What to do before, during, and after the freeze.
Jack Frost as a Union general, Harper’s Weekly, Oct 1861.
Jack Frost is on his way, and this visit is going to be a doozie. We’ve already gotten a taste of his chilly touch here during the last few days, but in this first week of 2018 things are going to get extreme. Luckily, this super-cold won’t last more than a couple days, but even a brief plunge into the low teens is enough to do damage to very new plantings and those plants that are pushing the limits of cold hardiness here in Zone 8/7b.
If you haven’t already taken steps to prepare your garden for the cold, here’s a battle plan to help you win this bout with Jack Frost:
Before the Freeze
Cover container gardens.
Anything planted in a pot above ground is more vulnerable to the effects of frost because the roots aren’t insulated underground. Take the following measures to protect your container gardens:
1. Put any containers, window boxes, or hanging baskets that are light enough to be moved into a garage or carport. At the very least, tuck them in close to the foundation of the house or under the eaves. This will protect against heavy frost and desiccating winds.
2. Cover container gardens with frost cloth that goes all the way to the ground. Secure the cloth with bricks or heavy-duty landscape staples so it doesn’t blow off (image from the Missouri Botanical Garden’s Kemper Center for Home Gardening website). The frost cloth must go all the way to the ground in order to protect the exposed roots of the plants. The frost cloth will also trap any residual heat rising off the sun-warmed ground further buffering the roots of the plants against the extreme overnight cold.
3. If you don’t have enough frost cloth for all your containers, mound up fallen leaves, mulch, or straw around the base of the pots. Anything you can do to provide a layer of insulation between the roots and the cold air will help.
4. The same protection measures that apply to container gardens apply to plants that are still in their nursery pots. Group them close together in a sheltered spot and cover with frost cloth or mulch. Tip evergreen plants on their sides before covering in order to protect their leaves from the wind. Resist the urge to bring these plants indoors. Putting them in an unheated garage is fine, but they really don’t want to spend the winter in your living room next to a roaring fire.
Cover certain agaves.
Succulents such as yucca, sempervivum (hens-and-chicks), and sedum (stonecrop) are very cold hardy and won’t be harmed by the low temperatures. There are also a number of agave species (such as the ones planted on the berm in our parking lot, pictured) that can handle the cold like champs. However, if you have some of the more tender species such as Agave americana (Century Plant), you should definitely provide protection. Use frost cloth, bubble wrap, a plastic tarp, or even a big cardboard box to completely cover the agave from its spines all the way to the ground. Remember to anchor the covering with something heavy to keep it from blowing off.
Unsure if the species of agave you have is cold tolerant? Cover it. Even the really hardy varieties will appreciate a little extra protection from winter rain and extreme cold.
Mulch or cover newly planted perennials.
Newly planted herbaceous perennials – this means perennials that die back to the ground or to a small cluster of leaves at the crown of the plant – will need some extra protection in an extreme cold snap like the one we’re about to experience. The best protection is mulch. Use nuggets, shredded bark, pine straw, or fallen leaves. Apply a thick layer to all the plants. If you’re in a hurry, you can even mound it over the basal leaves at the crown of the plant as long as you remember to rake it back once the temps warm up again. The important thing is to get the shallow roots of the newly planted perennials insulated from the cold.
If you can’t mulch, or for plants that are in a rock garden or an area where organic mulch isn’t used, cover any newly planted perennials with frost cloth, making sure to anchor the cloth so it doesn’t blow off.
Here at the nursery, we lay down plants that won’t be harmed by being horizontal (note: don’t try this with Japanese maples and other plants with tender bud tips). We group the plants close together to reduce the amount of surface area exposed to the cold. Then we cover them from tip to toe, making sure to anchor the frost cloth by tucking it under the pots or weighing it down with bricks.
Plants whose roots are above-ground (such all our plants at Garden*Hood) are much more vulnerable to cold damage than are plants whose roots are well-planted in the ground. Remember, plants have been surviving winters sans frost cloth for millennia. If your plants are happily rooted in the ground and mulched well, chances of them living through these cold snaps without supplemental covering are excellent.
Watering deeply before a freeze is beneficial in two ways: it provides valuable moisture that the plants won’t have access to should the ground freeze solid for an extended period; and, through evaporation, it helps raise the ambient air temperature immediately surrounding the plant. This rule applies to plants in the ground and in pots. The only exception is succulents. Keep them as dry as possible during cold weather, and protect them from snow and ice with a heavy cardboard box or other covering.
There’s no need to water plants once they’re already frozen. Sure, their leaves may look limp, wilty, and off-color – all signs we usually associate with dehydration – but in actuality this is the plant’s way of protecting itself against the cold. Resources are withdrawn from the furthest tips of stems and branches in order to conserve energy and protect the core.
Drain hoses and outdoor spigots.
Remember to unhook and drain your garden hoses so they don’t freeze and burst. Drain your outdoor irrigation lines and leave exposed spigots dripping so the pipes don’t freeze.
After the Freeze
Remove frost cloth if temps rise.
If you’ve covered plants with frost cloth it’s okay to leave it in place for days (even weeks) as long as the daytime temperatures don’t rise above 50. You should remove the cloth once temperatures rise into the 50s so that plants can remain acclimated to the cold and not get too comfortable in their makeshift greenhouses.
Prune with caution.
Once the temperatures warms back up a bit you may have the urge to get out and prune. Winter is a fine time to prune most plants, but there are a few that shouldn’t be touched until later. In general, salvia and anything with silvery foliage (Artemisia, Buddleia, and Peroskia, for example) should not be pruned until very late winter/early spring. Cut them back too early and you rob the plants of important insulation and stored sugars needed to feed it through the winter. Plus, these plants have hollow stems, and pruning exposes that inner tube to the elements. Once rainwater builds up inside, freezing temps can cause the stems to rupture. Keep these plants in good shape by pruning them at the beginning of March when the weather is just beginning to warm up.
Also, resist the urge to fuss with plants that are still frozen. Allow plants to thaw and to go through at least a two-week recovery period before you begin assessing damage and taking drastic pruning measures.
Remember that in many cases it’s better to leave cold-damaged stems in place to protect the plant from further damage by future cold snaps. If in doubt, leave things alone until late winter/very early spring. Then, as plants break dormancy, let their new growth guide your pruning efforts.
(Pictured: Spotted, off-color leaves are evidence of frost damage at the tips of the stems of Viburnum ‘Reifler’s Dwarf’ in our our display bed. Notice that the mature leaves further inside the shrub are green, glossy, and completely undamaged.)
Go natural with grasses.
Ornamental grasses like Panicum (Panicum virgatum ‘Northwind’, right), Muhlenbergia, and Miscanthus turn nice and coppery in the winter. Leave these straw-hued beauties up throughout the season so birds can eat the seed and use the stems for shelter.
Plus, cutting grasses in the middle of winter makes them vulnerable to rot at the crown. Wait until late February/early March to cut them back to just a few inches above the ground. This side-steps crown rot and avoids letting last year’s tan stems get tangled up in the fresh new green growth.
Remember, frost isn’t all bad. It shuts down the mosquitoes, cabbage moths, and aphids that plagued us all fall. Plus, it adds a touch of sweetness to kale and other winter greens. It also puts plants into full dormancy, giving them a chance to rest and rejuvenate for a fantastic spring show.