How . . . cold . . . is . . . it?




by Charles Kidder

How many of you recognize that line from a TV show of many years ago? (The answer, at the end of this month’s column.) Of more immediate interest to gardeners: how cold does it normally get in our area, and how does that affect your choice of plants for the garden?

If you’ve gardened for any time at all, you have probably heard the term “zone” tossed around, as in, “Albemarle County is in Zone 7.” These zones appear on a map published by the United States Department of Agriculture in 1990 and indicate the average annual minimum temperature experienced across the country. Each zone covers a 10-degree temperature range, e.g., from 0 to 10 degrees for Zone 7. (And you’re right if you say that’s an eleven-degree range. I have never been sure if zero degrees belongs in Zone 6 or Zone 7-or both. Take your pick.) The USDA map further divides each zone into “a” and “b” sections; we are in 7a, with an annual minimum between zero and 5 degrees.

The USDA map of 1990 is based on only thirteen years of record, from 1974 through 1986, and superseded an edition published in 1960 that generally showed a warmer picture of the U.S. Then in 2003 the American Horticultural Society put out a map based on the years 1986-2002, and this showed a return to warmer averages, with many areas of the country now in a higher zone. However, this map was pretty much booed off the stage. Many thought it put too much emphasis on recent warming; also, it eliminated the refinement of the “a” and “b” divisions.

I’d argue that the longer the temperature record, the better, in order to capture more extremes. Unless the climate is changing, that is. Then, the more recent records would be more relevant. So, global warming would seem to indicate that we could throw out those older/colder years. But consider: does Global Warming=Crozet Warming? (It appears to, but I would invite comments from the State Climatologist on that.)

Regardless of overall trends, climate still involves a lot of “noise,” or fluctuation, from year to year. And those fluctuations are what may end up doing in your plants.

For example, 10 degrees below zero is the lowest temperature on record for Charlottesville, with data dating back to the late 1800s. However, we hit that frigid temperature very recently-January 1994, to be exact. And just this past January, Charlottesville got down to -2, officially putting us into zone 6 for that winter.

But did all your Zone 7 plants croak this winter? I doubt it, although it may be a bit too soon to tell for some things. Although this zone system is handy, it’s based on only one statistic, the minimum temperature. It neglects many other factors: the duration of the cold temperatures, wind, and snow cover, for example. All of those can affect the viability of plants, but would be difficult to incorporate into one single parameter.

All these fine points of minimum temperatures aside, is there anything you can do to enhance the survival of your plants, other than either building a greenhouse around them or packing up everything and moving to the coast of South Carolina? Most assuredly, yes, and it gets back to the old mantra of “right plant, right place.”

Many plants come with a tag or label that indicates their hardiness limits, either by zone or by actual temperature, such as “Hardy to 0 degrees.” If you’re buying from a local retailer, it’s highly unlikely they would be stocking plants that are not hardy in this area. Mail-order outfits are another proposition, however, since they may be based in places like East Nowhere, Alabama. Still, they almost always quote zone hardiness in their catalogs, sometimes even getting down to such detail as, “Has survived -9 degrees in our garden.” So you should be able to make an informed purchase.

Let’s say you decide to buy a plant that’s on the edge of its cold-hardiness. Although widely grown in this area for years, I would put camellias into this category. Breeding has given us many varieties that are hardy even to Zone 6, but I would still place them in the landscape with care. Like many broadleaf evergreens, camellias are subject to “burn,” or a bronzing of the leaves when subjected to cold, sun and wind. Ideally, they should be planted where they get a lot of filtered light, but are shaded from the direct rays of the sun, perhaps under a canopy of tall pines. Also, use either a building or larger evergreens to shelter camellias from drying north and west winds.

If your property has a fair amount of relief, avoid planting sensitive plants in the low spots. Although temperatures decrease with elevation as a general rule, cold air is heavy and will sink to low areas, sometimes known as frost pockets. This phenomenon is especially noticeable on clear, still nights when no winds are mixing warmer and colder air. Mid-slope elevations on mountain ridges can often escape late or early frosts that hit lower areas. Such areas are known as thermal belts, and commercial growers take advantage of them.

Winter hardiness of plants is not an all-or-nothing proposition. Cold weather might blast either flowers or their buds, but leave the rest of the plant unscathed, so not a big deal. (Unless you were counting on fruit from those flowers, as in apple or peach growers!)

Damage might only affect the smaller twigs on a shrub, but leave larger branches intact. Exceptional cold might cause all the leaves on a broadleaf evergreen to drop, but the plant will recover come spring.

Moving further down the scale of cold damage, the entire above-ground portion of a plant can be killed, with only the roots surviving. Not good by any definition; still, a plant like a crape myrtle can send up shoots the following spring that will flower that same summer.
And ultimately, sufficient cold can kill the entire plant. Kaput. Gone.

But if you’re the adventuresome type, don’t always take published reports of a plant’s hardiness as gospel. With some careful planting, maybe you can grow a Zone 8 plant in your garden. Remember: plants don’t read.

And the answer to the question posed at the top: In its early days the Tonight Show came from New York. If the day had been unusually cold, Johnny Carson might start his monologue with, “It was cold in New York today!” (Dramatic pause.) Ed McMahon would say, “How…cold…was…it?”

Johnny: “It was so cold, the muggers in Central Park…” Well, you’ll have to furnish the rest of the punch line!