Much has been written in recent months on hardiness. The many facets of the term have been discussed with explanations of each, but I suspect there will continue to be unexpected losses.
Perhaps the ability of a tree, like a man, to withstand extremes of heat or cold depends upon general robustness, in addition to the factors of inheritance and environment.
Thin blood and a watery sap, unsaturated with plant food essentials, both may indicate an inability to contend with the weather’s extreme vagaries.
Tough Winter In The Northeast
Because of my interest in evergreen hollies, I have striven to assemble information about the qualities and factors that enhance their distribution and appeal.
Horticulturists in Pennsylvania and nearby states, who handled English and Chinese hollies in former years, speak of the winter of 1033-34, demonstrating that such species are not hardy in the Northeast.
That was a tough winter. February 1034 was the coldest month in Philadelphia, with an official low of minus 11° degrees Fahrenheit on the 9th.
In suburban areas where much English holly was growing, the mercury tumbled to 16 or more below.
Such an extreme is a test, and a plant’s ability to survive is one criterion of hardiness. Many evergreen hollies suffered in that cold.
Some trees died; others were killed to the ground line, later to grow again, while many more were damaged but not killed back. Occasionally, an English holly in the same general location showed little or no damage.
Other Plants Injured During Winter
Hollies were not, however, the only trees to be injured during that Winter.
Sweet Cherries
Sweet cherries, suffruticose boxes, and many hybrid rhododendrons that thrived for decades were killed.
Flowering Dogwoods
Even flowering dogwoods, native to Pennsylvania, died in that sub-zero blast, as also did Stayman apple trees, a favorite of Keystone orchardists.
Perhaps such an extremely low temperature for a given locality is a hardness test, though I doubt it is fair.
Certainly, it cannot be considered a test of tenderness, and else the indigenous Cornus florida would have to be classed as tender in its native habitat.
Plant life in Oregon took a beating last Winter due to periods of unusually low temperatures.
On a visit in June, I was impressed as much by the plants which did not suffer, such as the monkey puzzle tree, as by those that could not stand such weather.
American Holly
The American holly, Ilex opaca, which grows well around Detroit, in the Catskills, and through the rigorous Winters of Massachusetts’ North Shore, fared worse than the English holly.
To my knowledge, only one nursery specializing in English holly lost heavily. Others within five miles on similar contours showed little or no injury.
English Holly Orchards Of Oregon And Washington
Generally speaking, Oregon and Washington’s extensive English holly orchards suffered little.
Externally, the only variation from normal was an early drop of last year’s leaves, making the trees look sparse.
The argument is that English holly does well in the Pacific Northwest because the heavy and evenly distributed rainfall does not appear to be borne out by facts.
At Bellevue, Washington, across the lake from Seattle, where I saw a fine holly orchard, the annual rainfall averages six inches less than in New Jersey or Connecticut.
Annual Precipitation
While the winters are almost continually cloudy, with frequent drizzles, little or no rain falls during the Summer months.
In Portland, Oregon, the annual precipitation is about the same as on Long Island, with most occurring between October and May.
Artificial Watering
In Summer, artificial watering is sometimes resorted to in the holly orchards. While the average daily Summer temperature in Portland is not as high as in eastern Pennsylvania, with less Summer rainfall, semi-drought conditions are not uncommon.
Of course, there is more to hardiness than the ability to withstand low temperatures and deficiency in soil moisture.
Changes In Temparature
Rapid temperature changes are trying for man and plants. Some observations from a few years ago indicate why plants, such as English holly, exposed to the late afternoon sun fare worse than those facing the east or north.
On a March day, the thermometer stood at 39° degrees Fahrenheit at 7 A.M. With the bright sun.
The temperature rose rapidly, so the reading at 3 P.M. on the southwest side of the house showed 78° degrees Fahrenheit.
At the same time, the mercury stood at 53° degrees Fahrenheit on the east side of the house. That night there was a drop to 24° degrees Fahrenheit, making a temperature spread on the east side of 29° degrees Fahrenheit, while on the southwest, the change was 54° degrees Fahrenheit.
Perhaps these figures mean nothing, but I suspect a drop from 78° degrees Fahrenheit to below freezing in 10 hours would not tend to make a holly robust.
At least, that is my explanation for the fact that English hollies in Pennsylvania and New Jersey, exposed to the north or east, generally do better than those which receive the full heat of the afternoon sun.
Climatic Hardiness
Climatic hardiness is an indefinite, largely inherent, though I believe it can be acquired.
The acclimatization of plants out of their range is a complex process that may be determined as much by soil, exposure, and atmospheric variables as inherent tendencies.
44659 by H. Gleason Mattoon