In the past week, temperatures have plummeted to -50F in the Dakotas, and Florida's Citrus crops have been under assault by the cold. Many people will use a hot spell to try to prove global warming, and a cold spell to prove global cooling.
Climates and weather patterns fluctuate. That's what they do. Hardiness maps, such as the USDA Plant Hardiness Map, are meant to be used as a guideline for what plants can survive the coldest temperatures expected in an area. Since some years are colder than others, hardiness maps need to reflect what the coldest temperature may be over a period of years.
Woody perennial trees and shrubs, as well as herbaceous perennials, may live for many years, decades, or in some cases centuries. While years may sometimes pass with out a particularly brutal cold snap, hardiness maps need to reflect temperatures which may be incurred over the lifetime of a plant.
Southern magnolia (Magnolia grandiflora) is a wonderful evergreen tree native to the southeast United States. This native tree can become quite majestic and is planted extensively in the warmer parts of the United States. Because this is such a beautiful and versatile tree, people who live north of its native range try planting it, and they get away with it for many years in some cases when winters are mild.
Cincinnati is well north of southern magnolia's range which is listed as 7a (southern Tennessee). For years when winters were mild, southern magnolia was planted in Cincinnati, and some of these plants became quite large. Then, in 1977 and 1978, two brutal winters challenged the hardiness maps and any plant which dared to grow north of its range. Temperatures dropped to -25F in 1977, and bitter cold and snow prevailed for many months in 1978.
Southern magnolias in Cincinnati were devastated during these winters. A number of trees were killed, while the majority suffered major damage. Some recovered, but it was a stark reminder of what happens when we challenge hardiness zone designations.
Since the late 70's. temperatures have dropped to -20F or colder at least twice in the 1980's and again in the 1990's. Just two years ago, temperatures dropped to -14F at Hamilton-Fairfield Airport in Butler County. Even so, proponents of global warming are saying temperatures are warming and hardiness zones are moving north. In contrast others, including Cincinnati Weather Examiner columnist Rich Apuzzo, is writing about the prospects of global cooling. An article in a journal of the International Society of Arboriculture recently referred to global warming as if it was fact.
Meanwhile, it has been a number of years since devastating freezes visited Florida, but the cold there is brutal this year and the resultant damage is expected to be devastating. In recent years, Cincinnati gardeners have increasingly been adding marginally hardy plants such as crepe myrtle (Lagerstroemia sp.), and mimosa (Albizia julibissin) to their landscapes. At the same time, recognized experts are telling people that global warming is moving hardiness zones northward and inferring that it is a good practice to increase the use of these marginally hardy plants.
Only time will tell if climatological conditions truly warm or cool but, given the lessons of history, it is only a matter of time before a repeat of the brutal winters experienced in the 1970's makes another visit, providing a brutal reality check for those who have pushed the envelope.
In the years since the 1970's, research has marched on. The U.S. National Arboretum has conducted extensive research into cold hardy crepe myrtle varieties. While there is still not a reliably hardy crepe myrtle for Cincinnati, many crepe myrtles are considered to be root hardy, meaning the roots will survive the winter but the above ground portions of the tree are subject to partial or complete die-back.
Additionally, a southern magnolia is now available, named Bracken's Brown Beauty, which is considerably more cold hardy than those available in the 1970's. In fact, this writer feels confident in recommending Bracken's Brown Beauty in the Cincinnati area. In very cold winters, the leaves may brown and even fall off, but the twigs and buds survive and show no damage. Trees planted in Cincinnati have survived -20F without lasting damage.