How Cold Was It?

That’s a question that gardeners and farmers ask each other all of the time. The talk really heats up at several key times of the year: mid winter, when extreme low temperatures occur; late spring, when frosts can damage tender new growth and blooms; and again in the fall, when the first “killing freeze” signals the end of the growing season.

It is supposed to get a bit chilly later this week so I am sure there will be lots of conversation about the season’s lowest temperature, to date. The weather women (a major improvement over the weatherman) are talking about single digits on Thursday and Friday nights. Since most of Bucks County, PA is in USDA Hardiness Zone 6, that’s normal. In fact, if temperatures stay above minus 10 degrees F we are still within the normal average range for zone 6.

If you are not familiar with the concept of hardiness zones, click on the USDA link shown above and educate yourself. Zone designation is based on records of average low temperature that occurred from 1974-1986. The country is divided into 11 zones. The lower the number, the colder it gets.

Believe it or not, there is weather outside of the USA. I recall taking a stroll in Skibbereen, southern Ireland, a few years ago and admiring the Fuscia hedges. They were obviously hardy perennials there. This is Zone 9. The locals said it rarely went below freezing. The hardiness map says the average low temperature range is 20-30 degrees F. I note that my gardening pals in Nuremberg, Germany are zone 6, just like us. I would expect that the procedures I use for growing garlic would work there as well. Cold hardiness is a useful, international concept.

However, cold hardiness is only one important factor in plant growth. For instance, although the average low temperatures in southern Ireland look good for figs, I wonder if they get enough heat to ripen a crop. Rainfall, day-length and many other factors play a role in plant survival. I’m told that although Norway Maple is cold hardy in the southern states, it struggles under the high temperatures. Same thing with currents and gooseberries.

There isn’t much you can do about the weather but it’s important to consider winter hardiness when deciding what plants you are going to grow. Professional horticulturists and plant sellers use the USDA hardiness zones as an official and useful guide.

Of course, gardeners are always pushing the limits. I’ve got a fig tree and crape myrtle in my yard and they are both of these are zone 7 plants. I expect them to be severely injured by low temperatures now and then. Both have been killed to the ground. They re-grow from below ground parts that were not exposed to the killing temperature. Pushing the limits of hardiness zones is OK for the amateur but you’d be nuts to start a fig farm in zone 6.