knockout roses

March 24th, 2017 – Williamson County Weekend Gardener

Knock Out Roses quickly became a staple in many Williamson County landscapes when they were introduced about 15 years ago, largely because they were beautiful, low-maintenance, and virtually indestructible.But a blight called Rose Rosette Virus (RRV) has overtaken our area, and poses a real threat to these popular beauties.

Personally, I was a fan of a specialty variety called ‘Blushing’ that features shell pink blooms with white centers.The blight hit one of three groupings I’ve had for over 10 years and wiped them out.The devastating thing about RRV is that you can’t just spray for it; physical removal of the plant is the only course of action and RRV can remain in the soil for up to a year, so you can’t just replant.

How To Recognize Rose Rosette Virus

A walk through my Franklin neighborhood revealed all sorts of roses afflicted with RRV. The tell-tale signs include:

  • Leaves and twigs produced are a bright, rich, red color.
  • Leaves are distorted and twisted.
  • There may be a proliferation of leaves.
  • The stems grow slowly and produce excessive thorns.
  • There may be so many thorns that there is no stem available to be seen and the thorns are often red-tinged.
Rose Rosette Virus
The Effects of Advanced Rose Rosette Virus (RRV) Attacking a Knock Out Rose

Are Knock Out Roses Down for the Count?

With all the buzz about RRV, I was fairly shocked to see Knock Out Roses for sale recently at Lowes. It seemed like a bad idea. So we went to the UT Extension office to ask if it’s OK to be planting Knock Out Ross here in Williamson County or not.

The extension office says it’s OK to plant them, but offers this advice:

Currently, there are no effective controls for rose rosette virus disease. Research is being conducted by many universities, but it will most likely be several years before any positive control measures are reached. even though the disease is widespread, I still encourage people to plant Knockout roses.

Here are some suggestions if you still want to plant Knockout roses:
It is best to start by purchasing disease free plant material. When visiting the nursery inspect plant material and avoid buying roses that show symptoms of this virus.

When planting new roses in the landscape, leave enough space for plants to mature without overlapping stems or leaves of neighboring roses. This extra space will help prevent eriophyid mites from crawling from one plant to the next.

Next, remove wild multiflora roses that exist within 100 yards of the landscape.Start mite control early by pruning your roses hard in late winter (back by 2/3) to remove as many overwintering mites as possible and then spray with horticultural oil to kill any remaining mites. Organic pesticides such as horticultural oils and insecticidal soaps are recommended over other pesticides as these organic pesticides are less harmful to natural predators that feed on the problem mites. Apply weekly during the months of June and July paying particular attention to the new growing tips where the mites will congregate.

Refrain from using leaf blowers around roses as they can spread mites. Plant only a few roses instead of a large number of plants. Ornamental grasses planted between roses can act as physical barriers to help prevent the potential movement of the mites form one plant to another.

Knock Out? Or Not?

Given the amount of RRV obvious in my neighborhood, I’ve come to the conclusion that any new Knock Out Roses I plant are likely to become infected and I’ve begun researching alternatives. But if you’re not in close quarters with the disease, it seems as if you can still grow these beauties successfully.

Weekend GardenerWilliamson County Gardening Resources:

Williamson County Master Gardeners Association

Williamson County Extension Office

Williamson County is in USDA Hardiness Zone 7a (0 – 5 degrees)
USDA Plant Hardiness Zone Map

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5 Ways to Get Soil Ready for Spring

Stop Topping Crape Myrtles

5 Plants Anyone Can Grow

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