We have a tendency to baby our roses, and sometimes we forget that Mother Nature has her own processes for preparing for winter, in spite of all the preparations we may feel we have to make. The purpose of this article is not to give you pointers on what to do, but rather to give you a better picture of what your roses are already doing for you.
A Bit of Botany
First, a little botany background. During the growing season, leaves are actively converting water and carbon dioxide to carbohydrates through photosynthesis (6CO2 + 12H2O ? C6H12O6 + 6O2 + 6H2O). These carbohydrates are the building blocks for all other structures in the plant, whether they simply combine with each other to form sugars, or whether they combine with other nutrients to form hormones, cell walls, nuclei, whatever. The plant’s vascular system, a series of tube-like cells, moves the carbohydrates, diluted in water, to various parts of the plant, where they are incorporated into the processes occurring there, whether it be new growth, flowering, or root development.
Of all the different chemicals manufactured by the plant out of these carbohydrates, hormones are the ones that direct and regulate activity in the plant. These hormones include auxins, gibberellins, abscisic acid, ethylene, and cytokinins. Some of these hormones accelerate growth, some discourage it. For now, it is enough to understand that the plant manufactures different amounts of these hormones, depending on certain environmental signals, and the change in hormone balance triggers changes in plant processes.
The Effect of Fall
In the fall, a number of conditions set off changes within plants that slow growth. Decreasing temperatures and shortening day-length (or increasing night-length) are two major conditions that start these changes, although reductions in water and nitrogen will also stimulate changes. These events trigger the production of abscisic acid, which is a general growth inhibitor. Abscisic acid also affects the stomatal guard cells; an increase in production causes guard cells to collapse and close the stomata, slowing the expiration of water. Times of drought tend to stimulate production of abscisic acid, reducing expiration and increasing root development. (That’s why we recommend stopping supplementary watering in the fall; it helps trigger the production of abscisic acid. And since abscisic acid also promotes root growth, we recommend moving or planting dormant bushes in the fall.)
In roses, as in other woody, deciduous plants, the carbohydrates produced in photosynthesis are no longer needed for growth, so they begin to move from the leaves to the canes, usually in the form of sugars, leaving behind various waste products. Ethylene and abscisic acid (at least so some scientists believe) then affect a section of specialized cells at the point where the leaf attaches to the cane.
These cells are sensitive to the hormones, and as the concentration grows, the walls of the cells disintegrate, releasing the leaf, and leaving behind a protective layer on the plant where the leaf was attached. (This same process causes petal drop; if you store a rose in a refrigerator with apples, which give off ethylene, the petals will fall.)
Preparing for the Freeze
The next stage of preparation involves getting ready for freezing temperatures. If the plant did nothing, ice crystals would form within the plant cells, bursting cell walls or dehydrating plasma membranes. When spring arrives, the cell tissues would begin to rot, resulting in the brown pith we often find when we prune.
To prepare for freezing temperatures, plant cells have developed a structure that compensates. In a normal winter, the exterior of a cane would freeze first, with a thin layer of ice forming from water in the air. Then the water between cells would freeze. As more water between the cells freezes, water within the cells is drawn out through the cells’ permeable walls. But those walls allow water to pass much more easily than the sugars and other solutes, so what remains within the cells becomes much more concentrated, with a lower freezing point, and acts like antifreeze. The sugars that moved to the canes were taken up by the cells, increasing the initial solute concentration. Without the extra sugars, the freezing process would remove too much solution, resulting in cell dehydration and injury.
If the exterior of the cane should warm up, either under winter sun or during a brief warming period, the water between the cells would normally remain frozen, the concentration of the solution within the cells stay the same, and the cells would survive the temporary thaw. If the warm period lasts long enough for the water between the cells to thaw, water seeps back into the cells, diluting the solution and raising its freezing point. Then, if a sudden cold freeze hits, the diluted solution might form ice crystals, destroying the cell. So, the plant cell faces two dangers from freezing temperatures: dehydration if enough sugars are not present to retain a liquid “antifreeze”; and ice crystal damage if water dilutes the “antifreeze” enough to raise its freezing temperature.
When spring returns, the plant begins to manufacture auxins, stimulating new growth. The new growth taps the store of carbohydrates in the canes until leaves develop and the vascular system can deliver water from the roots to the leaves. If the cells have been damaged, the carbohydrates are still available in the nearby shattered cells, but with the vascular system destroyed, no water can be pulled from the roots to the new leaves to feed photosynthesis, so the new shoot withers and dies.
When we look at the purpose of winter protection, it is not to keep the bush “warm,” but to moderate the rate at which the bush freezes and thaws. A winter cover that allows water to seep through and freeze causes no danger to the bush, nor does frozen soil. (Frozen soil is more a problem when ice crystals freeze out of surface soil, causing frost heaves, snapping roots; since this occurs only where there is room for the ice to expand (i.e., the surface), mulch or winter cover would move this heave zone to the surface of the mulch or mound, well out of reach of the roots.) Should the water in the winter cover freeze, it would do so slowly, and as warm temperature or sun returns, it would thaw slowly, giving the bush time to adjust.
Water in soil (and on cane surfaces) will freeze at 32 to 23ºF (depending on what is dissolved in the water). Water between the cells (called intercellular water) freezes at 23 to 14ºF. The concentrated solute within the cells freezes at -4 to -40ºF. If a plant has time to prepare, it can withstand quite cold temperatures.
What about roses in containers? Roots are generally not subjected to the harsh temperature changes that the above-ground bush needs to withstand. Although roots do cold harden somewhat, they do not do so as dramatically as the rest of the bush. I found no hard figures for roses, but other species of woody plants suffer root kill at temperatures as wide as 23 to -9ºF. From my own experience, I have had little loss of miniature roses in unprotected containers (one- and three-gallon pots) in a “cold house” with temperatures down to 20ºF, but more significant loss of 1-gallon plants when night temperatures dipped to 15º F for an extended period.
All this discussion about moving gracefully into winter assumes your bushes have been healthy and vigorous all summer. If your plants have been under stress – lack of water during August and September, or leaf drop from blackspot or spider mites – the plants have not been producing normal amounts of carbohydrates. As winter approaches, the bushes will not mysteriously manufacture more. The concentrations of sugars in the canes will be lower than normal, and the potential for freeze damage higher. If such is the case for your bushes, get your winter protection onto your rose beds soon after the first couple of heavy frosts…and pray.
Otherwise, take your time with winter cover. Let the bush adapt naturally. Give the bush time to give up its leaves on its own, with a little assistance only if needed. Then give your bushes the extra measure of protection, especially the crown and roots, as winter progresses and really cold weather threatens. For bushes in the ground, I generally wait until temperatures threaten to drop below 20ºF, and then protect those bushes that are exposed to winter winds, covering the crown and about 6″ of cane. Many of my bushes, especially those on their own roots, I give no extra protection at all. And they usually do a fine job of winter protection all on their own.
From Rose Petals, the website of the Seattle Rose Society. Originally published in the November 1998 newsletter.