Once you finish planting your spring garden, feeling proud for not procrastinating this year, you can finally relax. Seedlings are poking out of the soil. Flower buds are starting to bloom. Then bad news strikes: a frost advisory! Giving your plants an early start in spring has many advantages, but the season’s unpredictable weather may threaten to ruin all of your hard work.
Should you worry about your plants when the mild spring nights are interrupted by a bitterly cold one? Is it important to take immediate action to protect your seedlings from frost, or should you just hope for the best? While some cool weather plants can handle a cold snap, others cannot.
Preparing for a surprise frost will save you the stress and worry of frantically rushing around at the last minute to save your precious plants.
What’s the Risk of Spring Frost?
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Knowing which plants are most at risk and how to effectively protect them from frost will save your garden and all of your hard work. Spring’s wild temperature fluctuations can damage seedlings, cold-intolerant plants, blossoms, new growth, and the newly planted. If it’s especially cold, your plants may die.
To ensure your plants survive those extra chilly nights, we got our hands dirty (figuratively) to find the experts. This article is rooted in research from horticulture experts, university extensions, and master gardeners across the U.S.—including the University of Kentucky, University of Georgia, Minnesota State Horticultural Society, North Carolina Cooperative Extension, and Texas Master Gardeners.
How Cold is Too Cold?
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Horticulture magazine reminds us that the National Weather Service only issues frost and freeze warnings after the (average) last frost date for your area, so consider planting your seedlings and annuals after that date. Cold-tolerant plants are usually fine, but even the hardiest among them may need help if the temperature plummets below 25°F.
Rick Durham, a horticulture professor at the University of Kentucky Extension, explains that if a cold snap occurs early in the season and temps drop down in the 30s, it won’t affect plant development much – except maybe small blooming shrubs and fragile new plants. It is more problematic if a frost or freeze comes later, like a mid-May freeze in Kentucky.
In general, cold-tolerant plants like lettuce, asparagus, and rhubarb can tolerate temperature drops as long as they don’t hit the 20s; if it gets that cold, covering them is your best bet. Blooming blueberries and strawberries need to be covered in the event of frost, but you don’t need to worry about berry plants that produce later. Some flowering plants, such as daffodils, pansies, and primrose, tolerate the frost very well. Even if wilted-looking after a frost, they typically bounce back as the temperature warms.
Why is Frost Bad?
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University of Georgia Extension horticulture specialist Paul Thomas points out that it’s not the cold air that is the issue but rather the ice itself. When frost occurs, icy crystals form inside the plant and shred it from within. Interestingly, woody plants that are dormant over the winter are safe from this icy death because the stems contain sugars that prevent ice from forming. Yet, they too are vulnerable once the temperature warms, water replaces the sugars, and new growth begins anew.
Cover your Plants
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The Minnesota State Horticultural Society recommends covering your rows of seedlings. This works for both raised bed gardens and in-ground. If you have raised beds, simply drape the sheeting over the vulnerable plants and staple it to the edges of the bed. Placing rocks or bricks around the edges works, too. Ultimately, you are trying to keep the cold air out while preserving the soil’s heat.
For an in-ground garden, try propping up the center like a tent with small garden stakes, so the sheeting doesn’t touch the plants. The key is to create air space between the seedlings and the cold temperature.
Another idea is to build an inexpensive framework around your garden in advance. Texas master gardener Diane Kufert suggests flexible PVC pipe over rebar to create an arch framework. Draping blankets or covers over the frame will offer great frost protection while keeping the heavy fabric off the plants. Wooden stakes or short-wire garden fencing can also be used for a DIY frame.
Which Material is Best?
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You have a choice of materials of varying thickness for covering your plants. One option is a lightweight row cover- a thin, gauzy material. The University of New Hampshire Extension says that the lightweight variety only provides about 2°F of protection from frost, while heavyweight row covers offer 6° to 10°F of protection.
For extra protection, frost covers are ideal. They can be anything from old blankets or bed sheets to burlap. Be sure not to drape anything too heavy over delicate plants or you risk damaging them from the weight. Blankets are perfect for wrapping around small bushes to protect them.
UGA Extension nursery specialist Matthew Chappell prefers nursery or tobacco cloth. The lightweight cloth retains heat well, protecting plants when temps dip down as low as 12°F. It can also be stored away and reused for many years.
Is Plastic a Good Choice?
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There is mixed advice on the use of plastic to cover your plants. You can certainly use plastic sheeting but it’s a bit trickier. Kufert cautions gardeners on the use of plastic, as it can burn plant tissue when the temperature warms back up. North Carolina Cooperative Extension agent Rebecca Bradley suggests not using plastic at all. The most important point to remember is that plastic should never make direct contact with plants.
Other experts, like the UGA Extension, recognize the benefits of plastic for retaining heat. Yet, even they highly recommend removing the plastic the next morning by 8 am, as soon as the sun is out. Plastic can also be used for an added layer of protection, on top of another covering.
Other Creative Ways to Cover Your Plants
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Take special care to protect small, fragile seedlings. One great method is to use plastic (or glass) cloches – small, individual greenhouses for plants. You don’t need to go out and buy these special coverings if you are in a pinch. Lots of things around the house can act as cloches.
Minnesota State Horticultural Society suggests these items as DIY cloches: large plastic drinking cups (that come in party packs), empty pots, buckets, and small cardboard boxes. You can even make your own with repurposed, plastic milk jugs. Trash cans and plastic tubs are also perfectly good coverings for plants.
Mulching is Also Great Insulation
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Mulching around the plant roots is another option for protection against the dangers of frost because it reduces temperature fluctuations. Seedlings, small plants like strawberries, and any other sensitive plants in your garden will do well with a thick covering of pine straw or leaves, according to Bradley.
Thomas agrees, explaining that if you surround new plants with a few inches of pine straw to help with evaporation, you simply need to top them with about 2 extra inches when cold weather strikes. That will trap heat from the ground and protect your plants from roughly 90% of damage due to frost, even if the temperature plummets to 20°F. Just remember to remove the straw over the plants when it warms up the next day because they need sunlight and air circulation.
What About Potted Plants?
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If temps are expected to drop between 29° and 32°F, known as a light freeze, consider moving potted plants together close to the house, preferably exposed to the south. Kufert also recommends adding mulch or straw to help insulate the roots; if not, covering them is suggested. Should a hard freeze (25° to 28°F) pop up in the forecast, it’s best to move potted plants into a garage, barn, shed, or other enclosed structure to protect them.
If you cannot move the pots, simply cover them where they are. A covering that is large enough to drape down on all sides and lie flat on the ground is preferred. That way, you can put objects along the edges to hold it down, trapping in the heat from the soil, and keeping out the cold air.
Tip: If your potted plants are left outside without protection during a frost, just hope for the best! Horticulture magazine advises against bringing them inside to warm up; they will suffer more damage with an abrupt change in temperature.
Water Your Plants
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If you have fruit trees, consider this surprising technique: watering the soil around the base of the tree before a freeze. The wet soil stays warmer, and as the water cools, it gives off heat which rises up – protecting the blossoms from frost. Minnesota State Horticultural Society says that orchards and even some berry farmers use this method.
It is also true that drought-stressed plants are more likely to be damaged by a cold snap, so it’s good practice to water your plants a few days before the temperature is supposed to drop. And since it’s spring, chances are – Mother Nature may take care of that for you!
Don’t Worry and Be Prepared
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With the season’s rollercoaster of weather, it may feel like your plants are doomed to a frosty death, but rest assured – they are oftentimes quite resilient in the face of cold weather. Many plants easily survive a surprise snow shower too, so there is hope!
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