The Knock Out® rose hybrids (Rosa "Knock Out®") began with "Radrazz" (Rosa 'Radrazz'), a cherry-red blooming, drought-tolerant rose with excellent disease resistance. More hybrids with different flower colors soon followed. While all Knock Out® roses are more resilient than most other types of roses, they still require supplemental irrigation when it does not rain.
Cultivars and Hardiness
Knock Out® roses are generally hardy in U.S. Department of Agriculture plant hardiness zones 4 to 11, although this varies slightly depending on cultivar. Fresh loamy soil should be piled up over the crowns of all Knock Out® roses to a depth of 10 to 12 inches for winter protection in USDA zones 4, 5 and 6.
'Radcon' (Rosa 'Radcon,' USDA zones 5 to 10) blooms in pink, 'Radcor' or rainbow (Rosa 'Radcor,' USDA zones 5 to 11) has dark coral pink flowers with yellow to pale coral centers and 'Radsunny' or sunny (Rosa 'Radsunny,' USDA zones 4 to 11) produces flowers that are gold-yellow in bud but change to bright yellow when they open and fade to cream-yellow. These Knock Out® roses all have single-form flowers with five to seven petals.
'Radtko' or double Knock Out® (Rosa 'Radtko,' USDA zones 4 to 10) produces cherry red flowers with 18 to 24 petals per flower. 'Radtkopink' or pink double Knock Out® (Rosa 'Radtkopink,' USDA zones 5 to 11) produces bubble-gum pink flowers with 18 to 24 petals each.
Knock Out® roses are shrub roses that grow to a height and width of 3 to 4 feet in cold-winter climates but can mature to 6 feet by 6 feet or more in mild climates.
When to Water
In mild-winter climates above USDA zone 8b, Knock Out® roses bloom all year round. This means they must be watered all year round in southern regions with warm, dry winters.
Knock Out® roses bloom from spring to first frost in cold-winter climates. In northern regions where temperatures drop below freezing in the winter, begin providing supplemental water in the spring when the ground warms and they begin growing, unless they are getting 2 inches of water per week from rain. Continue to water throughout the spring, summer and fall until the weather gets cold and they start dropping their leaves.
Amount and Frequency
Roses need to be watered more often when they are planted in sandy, fast-draining soil than when they are planted in loamy or clay soil that drains more slowly.
Newly planted Knock Out® roses need to be watered as often as necessary to keep the soil uniformly moist for the first one to two months until they get established. Give them 3 gallons of water, then check the top inch of soil every few days. When it begins to dry, give them another 3 gallons.
Give established Knock Out® roses planted in fast-draining soil 6 gallons or 2 inches of water each week when temperatures are between 70 and 80 degrees Fahrenheit. They may not need to be watered that often in slower-draining soil. Always check the soil before watering. If the top 2 inches is still wet, wait a few more days -- as a general rule, water when the top 3 inches of soil becomes dry. As temperatures rise to between 80 and 90 degrees Ft, Knock Out® roses may need as much as 9 to 12 gallons or 3 to 4 inches of water each week. Even more may be needed when temperatures exceed 90 degrees F.
How to Water
Water Knock Out® roses from below the foliage with a watering can, garden hose or soaker hose.Even though they are resistant to black spot and powdery mildew, it is better to keep the foliage as dry as possible.
Distribute the water evenly over the soil from 2 inches away from the stems and extending out about 1 foot beyond the drip line** or outer edge of the branches. Always water in the morning** so foliage will dry throughout the day if it gets wet, and the moisture will be available to the shrub in the heat of the day.
When using a soaker hose, set a 1-inch deep tuna or cat food can beneath the hose where water will drip into it to measure how much water is being delivered to the shrubs.
Spread a 2 to 3 inches of organic mulch over the soil around Knock Out® roses to help conserve moisture, keep the roots cool and reduce weeds. Pull the mulch a few inches away from the stems to prevent injury and disease that can result from the mulch keeping them too wet.