What Is a Frost?
When weather forecasters issue a frost advisory in late spring and early fall, that’s your heads-up to protect annuals and other vulnerable plants. You might think temperatures have to get to the freezing point (32˚F), but a frost can occur between 36˚F and 32˚F. Also, frost can be light or hard; a light frost around the upper end of that temperature range might kill the tops of tender plants, but the lower parts remain green. A hard frost happens when temperatures hover around 32˚F for a few hours, enough to kill all above-ground parts. Below 32˚F is considered a freeze, which is more destructive than a frost. Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours. A freeze warning often signals the end of the growing season in fall because temperatures are low enough to kill off annuals and begin dormancy for hardy perennials, trees, and shrubs. A freeze warning in spring is an indication that you should bring tender plants inside.
Which Plants Need Frost Protection
Generally, annual plants that fruit and flower in warm temperatures are most sensitive to cold weather. Think about which veggies and herbs you harvest in midsummer and which annual flowers are most colorful during the hotter temperatures. Frost protection for plants like these is needed either in spring when they’re young and tender or in fall if you want to keep them going as long as possible before winter sets in. Many of them come from frost-free tropical regions of the world, so play it safe and make plans to protect them whenever temperatures dip below 40˚F. In contrast, perennials (the garden plants that come back year after year), shrubs, and trees can usually withstand a sudden drop in temperature, as long as they are healthy and hardy in your region. A spring freeze might damage developing fruit and destroy flowers, but these plants will survive. Some edible plants are actually quite hardy, such as peas, lettuce, onions, cauliflower, broccoli, radish, and cabbage. These cool-season vegetables generally withstand temperatures as low as 26°F. Even hardier crops such as beets, Brussels sprouts, carrots, collards, kale, parsley, and spinach can shake off temperatures in the low 20s. A few cold-tolerant flowers like pansies and sweet alyssum don’t mind frosty weather.
Frost-Protection Strategies
Depending on the size and quantity of plants you need to protect, you’ve got a few options when a frost or freeze is on the way.
1. Move Containers Indoors
When possible, bring tender plants indoors. Small container gardens and any plants still in their nursery containers are generally easy to move indoors temporarily. A heated location isn’t always necessary. Moving plants to a space like a garden shed or growing plants in a garage will provide enough protection when it gets cold. However, when lows are near freezing, it’s time for an insulated indoor location.
2. Bring Out the Blankets
Round up old bedspreads, blankets, and large towels. Drape them loosely over plants, supporting the material with stakes as needed. Be sure the plant cover extends to the ground in all places to create a small dome of insulation. If the wind is a problem, anchor the fabric to the ground with bricks, stones, or anything heavy. Woven fabric provides better protection than plastic or paper. You can add plastic sheets on top of your fabric layer to shield it from precipitation that may also be occurring. Remove your coverings by mid-day, so plants don’t overheat, but keep them handy because there is often more than one frosty forecast per season.
3. Use a Cloche
French for “bell,” a garden cloche is usually a rounded cover that acts as a mini-greenhouse around a single tender plant. A super easy garden hack is to make a milk jug cloche by cutting off the bottom of a gallon-size jug and placing it over a plant, making sure to push the bottom of the jug about an inch deep in the soil. Tie the jug’s handle to a nearby stake to prevent it from blowing away. Keep the lid of the jug closed at night for maximum protection, but remove the lid to vent the cloche during the day to avoid overheating the plant.
4. Water Well
Did you know that moist soil can hold 4 times more heat than dry soil? The moisture in the soil will conduct heat to the soil surface, warming the area around the plant as much as 2˚-3˚F. When cold weather is forecast, water your plants well. A cloche or blanket probably will be necessary, in addition to watering, to fully protect plants.
5. Add Mulch
A thick layer of mulch, such as shredded bark or compost, can help insulate tender plants. Cover the entire plant with mulch the night before low temperatures are forecast. Remove it when the weather warms up again. Messy and labor-intensive, mulch may not be the best option for large planting areas. Reserve this method for a few small but sturdy plants (don’t try this with fragile seedlings!) or those in a place where you can spread out the extra mulch when the need for protection ends.
