An early or late frost can put an end to the best-laid plans for extending your gardening season — or stunt your garden before you even get a chance to start. When the outside temperature dips below freezing, ice crystals can form inside plant cells, disrupting the flow of water within the plant. This results in damage to trees, shrubs, and bushes and may kill annuals and tender perennial plants, which include many herbs and decorative plants. Understanding when and how to protect your plants from frost can help keep your plants healthy and thriving, even through unseasonably cold temperatures.
When to Cover Plants
Deciding at what temperature you should cover your plants is a little more complicated than listening to the local weather reports. In general, annuals and tender perennials — most herbs and vegetables — will be damaged by even a light frost.
If it's likely to dip below 32 °F (0 °C), get those plants under cover. Temperatures from 25 °F to 28 °F (-4 °C to -2 °C) will cause damage to most vegetation and heavy damage to fruit blossoms and tender hardy plants, such as broccoli, cauliflower, chard, and lettuce. When the temperature drops to 24 °F (-4 °C) or below, nearly all of your plants will need some protection.
Other Risk Factors
However, as the Farmers’ Almanac explains, there are many reasons that a frosty night might devastate your garden while your neighbor's garden survives unscathed. Those factors include:
- Soil type
- Sun direction
- North winds
- Relative humidity
- Amount of cloud cover
- If your garden is sheltered from cold
Understanding the weather patterns around your garden is the best way to predict when you should cover your garden, as well as to monitor conditions as you overwinter. Kathy Purdy, owner and editor of Cold Climate Gardening, explains how and why she uses an AcuRite Weather Environment System with Wind and Rain with AcuRite Access™ to predict frost and help her make decisions that increase the success of her garden.
How to Protect Plants from Frost
First, check out our blog post on Preparing Your Garden for Winter for tips on getting your garden ready to get through the winter, especially if you're planning an extended growing season.
Second, get prepped in advance so you're not searching for suitable plant covers as the temperatures start plunging. Stockpile "plant covers" and have them ready to use when you're expecting a frost. Remember that the point of a plant cover is to trap warm air around the plant as much as it is to keep cold air from getting to tender leaves and buds. Almost anything that covers the entire plant will work in a pinch. Consider these options:
- Cardboard boxes in assorted sizes to cover individual plants. Just pop them over the plant at nightfall to preserve the day’s heat. Remove them when the sun comes up to allow light to get to the plant.
- Plant cloches made from milk jugs, soda bottles, or plastic buckets. As with cardboard boxes, pop them over individual plants at dusk and remove them when the sun comes out in the morning.
- Quilts, blankets, bedsheets, or tarps. Place garden stakes or poles at the corners or along the edges of the bed. Drape sheets or blankets over the stakes so that they cover the row all the way to the ground. Weigh the edges down with stones, or use additional stakes to keep them from blowing away during the night. For extra protection, cover the fabric with a plastic tarp or shower curtain.
- Burlap, horticultural frost cloth, or bed sheets to wrap trees, bushes, or hedges. Bob Vila suggests sewing two flat sheets together to create a "bag" you can pop over young trees.
Is Covering Your Garden with Plastic a Good Idea?
If you use plastic to cover plants, make sure that it doesn't come into contact with the foliage or the plant. Plastic can trap moisture against the plant, causing even more damage when it freezes. Instead, build a framework of wood, PVC, or metal, and drape the plastic over the framework to create hoop tunnels.
Also, consider using an anti-transparent spray, available at most garden stores, to protect from light frosts. The spray lightly coats the leaves, preventing moisture loss. Keep in mind that these sprays don't provide protection from severe cold. If temperatures drop below 30 °F (-1 °C) for more than a few hours at a time, you'll still want to provide extra protection.
Tips to Prepare Your Garden for Frost
Finally, here are some general tips on getting your garden and your favorite plants safely through a frost — or an entire winter.
- Water plants well early in the day. It may seem counterintuitive, but keeping your plants well-watered gives them extra strength and may help them withstand the cold a little better. Water early enough that they have time to absorb the water.
- Plant cloches made from milk jugs, soda bottles, or plastic buckets. As with cardboard boxes, pop them over individual plants at dusk and remove them when the sun comes out in the morning.
- Bring potted plants indoors — but don't shock them. They don't like big temperature swings any more than most people do. Most will be happy in a cool corner of the basement or garage.
- Dig up tender bulbs. Calla lilies, gladiolas, and other tender bulbs should come out of the ground before the freezing temperatures arrive. Dig them up gently, rinse the soil off them, and allow them to dry completely, then layer them in a box of peat moss or straw. Store the box in a cool, dark room until spring.
The key to helping your plants survive cold snaps is preparation. Monitor conditions carefully, and keep a close eye on crop temperatures — particularly the ambient temperature at crop height, which could be very different from the temperature reported by your local weather reports. Watch for signals that frost is heading your way, tuck your plants and garden beds in snugly when it gets cold, and remember to remove the covers in the morning so they can get the light and air they need to thrive.