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Home & Garden

The ubiquitous Knock Out rose takes a hit as a tiny mite punches above its weight

September 11, 2018 at 10:00 AM

Roses at Georgetown Waterfront Park show signs of rose rosette disease, whose symptoms include deformed clusters of buds. (Adrian Higgins/The Washington Post)

I found myself walking along the Georgetown waterfront the other early evening and came across a scene that looked like a whole town had come out to play.

There were couples enrolled in a very public dance class, picnickers on the lawns and, at Washington Harbour, a tableau of outdoor diners casting glances at the motor yachts moored nearby and the beautiful people within them.

My thoughts, naturally, turned to rose rosette disease.

The Georgetown Waterfront Park, like many public landscapes these days, features mass plantings of landscape roses, particularly the ubiquitous magenta-red Knock Out rose and its close variants. As a repeat-blooming, flowering shrub that shrugs off black spot, Knock Out is a knock out. As a rose, it’s not the prettiest or most fragrant. As a target of rose rosette disease, it seems to be especially prone.

In any case, most of the blocks of Knock Out roses along the river appeared to be well acquainted with RRD, exhibiting the telltale rosettes, or distorted clusters of stems and deformed buds at the ends of branches. Other signs are stems fused together, excessive (and soft) thorns, and red leaves and stems. For the viewer, the aberrations are strange and ugly; for the rose, the disease means slow death, usually within three growing seasons. There is no cure, though the malady can be checked by pruning or, better yet, by removing infected plants entirely, bagging them and thus protecting clean roses in the vicinity. You can plant another rose in the same spot, as long as you have removed all the roots of the old one.

The extensive planting of otherwise robust landscape roses in recent years has contributed to the spread of the virus, which is transmitted by a mite. (Adrian Higgins/The Washington Post)

The disease is caused by a virus transmitted by a tiny mite. The featherweight eriophyid mite is so small that it moves from rose bed to rose bed on the breeze.

The last point worries Lois Petzold, who grows roses in her townhouse garden in Alexandria and was aghast to see the extent of RRD along the Georgetown waterfront. In her own garden, she is quick to remove infected plants, she said, and fears that the public plantings in Georgetown (and elsewhere in the city) will cause problems for home gardeners. “They’re basically beds of infection waiting to float away,” she said. “I think it’s the one disease that can wipe out domestic rose growing.”

Related: [Why every garden deserves a rambling rose]

Plant pathologists don’t quite share that view, but they do say that the key to controlling the disease is vigilance. “Symptomatic plants should be rogued out as soon as possible, since infected plants may harbor large populations of eriophyid mites,” write Mark Windham, Alan Windham and Frank Hale of the University of Tennessee’s Institute of Agriculture. The disease “will prove to be controllable,” they say. “However, these efforts will take time, require increased levels of research funding and a lot of hard work.”

Knock Out is no more susceptible to the disease than other rose varieties, the experts say, but its success works against it. Because it is so tough in other respects, it has been planted by the millions over the past two decades in public and commercial landscapes that inherently get less care and attention than places with dutiful gardeners. As shown at the Georgetown waterfront, the disease can become widespread.

Thomas Evans, a plant pathologist and RRD expert at the University of Delaware, said the landscape industry has moved away from Knock Out and other shrub roses in recent years as the disease has spread. “When you have half an acre of roses in a landscape, that’s a big target for the eriophyid mite,” he said.

Evans is part of a research team conducting a five-year, four-state trial of roses showing resistance to the pest and disease. Of some 600 rose types, the trial has identified approximately 30 with some promise of resistance that breeders can then use in their programs, he said. A related public website, roserosette.org, has been established where home gardeners can share pictures of symptomatic roses.

The waterfront park is owned by the National Park Service. Spokeswoman Jenny Anzelmo-Sarles told me via email that “we’ve been doing our best to keep the roses alive by pruning out the diseased growth during the growing season and doing a hard cut back of the shrubs in late winter.” In addition, she said, gardeners have been dividing ornamental grasses and planting them where roses have been removed.

It is the presence of another rose, a species named Rosa multiflora, that caused rose rosette disease to spread across much of the United States. The multiflora rose was planted widely in the past century but is now considered an invasive weed. It is a prolific seeder, and as it advances through natural areas, it takes the mite and virus with it.

The underlying dynamic is the propensity of plant breeders, growers, wholesale and retail nurseries, designers, landscapers, and consumers to rely too heavily on too few plant varieties. When you have mass production of a handful of clones, you set yourself up for problems if and when a pest or disease arrives, or the plant itself turns invasive. We ran into trouble, for instance, with such overplanted trees and shrubs as red-tip photinia, American elms, Bradford pear, English ivy and barberries.

Other plants are pest-free (so far), well-behaved (so far) and undeniably handsome, but do we need to plant any more Green Giant arborvitae, Natchez crape myrtle or cherry laurels?

The garden is the place to counter this herd mentality. There are few, if any, plants I see in commercial landscapes that I would want to grow, with the exception of some ornamental grasses and prairie perennials.

One thing that continues to astonish and delight me about gardening is that after decades of discovery I still find new plants every year. Obtaining them may take some effort, but that’s part of the fun, too. In gardening, as in life, you shouldn’t rest on your laurels.

@adrian_higgins on Twitter

Tip of the Week

If your pot of petunias is tired, pull the plants, freshen the soil and sow some seeds of arugula for a fall supply of peppery greens to add to sandwiches. Thin seedlings as needed.

— Adrian Higgins

More from Lifestyle:

5 toxic plants you should know — and avoid

D.C. summers can be a slog. The gardener’s antidote? A little fall planning.

The best gardens aren’t static. Case in point: Chanticleer.


Adrian Higgins has been writing about gardening, landscape design and related environmental topics since the late 1980s. He joined The Washington Post in 1994. He is the author of several books, including the "Washington Post Garden Book" and "Chanticleer, a Pleasure Garden."

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