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Last week’s column presented the grim effects of severely cold weather on landscape trees and shrubs. Our region, like much of the country, logged record-breaking temperatures – below zero for several days – and extreme winds that drove the wind chill factor to as low as minus 20 degrees in some locations.

Wind is a threat that we should never underestimate. To people and animals, the wind chill factor means that the wind makes the temperature feel like that low number. The wind chill factor describes how uncomfortable we feel and how seriously the cold weather could put us at risk for hypothermia or frost bite. While we don’t have an exactly parallel wind chill measurement system for plants, wind greatly exacerbates the dangers of extreme cold. Evergreen plants (whether they have needles or broad leaves such as rhododendrons and hollies) are especially vulnerable to wind damage.

The big picture

Remember – a quick review – the several ways that winter conditions harm plants: Frost heaving, blow-overs, ice or snow smashing or breaking them, bark cracking or splitting, needles browning and dying and root death. Plants in the right site, well-established, that were well watered before winter are normally the least likely to be damaged. Anything recently planted, stressed by drought or changes in its environment (construction, utility trenches), or marginally hardy, is more likely to show winter damage.

Understanding that, which plants are most at risk?

After normal winters (with a few days below 10 degrees), landscapers and nurserymen can predict questions and complaints about certain plants. Arborvitae, boxwood and hollies often show brown patches with dead needles/leaves on the side facing the wind. Japanese maples and other thin-barked trees often have cracked bark. Yews, junipers and dwarf conifers are split or crushed from ice crashing down. Many hydrangea buds are killed by a late frost, magnolia buds ruined in a windstorm. Poorly pruned or weak-branched trees such as silver maples or some pears lose branches during storms. Many of these typical landscape problems are normal for the plant. Many can be prevented by better plant placement, selection, proper pruning, or by using anti-desiccant products, shrub covers or other barriers.

A different winter

This was not a normal winter so far, and it threatened plants that have flourished for years in our landscapes and professional nurseries. At MANTS (the mid-Atlantic Nursery Trade Show) in Baltimore last week, growers from North Carolina to New England discussed temperatures and wind chills they hadn’t seen in decades – a minus 6 in New Jersey, for instance. They asked each other, “How bad was this? Which plants do you think you’ll lose?”

Russ Gullo, a CNLP and owner of Russell’s Tree & Shrub Farm in East Amherst, the largest distributor of nursery stock in Western New York, was in several of those Baltimore discussions. He told me that most of the professionals said they just don’t know yet which plants will make it, which will die and which just won’t bud and flower.

“We know from long experience which of the tried-and-true plant species and oldest cultivars are the most winter-hardy, but we’re using thousands of newer cultivars and we’ve all introduced plants we used to consider just marginally hardy. (Our zone’s been reclassified, and consumers want them). So we’ll see ... spring will be very interesting across the whole Northeast,” he explained.

You might have experimented with the hardiness zone limits in 2013, and possibly the crape myrtle or Kalmia won’t make it. You might have bought an improved dogwood, crab apple, redbud or magnolia tree or one of the newer azaleas. If yours dies or loses its buds, don’t blame the seller or necessarily yourself. Severe temperatures, that came on so fast so that the plant couldn’t acclimate, are a likely cause.

Teresa Buchanan, a CNLP and Lockwood’s Greenhouses manager, also warns us to be patient in spring about what’s dead or alive: “I fully expect some shrubs such as Buddleia (butterfly bush), Vitex and Caryopteris to be killed right back to the ground – sometimes they leaf out high on the stems. Don’t give up on them.”

One encouraging note: The horticulture industry continually works to develop plants that can withstand extreme cold, including some rhododendrons, roses and even hardier (and bud-hardier) hydrangeas. Spring won’t be all bad news.

Potted plants

Last fall we gardeners rushed around storing all the potted plants we hadn’t planted. We put them against the house out of the wind, in the garage or sunk them in trenches. A blanket of snow gave us some consolation when we heard the deep freeze was coming. Maybe we dumped extra mulch and later the Christmas tree boughs on top. There wasn’t more we could do.

Now imagine if you had an acre of those plants, your garden center’s investment in the season to come. Most plant businesses can store only a percentage of potted trees, shrubs and perennials in a poly house (an unheated greenhouse covered by plastic). Such houses keep the temperature 10 or more degrees above outside temperatures, slow temperature fluctuations, and – even more important – keep the wind away. Russ Gullo used an insulation blanket over some of the most vulnerable plants, gaining 6 degrees. Professionals have to gamble. It’s a science and an art to decide which plants to store in the limited protected space available.

Most of a nursery’s stock is outside, the root balls or pots healed in (sunk into) the ground or covered with wood chips or clustered together. When roots are just inches away from outside temperatures, plants can die quickly if the cells are exposed to temperatures lower than their tolerance. Ultimately, without protective measures these plants are vulnerable to whatever nature throws at them.

So what will spring present to us and the nursery professionals we’re counting on? How many plants will have died in our yards and garden centers? Good gardeners and smart professionals tried to decrease the risks, but nature dealt our plants a tough hand this January. We will simply have to wait and hope. Only time will tell.

Sally Cunningham is a garden writer, lecturer and consultant.