Excerpted from the book Year Round Gardening Projects Excerpt by Oliver E. Allen line drawings by Elayne Sears.
While wild roses and other native species are assuredly programmed to endure the wide temperature swings of their areas, hybrid teas are for the most part descended from plants of frost-free or moderate regions and are thus naturally vulnerable to the onslaught of cold. Deep freezing breaks their cell walls and dries out their canes; overnight they turn black and are gone. So the prudent gardener must protect them.
The amount of protection a hybrid tea rose needs depends on the climate. If your temperatures stay below 20° F (7° C) for considerable periods of time without a predictable snow blanket to shield plants, protect your roses. If the ground freezes solid for most of the winter and temperatures are likely to drop below 10° F (-12° C), again without consistent snow cover, your hybrid teas will need covering. But if very low temperatures are infrequent and heavy rains keep the ground wet, the bushes are better left uncovered, as wetness encourages fungus diseases and other ills.
The time-honored method of protecting hybrid tea bushes is to pile up dirt around the base of the
bush. The process is known as hilling and is done just before the ground is likely to freeze solid. The hill should reach a height of six or eight inches (15 or 20 cm) if you are in a moderate zone, but at least 12 inches (30 cm) in colder areas. What the dirt does is conduct heat up from the ground (which even if frozen will be warmer than air whipped by icy blasts) to the crown and lower reaches of the bush; if that much of the bush survives the winter, the rest (even though blackened) can be pruned away in the spring and the bush will swiftly put out new growth to replace it.
As with most plants, the best guarantee of a rosebush’s survival through the winter is good care in the summer, particularly pest control. A vigorous bush will withstand cold’s rigors far more readily than a weakened one.
1. Prune back. A couple of weeks after the first frost in your area, but before deep freezing sets in, prune your rosebushes roughly to eliminate any dead or weak shoots or any that show signs of disease. Trim long canes by half and shorter ones by about a third. The aim is to reduce each bush’s overall bulk so that it can be protected most efficiently.
At the same time, to help keep your bushes from drying out in the winter wind, spray them with an antidesiccant not long after you have pruned them. By sealing in moisture, the antidesiccant minimizes the damage that can come from cycles of freezing, thawing, and refreezing in midwinter.
2. Bring dirt from elsewhere. In late November or early December, or whenever you sense that the ground is about to freeze, bring dirt from elsewhere in the garden and pile it around the base of your plants to the desired height. Lean, sandy soil is better than a humusy mix. Do not obtain it from between the plants, as that risks exposing roots and crowns to freezing and so defeats the purpose of hilling. Pat the mound firmly to make sure it encloses the canes snugly.
3. Add a mulch. Especially if you are in one of the colder zones, you will want to add some kind of mulch -- salt hay, bark chips, pine needles, leaves -- as extra protection, holding it in place perhaps with evergreen boughs or other branches. The mulch further minimizes the possible damage that can come from abrupt temperature swings in midwinter.
4. Be cautious about removal. In the spring, wait until the ground has thawed for good before removing the dirt. But then remove it promptly so that new growth is not damaged. Be sure to keep some extra mulch on hand to pile temporarily around the base of your plants in the event of a sudden late frost.
Excerpt from Year Round Gardening Projects by Oliver E. Allen
line drawings by Elayne Sears.