SHOP PRODUCTS
Houzz Logo Print
michaelg_gw

How winter affects roses--dormancy etc.

michaelg
10 years ago

There's a lot of misinformation and misunderstanding out there on this topic. Basically it derives from not getting the distinction between two basic types of roses.

1. Once-blooming roses such as gallicas, albas, and temperate-zone species: These are fully deciduous and go into deep dormancy. They respond to the shortening days of autumn by withdrawing nutrients from the leaves. The leaves turn color (bright color in some cases) and drop without being frozen. These roses need to experience some degree of dormancy in order to bloom. Only those canes that have been through a winter are able to bloom. ("Blooming on old wood.") In a temperate climate, you cannot stop them from going dormant. They will stay dormant until mid-spring. In a tropical climate, you cannot make them go dormant.

2. Typical modern repeat-blooming roses, such as hybrid teas and those shrubs that repeat freely: These derive their basic life-pattern from the species R. chinensis, which is sub-tropical. They are naturally evergreen and will happily grow and bloom all year if the climate permits. These roses do not need or want to rest, and you cannot make them rest except by withholding water, which is not good for them.

Leaves of china-derived roses do not naturally drop in fall. In severe winter climates, leaves drop after they have been killed by the cold. In zones 8-10, old leaves drop gradually in spring and early summer as new leaves replace them. If you strip healthy leaves during mild (zone 8b-10) winters, you are just wasting some of the plant's energy. (If leaves carry fungus, that's a good reason to strip.)

Obviously, china-derived roses are not strictly subtropical, but are reasonably well-adapted to winters down to around 5 degrees F. In response to a few weeks with low temperatures in the 30s and 40s, they raise the sugar content in their stems to provide antifreeze. I know of no evidence that any cultural practice can help roses prepare for winter, beyond keeping them healthy and preventing defoliation by blackspot.

Modern roses do not rest until they are bludgeoned into inactivity by cold weather (mid 20s and below) that damages the leaves and kills the soft new growth. They will stay semi-dormant as long as the average daily temperature stays below around 40 degrees. They will start growing after a couple of weeks of milder weather at any time. In zones 6-8 you may have a problem with roses growing out in winter or early spring and wasting energy on new growth that will be frozen later. But there is nothing the gardener can do about that.

With typical modern roses, soft new growth is injured at around 25-27 degrees, woody green canes at around 5-10 degrees, and old canes at around 0-5 degrees.
...........

Of course, not all roses fit into these two categories. There are once-blooming roses that are not fully deciduous and hardy, and there are hardy deciduous roses (rugosas) that give repeat bloom.

This post was edited by michaelg on Sat, Sep 7, 13 at 11:18

Comments (31)