Introduced in 2000, 'Knockout' rose quickly became the best-selling landscape plant in the country. It had everything -- showy, continuous blooms; compact growth habit; tough-as-nails constitution; and, best of all, no need to spray for black spot disease. But now, nature has tossed green kryptonite into Superman's garden. And 'Knockout' rose may just get its bell rung.
A Deadly Threat 'Knockout' rose (the original single red, shown above, plus a bunch of newer colors) owes its uber-popularity to the belief that it's the first "no maintenance" rose -- perfect for the lazy gardener in all of us. People think it needs no watering, spraying, pruning, or fertilizing -- EVER. It's like an actual living plastic plant. You just stick it in the ground and it will bloom, bloom, bloom with zero care from you. How marvelous.
Unfortunately, this belief is dead wrong. 'Knockout' does need water, fertilizer, and pruning. And now it's facing a disease so serious that its very survival is in question. Rose rosette disease.
This is what rose rosette looks like and it's not pretty. A formally healthy plant starts producing Medusa-like bunches of bright-red new shoots. The shoots bloom, but the flowers look distorted. As rose rosette spreads through the plant, the rose gradually dies back, until it completely croaks. Down for the count.
What Causes Rose Rosette? Rose rosette is caused by a virus first discovered in the western U.S. around 1940. The virus is principally spread by tiny eriophyid mites -- so tiny, in fact, that they literally blow into gardens on the wind. When they feed on a rose, they transmit the virus. At that point, the jig is just about up.
Now here's a surprise. There was a time when rose rosette was considered a savior, not a plague. Any of you remember the infamous "living wall," aka the multiflora rose? A vigorous, arching import from Japan, it produced pretty white flowers in spring and thousands of small, bright-red rose hips in fall. It grew so thickly that highway departments in the East and Midwest actually planted rows of it down highway medians. Even a tractor-tractor couldn't smash through. Cattlemen also used it to contain cattle.
But you know what they say about good intentions. Birds ate the red rose hips and spread multiflora rose everywhere. It proved to be an awful, noxious weed. States banned it, but it was too late. The entire eastern U.S. was destined to be smothered by the stuff, unless a control could be found.
It was. Rose rosette disease.
Wahoooo!!! Rose rosette killed multiflora rose faster than a van filled with nuns kills a good kegger. Unfortunately, when rose rosette ran out of multiflora roses, it looked for something else to feast on. The target? 'Knockout' rose and other shrub roses. The first 'Knockout' roses to show symptoms were located where the highest concentrations of multiflora roses were growing -- the East and Midwest. 'Knockout' roses in the South have it now too.
Can Anything Stop Rose Rosette? Because rose rosette is caused by a virus, it eventually spreads internally to every part of the plant. Promptly removing the bright-red shoot clusters by cutting through healthy green wood below them may save a rose. But once a rose gets full-blown rose rosette, turn out the lights. You must pull up the rose, roots and all, bag it, and throw it out with the trash. Spraying will not work.
Conard-Pyle, the respected Pennsylvania nursery that introduced 'Knockout' roses, suggests pruning back the plants by 2/3 while they're dormant in late winter to remove any overwintering mites and eggs in the bud crevices. This is especially important for large landscape plantings of 'Knockout,' because the more bushes you have, the more mites you have, and it's easier for the virus to spread.
Now For Some Really Bad News According to Grumpy's sources, most rose species and their selections are vulnerable to rose rosette -- not just 'Knockout.' So if your love your roses, keep your eyes peeled for weird-looking, bright red shoots. Don't leave yourself open to a 'Knockout' punch.