American Rose Annual pp. 54-56 1924
The Principles of Plant Physiology Relative to Frost Damage
By Dr. R. B. HARVEY, Associate Professor of Plant Physiology
University of Minnesota, St. Paul, Minn.

THE winter-resistance of rose varieties is one of the most important factors in their growth in northern climates. All too frequently the descriptions of varieties are too optimistic in stating the hardiness of the plants. These statements are probably based on enthusiastic reports from northern states of the varieties living through mild winters. They do not take into account the cold, barren test-winter which seems to recur about every ten years. To be listed as hardy in a region, varieties should be able to withstand these test-winters. There must be more accurate information on the hardiness of rose varieties, and in obtaining this the physiological reactions of the plants must be taken into account.

Of the factors concerned in winter-killing, the fluctuation of temperature is often more important than extreme low temperature. Especially is this the case where the plants are actually exposed. We suppose that plants from northern Canada will be more hardy than plants from the plains area, yet this is not true, for a heavy snow-cover may allow plants to survive which would die under open and snowless exposure in the northern plains area. The hardiest native plants are those found in the cold continental interiors where the snow-cover is light in winter.

FIG. 1. Graph showing rapid fluctuation of temperature, with freezing and thawing of the live bark on the sunny side. The air temperature is relatively constant.

Rose plants exposed to the sunshine in winter are subjected to alternate freezing and thawing. When the temperature is a few degrees below freezing, ice will form in the tissues of the stem. The south side of the stem, being exposed to the sunshine, is heated by the absorption of the light energy and will quickly thaw. With every cloud which obscures the sun for more than a minute, the live bark will freeze, only to thaw in the next few minutes after the cloud passes. The north side of stems, not being exposed to the sun, is not frozen and thawed in this manner in rapid succession, and the tissues are not as frequently killed.

The temperature in the thin layer of bark can be accurately determined by thermo-couples. The graph of temperatures (Fig. 1) obtained in this manner will give an idea of the temperature fluctuations which exposed black stems must withstand in the Twin Cities.

*Erlanson (1934) reported on segregation of stem color
and waxiness in self-seedlings of some wild roses.

*The color of the stem determines the amount of light energy which will be absorbed and the heat which will be produced in the bark as a consequence. Not all colors of barks can be had from one variety of shrub or tree. In comparing the heating effect of sunlight on different colors of bark it is necessary to use different types of plants. A white bark, such as that of the paper-white birch, reflects a large part of the light and does not thaw as easily as a bark such as black plum. Green-colored barks absorb the red and violet rays and reflect green rays. Red barks reflect red rays. Most of the energy of sunlight is distributed in the red rays, consequently red barks are less heated in sunlight than green barks.

Black or brown barks, which absorb nearly all of the light, are heated most of all. Shiny mirrored surfaces on the bark, produced by waxy coverings, aid in light-reflection and tend to decrease the injury.

Many varieties of stems have the ability to produce red pigments in the bark of the south side in winter. This is a distinct protection against winter-killing which has enabled many shrubs of the northern plains area to survive.

Coverings of wax on stems protect them from excessive desiccation by dry winds at times when moisture cannot be taken from the frozen soil. Such coverings also protect against freezing, since they allow the tissues to undercool below the true freezing-point without ice-formation in the tissues. The development of woody tissues and the production of gums in tender shoots during the autumn make them more resistant to freezing and to drying. The production of sugars from starches of the stem in the autumn also tends to prevent injury.

Some varieties are easily stimulated to growth during warm periods in spring. Subsequently, such varieties may be killed by a single night with low temperature. This break in dormancy can be prevented or decreased by coverings which do not allow the plants to warm up in a short time.

The question of varietal differences in hardiness rests upon a great complex of physiological adaptations of plants to the environment. More accurate determinations of the hardiness of varieties must be made under exactly the same conditions of exposure. The temperature zones of this country have been recorded for many years. We should have equally well defined the hardiness of each variety of rose so that we may know the exact temperature limits in which the varieties are suitable for planting. The determination of hardiness of varieties has usually been made by field-trials. These tests are reliable only when they have been carried through many years.

A certain winter may be mild, so that only the tender varieties are killed, or it may be so severe as to kill all varieties. Obviously, small differences in hardiness would not show up under these conditions. Definite results on hardiness of varieties can be had only by exposing the plants to an artificial winter which can be exactly controlled, and in which the treatment of the plants before and alter the trial can be controlled. Such tests in Minnesota have given very reliable results.

CybeRose note: Hardiness in roses is not a simple trait. To be fully hardy in a given climate a rose must go dormant before the first killing frosts of Autmn, remain dormant during warm warm spells that may occur in Winter, and continue to sleep (or grow slowly) late enough in Spring to avoid late frosts.

Early to bed …
Bugnet: The Search for Total Hardiness (1941)
"I have often noticed that half-hardy plum or apple trees here, unhurt by December 1, passed unharmed through the rest of the winter no matter how intense the cold.

Late to rise …
Rosen: Resistance to Spring Freezes (1956)
"In general those varieties which have an inherent tendency to break their dormancy ahead of other varieties are apt to suffer more from late spring frosts and freezes."

And grow slowly at low temperatures.
Risley: Male Controls Sprouting (1958)

Bark is also a consideration.
Harvey: Rose physiology and hardiness (1924)
Red barks reflect heat; wax protects against desiccation.

Heat and Growth Bibliography