Pruning Knock Out roses

CorrespondentMarch 6, 2010 

After an unusually cold and dreary winter, gardeners are starting to think about spring. The first item on the minds of some is pruning their Knock Out roses. These new landscape roses, introduced just a decade ago, have proved their worth in the Piedmont, resurrecting interest in rose growing

It is their strong performance that has gardeners asking about pruning them. Our long growing season encourages vigorous growth among Knock Outs, and I have seen some 5 feet tall and almost that wide. And that tells you this is no simple rose bush, but a major shrub that is an important feature in a sunny landscape.

And, unlike some other flowering shrubs such as azaleas, forsythias and rhododendrons, this is a plant that bears pink, yellow or cherry-red flowers on new growth produced this year.

This new growth will come out early, which is why late winter - sometime in the next two weeks - is prime time to shorten and shape up your Knock Outs. Even if yourshaven't hit 5 feet, leaving these vigorous shrubs alone now could result in a plant that is too big for its space, and sooner than you think.

So take a critical look at your plant from all directions and evaluate the shape. You will want to bring the plant down to 2 feet or a bit higher. But before you cut, clean your pruning shears by dipping the blades in a solution of water and 10 percent household bleach. While Knock Outs are resistant to rose diseases, they are not immune from them. Dipping the blades in this water and bleach solution after each cut will reduce the possibility of putting disease organisms onto the open wound you create when pruning.

Pruners in hand, start with the longest canes (those are stems in rose-speak) and cut them back to just about one-fourth inch above a bud. Buds are easy to see along the canes, especially when they begin to swell in late winter. Make a slanted cut downward, starting about one-fourth inch above a bud.

Your goal should be a pruned plant that is 24 to 30 inches tall.

When you have the plant down to this manageable size, look for any dead, diseased or twiggy growth in the interior and remove that. Study the overall shape. If it seems one-sided, do some evening-up all around.

Pruning will encourage fresh growth, and Knock Outs should be roundish shrubs, well-filled with foliage that bears roses all around. On the largest canes, a sealant of white glue popped on just after cutting will prevent intrusion of borers. Think of it as insurance that doesn't cost much.

Fertilizer will help, too. Pull back the mulch and apply rose fertilizer at the rate directed on the package. A timed-release fertilizer that works over a long period such as three months will save you time and energy.

Fertilizer is essential on roses, which are heavy feeders and grow vigorously. Epsom salts work with roses, too. It encourages vigor and good green color.

Epsom salts are magnesium sulfate, and you don't need much - a couple of tablespoons per plant worked gently into the soil before you put the mulch back on. Many gardeners replace mulch on roses each spring.

I think this is essential for types that are prone to diseases because their leaves drop onto the mulch. But Knock Outs tend to be healthy and you can replace the pineneedle mulch when you think it looks tired and worn-out.

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