Double Knock Out Rose (Rosa x "Radtko") is a compact deciduous shrub that grows up to 4 feet tall, continually producing firetruck red double roses throughout the growing season. Compared to other rose species, Double Knock Out Rose requires little maintenance, and offers the added benefit of being resistant to common rose diseases such as downy mildew and blackspot. Care requirements are the same for Pink Double Knock Out Rose (Rosa x "Radtkopink"), a deep pink-flowered cultivar.
Plant Where Hardy
Both Double Knock Out Rose and Pink Double Knock Out Rose are hardy in U.S. Department of Agriculture plant hardiness zones 4 to 9, where they require at least 6 to 8 hours of direct sunlight per day. If growing multiple shrubs, provide at least 5 feet of space between roses to allow for ample air circulation.
Maintain a year-round 2- to 3-inch layer of organic mulch, such as leaves or pine boughs, around the shrub. Keep the mulch about 6 inches away from the plant's base, and replenish before winter frosts to ensure winter protection. In freezing climates with dry, windy weather, you can also wrap the shrub in burlap, removing the fabric in spring after frosts have passed.
Prune in Spring
Prune in early spring to remove winter damaged canes. Using hedge shears, you can also cut the whole shrub down close to the base to promote a bushier plant, which may be beneficial for severely frost-damaged shrubs. If desired, you may lightly prune throughout the growing season to control canes that are sticking out. Disinfect pruning equipment afterwards by soaking tools for 5 minutes in a solution that is equal parts rubbing alcohol and water. Rinse with water and air dry.
Plant Double Knock Out Rose in well-draining soil. You can enhance soil and boost growth by fertilizing the shrub in the spring after the first flush of flowers are spent. Use 1/2 to 1 cup of a 10-10-10 or 12-12-12 fertilizer per plant, applying fertilizer in a band 6 inches away from the plant that extends out to 18 inches. Water well before and after fertilizing. Optionally, you can fertilize a second time in late spring and a third time in mid summer. Do not fertilize in late summer, as this will create tender new growth that is easily damaged by fall frosts. Store unused fertilizer in a secure location away from children and pets.
Water deeply every week or so to keep the soil moist, but not water logged or flooded. It is time to water whenever the top 3 inches of the soil are dry. Avoid splashing the leaves and canes with water as you irrigate, as wet foliage attracts fungal disease. In addition to providing winter protection, a year round layer of mulch also retains soil moisture and suppresses competitive weeds.
Monitor Plants for Pests
Check foliage occasionally for problems such as Japanese beetles, metallic copper and green hard-shelled insects that like to feed on Double Knock Out Rose foliage and flowers. Leaf veins may be left behind, giving leaves a skeleton-like appearance. Pick off beetles by hand as they appear and drown them in soapy water. Also called sawfly, the rose slug may also leave holes in foliage, though mild to moderate infestations do not harm the overall health of the shrub. Putting a bird feeder near the shrub may help keep pest infestations at bay. You can also spray the tops and undersides of leaves with a direct stream of water to dislodge the pests.
Treat Mildew With Oil
Double Knockout rose is susceptible to common fungal diseases such as powdery mildew, which results in grayish or white spots on leaves, flowers and buds. Fight powdery mildew by mixing 2 to 5 tablespoons of horticultural oil per 1 gallon of water and spraying the tops and undersides of leaves with the solution until wet, using a handheld or hose-end sprayer. Store unused oil out of direct sunlight, and do not store with food products.