Is there anything more amazing to a northern gardener than the start of spring? And does anything induce more panic than news of an imminent cold snap?
After the panic subsides, your first inclination is to cover vulnerable plants. But how does covering help? If you wrap a blanket around yourself, you start to warm up because your body generates heat that the blanket traps close to your body.
Plants don’t generate heat, but the earth absorbs heat from the sun. When the air temperature is lower than the soil temperature, the soil releases heat into the atmosphere. This is the heat you are trying to trap. That means whatever you use to cover the plants must reach all the way to the ground. There’s no point draping an 8-foot shrub with a sheet unless you can secure the sheet to the ground with rocks or landscape pins.
Deciding what to cover depends on your priorities and such factors as plant type, age, weather the past few weeks and how far along in the growing season this cold snap will occur. Below are things to consider as you make your decisions.
Unless the tree or shrub is very small, there is not much you can do to protect it. You may see evergreen shrubs wrapped in burlap—that keeps wind from drying them out. You may see shrubs sheltered with wooden A-frames—that protects them from heavy snow loads. Neither protects against cold.
Most perennials—including bulbs—that emerge in early spring are used to dealing with cold snaps. A severe drop in temperature may cause damage to leaves or flowers but will not kill the plants. In the case of bulbs, if the flower gets ruined or the leaves suffer damage, they won’t grow new ones. Perennials will send up new leaves and, in a month, you won’t know they were damaged.
Cold-hardy annuals can take a light frost. Self-sowers, such as poppies, larkspur and alyssum, usually won’t sprout before it’s safe to do so. The early-spring vegetables, such as lettuce and peas, should also be fine. You shouldn’t plant tender annuals or vegetables before your area’s last frost date, but sometimes gardeners are seduced by mild weather and gamble. Did you hedge your bet and keep some plants safe in their pots? Good, they can wait out the cold in the garage. If they’re all planted in the ground, follow the protection guidelines below. Other factors to consider:
Age: Anything that was planted the previous fall doesn’t have a strong root system and is vulnerable. These plants can tolerate cold as well as their more established brethren, but if they are damaged, they don’t have reserves to fall back on. They deserve top priority for protection.
Recent weather: Plants have many cold-protection strategies that they gradually relax as the weather warms up. If there is only a week or so of warm weather before the cold returns, the plants that are used to emerging early will do fine. If there has been a month of frost-free weather and frost is returning, that’s a problem, especially if it’s a bona fide freeze. The general principle is, the more plants have been tricked into thinking it’s spring, the worse damage they will sustain. Also, the wider the spread between the temperature extremes (for example, going from 60 degrees F to -10), the greater the damage. Let’s face it: there’s not much you can do against a catastrophic shift in the weather. Frost protection will afford—at most—8 degrees of protection.
Snow and ice: Snow will trap heat to the ground for you. As long as the snow falls before the temperature plummets, your plants are protected. Ice causes damage from its weight, not its temperature. Trees and shrubs are most vulnerable, especially if they’ve already leafed out. Don’t try to knock the ice off. Wait until it melts and then prune broken branches as needed.
How much do you care? Plants with sentimental value, exceptional financial value or great amounts of time invested in them should get top priority when you’re considering what to protect.
HOW TO PROTECT PLANTS
Self-sown hardy seedlings can be given protection with evergreen boughs or the stalks of trimmed ornamental grasses. Cloches are the elegant and time-tested devices for protecting plants, but if you haven’t any on hand, improvise with an overturned flowerpot with a single hole in the bottom. Cover the hole with a rock and you’re all set.
For bigger plants, use a 5-gallon bucket. Anything too big to be covered by a 5-gallon bucket had better be worth the fuss and bother. For larger areas, try row covers made of spun-bond polyester, which lets in sunlight, or in a pinch, old bedsheets. Use a tomato cage turned upside down with a sheet draped over it, making sure the edges of the sheet are snug to the ground and secured against blowing wind. For any plant larger than that, or large areas of a vegetable garden, you will have to rig up a network of stakes or hoops and find a big piece of row cover.
WHEN THE COLD SNAP ENDS
Remember to remove the protection (unless it’s spun-bond polyester) the next day if it’s sunny—even if you think you’ll need to use it a second night. The plants will overheat under their covers and cook. How ironic (but still sad) to lose a plant to heat when you were trying to save it from cold.
Then, survey the garden and make repairs. Trim any flowers or foliage that turned brown or mushy, but leave any part that’s still firm and green, especially bulbs. What’s left of their leaves is all they have to nourish the bulb for next year. If the leaves of woody plants were damaged, don’t assume the branch itself was killed. Give it two months to see if it will push out new leaves. Broken branches should be pruned, of course.
A cold snap can teach you about your garden. If possible, take pictures of your garden before the cold arrives. Pay attention to the flower buds on spring-blooming shrubs and anything you think will take a hit. Then take another set of photos after the cold has gone.
Were plants damaged because of their location, how far along in growth they were, or are they just not able to sustain hard freezes at all? Does it happen frequently, such as with my Cynanchum, or rarely, as with daylilies? And if it happens frequently, is it a plant you really want to keep? Record in your garden journal any plant that you wished you’d covered but didn’t, and anything you suspect was covered unnecessarily.
The next time an unexpected cold snap is predicted, you won’t panic—you’ll know exactly what to do.
COVER OR NOT?
When choosing which plants to protect, give priority to these perennials.
Hostas – Cover any special favorites.
Old-fashioned bleeding hearts – You might lose this year’s flowers.
Astilbes – Frost damage delays them, but they do grow back.
Rodgersia (Astilboides tabularis) – I moved this to a colder spot so it takes longer to emerge in spring.
Agastache – Anise hyssop is less cold-sensitive than the hybrids. Cover them if they are in growth.
Barrenwort (Epimedium) – Flowers could be damaged.
Sedums – The groundcover types tolerate cold, but the taller ones might benefit from being covered.
Lilies – These always seem to be up too early for me, and if the tips get damaged it can ruin the flowers. I always cover them.
Big-leaf hydrangeas (H. macrophylla) – If their leaves have emerged, they will be damaged by frost.
WHAT'S A FREEZE?
Gardeners refer to light frost (29 to 32 degrees F), hard frost (25 to 28) and freeze (below 25). The National Weather Service has its own definitions:
- A Frost Advisory is issued when the minimum temperature is forecast to be 33 to 36 degrees on clear and calm nights during the growing season.
- A Freeze Watch is issued when there is a potential for significant, widespread freezing temperatures within the next 24 to 36 hours.
- A Freeze Warning is issued when there is an 80 percent chance that the temperatures will fall to 32 degrees or lower in the next three to 30 hours during the growing season. If the temperature is expected to fall below 28 degrees, this is considered a Hard Freeze.
Philip Harnden’s A Gardener’s Guide to Frost (Willow Creek Press, 2003) offers a deep dive into frost and all its ramifications for gardening in both spring and fall.