Hartley Magazine

All the latest news, hints, tips and advice from our experts

Weather lands a good punch on Knock Out roses

Everybody loves the hardy, disease-resistant shrub roses that have become so popular in the last couple of decades for their easy care and nonchalant reblooming. Many will bloom – and often rebloom after June – without deadheading, and survive even cold Midwestern winters without being swaddled. Some slow-release fertilizer in spring is welcome, but many shrub roses do just fine without it. When you see rose bushes in parking lot islands, you know they are proven to be tough plants.

The shrub rose revolution has been led, if not pioneered, by the monster hit Knock Out, with its cherry-red single blooms mo nth after month. these plants have been bred to be resilient and disease-resistant.
But they’re not made of steel, as I learned this spring. If conditions are extreme enough, any plant is vulnerable. And this season extreme weather conditions have brought some shrub roses to their knees.

Several years ago, a friend asked me to help her rework her back yard for lower maintenance. She wanted roses, so for a sunny spot along a sidewalk I recommended a row of Knock Outs. For several years they performed just as promised – blooming and reblooming all year with only as much pruning as it took to keep them from blocking the path, with no disease or insect problems.

Late this spring, though, she called me over there with a puzzle. The rose bushes’ leaves were covered with yellow spots, many of which were starting to develop orange fuzz. I consulted experts at the Chicago Botanic Garden to confirm that it was what I suspected: rose rust, a fungus disease I had read about but had never seen in the Midwest. It had appeared for the first time at the botanic garden this spring too, and now it was on my friend’s roses. On disease-resistant Knock Outs! I was flabbergasted.

Rose rust is a big problem in the cool, rainy Pacific Northwest, where conditions are just right for it. It’s a special threat in the closed environment of a commercial greenhouse, where many young roses are grown there. And that’s a reminder that the owners of private greenhouses need to be careful about hygiene.
Rose rust is rarely a problem in the Midwestern gardens, though, because winters usually are cold enough to kill the fungus spores off and moist springs don’t usually last long enough to nurture it.

But this spring we offered it an opening: After a mild winter, we had a record-setting 10-day stretch of above-80 days in March that caused many plants, including roses, to push out their foliage four to six weeks early. Then, while the foliage was still tender and vulnerable, the weather cooled off and we had a relatively normal April, with days in the 40s and 50s and occasional nights below freezing. Meanwhile, we were getting little rain and many gardeners had started watering early. The rust pounced.

When you see those spots, it’s too late for a fungicide to do any good. So I suggested to my friend that she cut the roses back hard and remove all the affected foliage from her yard. Most likely the roses will bounce back next year.

That’s if we survive a summer that is drifting toward drought. Occasional gullywashers to the contrary notwithstanding, rainfall has been running about 50 percent below normal and temperatures have been staying in the 90s – even 100s – day after day, breaking records right and left. It’s like no year in my gardening lifetime.

I have not seen rust on my roses (fingers crossed). But I know I’m going to be doing a lot of watering this summer, so I’m taking measures to avoid getting the foliage wet, since that’s a major invitation to fungus diseases. I’ve twined soaker hoses all around among my rose bushes and many other shrubs, including all the ones I planted this year or last. That way, I can water right at the root zone without ever getting the leaves wet. A long, slow watering every week or so, allowing the moisture to sink deep into the soil to encourage roots to grow, is best.

Altogether, it’s a reminder that even disease-resistant plants are not disease-proof. Drought-resistant plants are not guaranteed, either. But if gardeners refrain from panicking and wait until next year, many plants are surprisingly resilient. And even if they don’t bounce back next year, we can: We can try something new.