Knockout roses were created and introduced to the market in 2000. Because of cold hardiness, disease resistance, nice form and continuous blooming, they rapidly became the bestselling landscape plant in the country.
Knockouts are the easiest of all roses to grow. They are winter hardy to USDA zone 5 (-20° F) and are heat tolerant throughout the US. They have utility as a stand-alone shrub or as a beautiful hedge. If unpruned, they will reach a height of about 4 feet and as wide.
Blooming is from spring until frost. There is no need to deadhead the blossoms, they are self-cleaning. Another asset is that they are resistant to the blackspot fungus, needing no routine spraying as with other roses.
Pruning knockouts, as with most roses, improves blooming and overall health. Also, like others, pruning is best performed in spring, usually in early to mid-March as their buds begin to swell. Some rosarians suggest pruning when the forsythia is in bloom, as a guide. It is often recommended that knockouts not be pruned until the second or even third season, to allow establishment.
Knockouts do well when pruned back by 1/3 to 1/2. Some suggest pruning back by 2 feet from desired height. Severe pruning may reduce numbers of blooms temporarily while new foliage is produced, however, they are so vigorous, that this may not be noticeable.
There are now seven varieties of Knockout’s with a spectrum of colors.
Original Knockout: This rose blooms from spring until the first frost with medium red blossoms, often with a pleasant fragrance. It is still the most popular of the seven.
Pink Knockout: Nice pink color, otherwise similar to the original.
Blushing Knockout: Pink blossom, paler than the pink Knockout. May spread to 6 feet in diameter. Same blooming characteristics as the original.
Double Knockouts: Cherry red rose similar to the original except each stem has two booms. There is no need for deadheading, these plants are self-cleaning.
Pink Double Knockout: Similar to the red double, but has two pink flowers on each stem.
Rainbow Knockout: Has blooms which are light pink on the outside edge with centers which are varieties of yellow.
Sunny Knockout: This rose has a bright yellow bud and flower initially which fades to a pale yellow creamy color.
The idea of Knockouts being disease free lost some of its rosiness when the viral disease “rose rosette” appeared. Knockouts are very susceptible to the disease and once infected it usually is fatal. While knockout roses are very susceptible to this disease, all roses may develop it, but much less frequently.
The disease is caused by a virus spread by a tiny eriophyid mite. The virus causes the plant to develop many small deformed stems (witches broom) on the end of a limb. Many small reddish leaves develop and the stems produce large numbers of small thorns, almost appearing like a brush. The appearance is diagnostic and once developed, the rest of the plant is already infected. There is no treatment at this point, the best approach is to remove the infected plants, roots and all and send them to the trash. Photos of the disease may be found in Google Images.
The mite feeds only on meristematic tissue. This is specialized actively growing tissue found at the tips of stems and in developing flower buds. Since Knockout roses form many new flower buds continuously all season, they provide more food for the mite and thereby get more disease. There also may be some genetic factors contributing to their susceptibility. Other roses, such as hybrid teas, may get the disease, but they are much less attractive to the mite because of fewer meristems.
Most of the Knockout roses in the US are produced on the west coast. OSU plant pathologists point out that there is no Rose Rosette disease in plants originating from this area. They have no mites and no disease. The disease comes later when the plants enter a mite infested area.
Prevention of the disease is complicated, and basically involves trying to control the virus carrying mite. Only the mite can transmit the disease, one cannot infect a healthy plant by transferring sap from pruning shears, so there is no need for sterilization of pruning equipment.
The mite is very small and cannot be easily seen without magnification. It is transmitted from plant to plant by wind, animals and gardeners. The mite may get in the clothes of gardeners and be carried, unknowingly, to a healthy plant. A brief tumble in a clothes dryer may eliminate this contamination.
No official OSU recommendations for pesticide use is yet available. However, it is thought that the use of horticultural oils and soaps may be helpful. Stronger insecticides such as Sevin or brands containing the pyrethroid bifenthrin should also be effective, but these chemicals also kill the beneficial insects.
The time to spray should be both while the plant is dormant in late winter (to try to kill over-wintering mites) and regularly in spring during rapid growth of the rose. Thorough coverage is important, the tiny mites are adept at hiding in the nooks and crannies of plants.