Sunday, May 15, 2011

Cold Hardiness- What does that really mean?

A winter like the one we just had puts roses through their paces. As I was pruning this spring, I found it fascinating to look at the range of winter damage in the cold hardy roses I grow. Some had no dieback at all. At the other end of the spectrum are the few that die back to within inches of the ground. Why is that, when these roses are listed hardy to zone 5?

Cold hardiness is the lowest temperature a rose can live through without injury. Varying amounts of dieback could the result of growing conditions the summer or fall before, as well as what the spring is like. Roses are better able to withstand a cold winter after a good growing season. It allows them to develop good rooting and branching systems and store food for the coming year. Roses that produce lots of new growth late in the growing season are more prone to damage. New wood does not have time to harden off. Roses need a gradual cooling in the fall to acclimate to the cold. See saw temperatures interfere with this process. A gradual warming in the spring is just as important to minimize damage. Early warming triggers some roses to bud out earlier and those buds can be damaged by the colder temperatures that soon follow. So the answer to the question is- some varieties are programmed genetically to adjust their physiology to cold weather better than others.

Some cultivars have no dieback at all. Roses that lose no more than ten per cent of their growth are called tip hardy. Once-blooming roses need to be tip hardy. They bloom on old wood (last year’s growth). If too much of that wood is destroyed, they will not flower. Roses that repeat bloom flower on both old and new growth. They can withstand more dieback and still flower well. Vigorous repeat blooming roses can die back to the ground (crown hardy) and still flower on the new wood produced that year. One needs to ask/know not only whether a rose is hardy in a certain zone, but whether it is tip hardy or crown hardy in that zone and whether it is a once bloomer or repeat bloomer.

I find that using crown hardy roses for groundcovers makes spring cleanup much easier. I cut off the damaged canes within inches of the ground, do whatever weeding needs to be done and reapply a heavy layer of mulch. The weeding and mulching would be painful operations if those same plants were tip hardy, as I'd have to be working under and around them. By June the ground between the roses will be covered with canes and flowers. I like the rest of my roses (shrubs, climbers and ramblers) to be at least tip hardy, as I like substantial plants. There are always exceptions, of course. Antike 89 by Kordes is the one climbing rose I grow that loses many/most of its canes each winter. It has very double, bi-colored blooms and shiny, dark green foliage that stays clean all season. This year, it died to the ground. By June, it will be at least 6 feet tall and covered with blossoms. By September, it will be 8-10’ tall. The new growth is extremely vertical, so it works well as a pillar. The canes are too stiff to fan out on a trellis or wind around a chain for a catenary.

It’s all about knowing the right rose for the situation you have. If you have any questions about what is the right rose for you, please ask.

1 comment:

  1. Cold Hardiness seems to be a little different for roses than other perennials. Not sure why though. Do most roses keep their foliage through the winter or do they all shed? Is there any relationship between keeping the rosehips on the plant and its ongoing health? Does it strain the plant at all through the winter.