to colder temperatures than perennials.
For example, impatiens and begonias don’t appear damaged while frozen. However, as temperatures warm, the plant turns to mush due to a loss of structural integrity. Other plants, like coleus, will completely drop their leaves as temperatures increase after a frost.
Left unprotected, tender perennials like salvia can wilt and, in cases of extreme damage, may never recover, according to Schmitz. Tender perennials are plants that can live for years, but may not be suitable for your particular climate. Many tropical plants, for example, thrive for years in warm temperatures but can’t handle exposure to the cold.
Frost isn’t as damaging to hardy perennials, shrubs, and trees. But colder temperatures, like a frost that occurs in the spring, can still damage certain plant tissues like fruit tree blooms. This can then impact fruit production in the same year, says Schmitz.
How to protect plants from frost
You can prevent frost damage with a bit of planning and quick thinking. These methods work for frosts but aren’t suitable protection for when winter sets in and temperatures dip consistently below freezing.
1. Cover the plants with fabric
Covering plants effectively prevents frost damage since plant covers provide a physical layer of protection against cooling air temperatures, says Schmitz. “Cover them with lightweight sheets before it gets anywhere close to 32 degrees [Fahrenheit], then remove them in the morning so the plants can be warmed back up by the sun’s rays.”
Opt for fabric sheets instead of plastic since plastic can worsen frost damage if it ends up touching your plants. The easiest way to keep a cover from blowing away is to drape the fabric over plants and secure it with heavy rocks or other hefty objects.
2. Warm the plants up with water jugs
According to Schmitz, this method is akin to using radiators to heat up the surrounding air. Fill any size bottle or jug with water, making sure to place containers out in your garden while it’s still warm outside. One large gallon jug per plant should be sufficient. Water-filled jugs or bottles absorb the heat during the day and release it at night, protecting plants from a potential frost.
Schmitz explains that this method works best for young seedlings or transplants, like newly set out tomatoes or peppers. Using warmer jugs allows you to set plants out and get them established earlier. You can also use this method to protect vegetable plants from frosts that occur late in the season.
3. Cover with a cloche
Cloches are typically made of glass or rigid plastic. “They work by creating a mini-greenhouse environment around an entire plant,” says Schmitz. The warmth gets trapped inside and prevents frost particles from touching and damaging plant tissue.
These bell-shaped covers are a very effective frost protection measure. However, they take up a lot of space and can be quite expensive, says Schmitz. Additionally, because of their compact size, they’re best for covering tender young transplants in the spring (when they’re just beginning to grow) as opposed to large, fully-grown plants.
4. A simple fan
Schmitz also recommends using a fan when expecting a light frost. “It may seem a bit counterintuitive to employ an item generally used for cooling to prevent frost, but by reducing the relative humidity and mixing the cold air at the surface with slightly warmer air just a few feet above the plants, you could essentially create a microclimate where frost no longer has the proper conditions to form,” says Schmitz.
You don’t need much air movement to keep frost from forming, so even a gentle breeze will do the trick. A single fan per four-foot by four-foot garden bed should provide enough air circulation. Place the fan a few feet higher than the garden bed so warm air gets pushed downward. This method is best for greenhouses since you’ll need to protect the fan from the elements.
What to do if the plants experience frost damage
What you’ll do if a plant gets frost damage will depend on the type of plant you have. “If they are annuals and have been reduced to a pile of mush, toss them out and start again,” Schmitz says.
If perennials or shrubs incur frost damage, wait a week or so to see whether they recover. Schmitz points out that leaf loss doesn’t necessarily mean all is lost, and that stems and dormant buds may still be intact. Make sure to protect them from any additional frosts since they may be more vulnerable at this point. If plants don’t recover within two weeks, remove the damaged plant parts — most of the time, the rest of the plant will be fine.
Schmitz explains that flowering and fruit trees that experience blossom and bud damage may not produce fruit or flowers that year, but will usually continue to flower and fruit again in the following years.
Frost can cause irreparable damage to annuals and tender plant tissues like blossoms. But methods such as covering plants, running a fan, and using cloches or water jugs, can help protect vulnerable plants from light and hard frosts.
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