The Trouble with Out-of-Whackness

February 7th, 2012

So here it is the second week of February, and Punxsutawney Phil – that famous weather-forecasting rodent – is telling us to expect 6 more weeks of winter.

No more snow-covered winter snoozes for central-Pennsylvania plants. (Photo by Netherlands Flower Bulb Information Center)

I’m wondering when winter is going to start, not when it’s going to end.

We’ve been hovering mostly in the 40s and 50s since November with just one half-hearted snow. That’s drastically different from the winters I remember when we’d have a snow cover most of the time from December through early March and at least a few bouts of zero degrees – or less.

This warmer weather is becoming the new norm, a change verified 2 weeks ago when the U.S. Department of Agriculture updated the official Plant Hardiness Map for the first time in 22 years.

Based on the more recent temperature data from 1976 to 2005, the map shows most of the United States is a half of a growing zone warmer than it used to be.

That means we probably can get away with growing some so-called “borderline-hardy” species that previously wouldn’t even have been sold here – something alert gardeners have noticed for years.

In other words, Harrisburg is the new Baltimore.

The USDA hardiness map is important because its zone ratings guide garden-center buyers and plant sellers on what’s likely to survive in a given area’s winter.

I just got done writing about what that means to gardeners for my Feb. 23 Patriot-News garden column, which you can also read here.

But what I want to talk about is what the hardiness map doesn’t tell you, and more importantly, how this out-of-whack weather can be a problem for plants.

While the map is a good general measuring stick and probably the most accurate version yet, its value is limited. It really only measures one thing – the average winter low in a given area.

It doesn’t tell us how long the temperature stayed that low or how often it got down to that point.

A borderline-hardy plant might well recover from a few hours of, say, single-digit readings. But if the temperatures go that low several nights in a row and stay that low for 8 or 10 hours each night, you’re probably looking at a dead, brown blob.

The map also doesn’t take into account how a plant might actually fare better in a slightly colder but snowier area than a warmer, drier one. Snow makes excellent insulation.

And it doesn’t measure how hot summers and adequate growing-season rain can prepare plants much better for the stresses of a cold winter.

What has me way more concerned than any of that is the weather extremes we’ve been seeing.

Plants are pretty good at adapting to gradual changes in weather and climate. They’re not very good at dealing with shock.

We saw that graphically last fall when that sudden pre-Halloween snowstorm came along and snapped apart trees that hadn’t yet lost their leaves.

We also saw what violent windstorms and 13 inches of rain in one afternoon can do to a landscape.

Wild and sudden temperature swings like we’ve also been seeing are much more harmful to plants than any extended mild spells or half-zone shifts in growing zones.

Plants take on their full winter hardiness when temperatures drop gradually on the front end and rise gradually on the back end.

This is the reason why we sometimes get no fruit on apricots and cherries. What happens is that the fruiting buds start to break dormancy as temperatures warm up in spring. But when we suddenly go down into the teens one cold night in early April, that can kill buds that otherwise could’ve withstood single-digit temperatures in their fully dormant stage.

A late cold snap can brown the tips of emerged bulb foliage. A really, really late freeze can take out the whole flower bud.

A lot of people are worried now about the extended mild weather that’s caused their bulbs to poke up. If we stay like this, no problem. We’d get a beautiful bloom season, probably just a little earlier than usual.

The problem is if we end up getting our “real” winter next month. If we get a sudden cold snap down into the teens or (perish the thought) into the single digits for a few nights, that most likely will brown out the leaf tips.

The worst combo is dual extremes. If the temperature gets warm enough long enough to cause the flower stems to poke up, and then we get that same cold snap, you can kiss your buds goodbye.

In fall, it’s not a bad thing for the temperatures to stay warmer longer, as they’ve done. Again, the threat is the shock factor. Harrisburg-hardy plants that go into dormancy as the temperatures gradually drop in fall are well prepared for a single-digit January. But that same plant hit with 5 degrees before it’s gone fully dormant could well see dieback.

There’s really not a whole lot we can do about these wild swings since even sticking with so-called “hardy” plants isn’t completely fool-proof.

The best strategy is knowing the microclimates in your own yard and taking full advantage of them.

Cut-leaf Japanese maples, for example, get beat up a lot less in cold early-spring winds when placed in a courtyard or on the leeward side of a building.

Passion vines, cannas and colocasias may actually overwinter when planted next to a west-facing stone wall instead of in a frost pocket at the base of your back-yard slope.

And you may find lady’s mantles and primroses do best in the north-foundation shade where they stay a little cooler in a hot summer.

But that’s just good gardening – the kind of thing that veteran gardeners have fun figuring out.

And that’s something that no USDA Hardiness Map can replace.