You’d think that an old-school horticulturist with decades of gardening experience would know how to keep his own plants from freezing. But, as in most cases in life, it ain’t what you know that counts, or relying on the nearly useless “hardiness zone” map; sometimes it’s what you actually do.
And sometimes even that isn’t enough. Though the vast majority of my winter blooming plants - the paperwhites, mahonia, kale, agaves - did just fine, last winter I lost a camellia, sago, three ligustrum, an 80-old cleyera, a trellis of confederate/star jasmine, nandinas, and a handful of Tennessee-hardy perennials. Those normally dependable plants were killed by a combination of a hard early freeze followed by a mild winter and then a late hard freeze that caught them off guard. All good plants planted well.
Here is what happened, based on pure plant physiology: Most non-tropical trees, shrubs, and flowers produce a sort of anti-freeze in their cells. However, it takes time for this material to be produced, for plants to “harden off” before extreme cold; this is triggered by shortening days in the fall and exposure to gradually-cooling temperatures. Trouble is, weeks of mild midwinter temperatures - like we have - cause cells to lose this protection, making them susceptible to temperatures they could normally tolerate.
Which proved again that, when it comes to garden plant choices, the newly updated USDA Plant Hardiness Zones map is of little practical use. It only indicates that on average our coldest winter temps will be about 5 degrees warmer, not the lowest possible, or how sudden it dips down, or how long it may stay there.
“Furthermore,” as stated in the USDA web page, “many other environmental factors contribute to the success or failure of plants. Light exposure, wind, soil type, soil moisture (or lack of), humidity, snow cover, winter sunshine, duration of cold, and even all-night summer heat can greatly affect the survival of plants.”
So here is what I am doing this week, after of course bringing in cold-sensitive plants, to moderate plants from this looming hard freeze: Set cold-hardy potted plants on the ground up against the house, water them and cover pots with mulch to better protect roots. Spread a decent layer of leaf or bark mulch on top of the ground under shrubs and around perennials. Soak flowerbeds and shrub roots beforehand to insulate roots and for the soil to radiate warmth. I usually cover a few extra-tender plants such as borderline cold-hardy perennials, heavily budded blueberries, roses and mums that were pruned but have new growth, etc.; but I am being smart about it - simply tossing a blanket may make my neighbors feel better, has very little actual benefit to their plants.
True, burlap and blankets can protect to a certain extent from drying sun and cold wind, and keep frost off leaves, but they don’t retain heat, which is important if it dips into the lower 20s. Go with impermeable plastic sheeting, using stakes to keep it off tender foliage and making sure it goes all the way to the ground to trap warmth radiating up from the soil. Use boards or bricks to keep the bottom on the ground. And vent the plastic a little when the sun comes out to keep from steaming tender buds.
Summary for today: Wishful thinking and horticultural expertise aside, to protect normally hardy plants from damage due to sudden steep drops following a spate of mild sunny weather, bring in valuable plants, set others on the ground, water and mulch everything well, cover sensitive plants with plastic overnight. And keep fingers crossed.
Felder Rushing is a Mississippi author, columnist, and host of the Gestalt Gardener on MPB Think Radio. Email gardening questions to rushingfelder@yahoo.com.
