Cold winter may have hurt your roses, and you may need to take action this spring

It’s going to be interesting to see how our roses and fruit trees endured this prolonged cold winter. Most gardeners in this area don’t do anything special to protect roses against winter because our winters are usually pretty mild, with super cold snaps lasting only a day or two.

The blanket of snow may protect more than we expect, though. Some studies have shown that the temperature beneath a snow blanket is up to 25 degrees warmer than the air above. Since roots are more vulnerable than above-ground canes and foliage, the persistent blanket of snow may be a life saver as far as roses are concerned.

In spring, when it’s time for rose foliage to begin unfurling, and some canes don’t appear to have viable leaves emerging, use a fingernail to scratch a cane. If it’s dry and black beneath a green exterior, it was winter killed. If it’s reddish and wet, the bush has the pseudomonas disease that has appeared in this valley a few years ago. We call it bacterial rose cane blight.

If your rose is growing on its own roots (that is, not grafted onto a hardier root), it could come back if killed to the ground. If it was grafted, the root may send up suckers that will bloom quite differently than the rose blossoms you expect. That would be a good time to replace the rose, excavating quite a bit of soil with the killed shrub roots to avoid a condition rose growers call replant disease, and replanting in a generous amount of compost. Plant pathologists deny there is such a phenomenon in growing roses, but veteran rose growers say they’ve had shrubs fail to thrive after they’re planted in the same soil.

Some think this replant disease or “rose sickness” is due to nematodes, previous shrub’s exhausting the soil of needed nutrients, soil compaction, microbes or toxins. One solution that appears promising is planting African or Mexican marigold (Tagetes erecta) where the shrub was removed, thus giving the soil a year’s rest before the next rose is planted.

Andrea Wurtz, Boise native and former Advanced Master Gardener with the University of Idaho Extension service, is now one of the people in charge of the rose garden in Julia Davis park, working for Boise City Parks and Recreation dept. She says they’ve just replanted roses where the ones were that were winter killed or more often victims of bacterial rose cane blight.

Until this winter, the past few years that department has had good success in killing that bacterial disease in the rose garden with an August soil drench of Phyton 27. They used to think of that blight as a spring disease, but when they noticed the red tell-tale blotches on canes in August, they realized it was not a disease confined to spring eruption.

As far as replant disease is concerned, apple trees and some other fruit trees. all part of Rosa, the same genus as roses, are also susceptible to replant disease. It’s common in orchards, where an orchardist loses one tree in an orchard, and wishes to replace it, he/she must consider remediation, since it is an economic blow to the orchard’s output. One solution they’ve found is to plant grain where they’ve lost a tree, and wait a year before replacing it.

Rose growers who rely on renewed soil or abundant compost in which to plant new roses may think they’ve overcome rose disease, but perhaps not. One expert who had, over time, replaced roses in a dedicated rose bed and considered them healthy and growing, expanded his bed, removing lawn grasses and planted roses in that part of his yard. He was quite surprised to see the enhanced vigor displayed by the roses in the former lawn area, and now wonders if the roses in his bed were not growing and producing as they should be.

This story was originally published January 25, 2017 at 12:51 PM with the headline "Cold winter may have hurt your roses, and you may need to take action this spring ."