Knockout roses provide a stunning display of blooms from early spring until the first frost. These plants are chosen because they are hardy and resistant to infections. Even the hardiest plants will develop problems from time to time. The leaves of a knockout rose will turn yellow when the plant is affected by various climate and soil conditions.
Too Much Water
Knockout roses grow best with a steady water supply. Too much water will cause the leaves to turn yellow. Once the moisture level is balanced, new green growth will appear.
Lack of Ventilation
Roses prefer room to grow. If the plants are spaced too closely together, the leaves may turn yellow. The plants must be thinned out or pruned to provide better ventilation.
These tiny pests love to attack the underside of the leaves. To remove them, spray the underside of the plant leaves with a steady stream of water. This will knock them off the plant and remove their webs. To prevent spider mite infestations, keep rose beds free of weeds and decomposing leaves.
This dreaded condition affects the whole plant. A rose infected with black spot will display leaves with jagged black spots; these spots may be surrounded by a yellow halo. The leaves will turn yellow and drop. Completely remove and discard any infected leaves. Black spot can develop when water is allowed to sit on the leaves. To prevent black spot and limit its spread, water plant at the base and avoid spraying the leaves directly.
Low Levels of Nitrogen in the Soil
Soils lacking nitrogen will cause the knockout rose's leaves to turn yellow. Apply a nitrogen-rich fertilizer in early spring and again at the peak of blooming.
- "Roses: Placing Roses, Planting & Care, The Best Varieties"; Time Life Books; 1989
- "Ortho's All About the Easiest Roses to Grow"; Ortho Books; 2001
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