Knock Out roses, such as the original Rosa 'Radrazz,' have been around since 2000, and this popular, disease-resistant family of roses is known for producing blooms prolifically. But when it comes to successfully growing Knock Out roses, which are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 11, depending on cultivar, it's good to know when to plant them.
Planting Knock Out Roses
According to Star Roses and Plants, you should plant Knock Out roses in the spring or fall. If you're planting in spring, confirm that there's no danger of frost in the 10-day forecast. If transplanting a Knock Out rose, the best time is in late winter or early spring while the plant is still dormant. Plant the Knock Out rose in a spot that gets six to eight hours of sun each day.
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When you plant a newly purchased Knock Out rose, make sure your soil is balanced, with a pH of 5.5 to 6.5. If your soil is more acidic or more alkaline, you may need to amend it, based on the results of a soil pH test.
Dig a hole that is twice as wide as the plant container to aerate the soil. Plant the rose, taking care to gently loosen the roots at the bottom of the plant. Make sure the base of the rose is level with the ground. Return the crumbly soil to cover the roots. It's best not to fertilize Knock Out roses until the shrubs are established and have been through a bloom cycle.
Other Rose Planting Tips
You can plant Knock Out roses in containers as part of a container garden, but transplant each plant into a container that is twice as big as the original pot so the plant has room to grow. Make sure you place the container in a sunny spot and keep it watered. You may have to bring it inside in the winter.
Make sure your in-ground Knock Out roses have at least 3 feet of space around them. This allows for plant growth, good air circulation and prevention of moisture buildup on the foliage and blooms.
Once you plant the rosebush, water it well, allowing water to soak into the soil around the base of the plant. This helps it adjust to the shock of being transplanted. Thoroughly water the base of the plant several more times over the next few weeks after it's planted to make sure the plant has enough water while adjusting to its new surroundings.
Knock Out Care Tips
Prune Knock Out roses in late winter to early spring. If, however, you've just planted the rose — especially if it's beginning to bloom — it's better to deadhead the blooms than to prune the entire plant. After the rose is established, use hedge loppers to prune your rose stems back to 12 inches. By the end of the growing season, those canes should be 3 to 4 feet tall.
To prevent black spot, especially if you live in a humid area, make sure you water at the base of the plant to avoid wet foliage. Watering overhead could encourage fungal disease. Powdery mildew often occurs in spring and fall in humid areas, but this usually goes away with the heat of summer. It's best to be patient if your plant has powdery mildew.
For winter, if you live in an area where the ground freezes regularly, put a 2- to 3-inch layer of mulch around the rose to within a few inches of the base. If your winters are very cold, you may want to wrap the rose in burlap. Heavy snow cover will also insulate the plant. In spring, remove excess mulch from around the main stem and trim dead canes.