Why Are My Knockout Roses Turning Yellow?

Water

• The first factor to check when diagnosing Knockout roses is the water supply. Too much or too little water causes the leaves to yellow. Knockout roses require a moist but not saturated soil, and some growers prefer to plant them in raised beds so that moisture enters easily and drains efficiently.

Insects

• Aphids and spider mites are both common garden pests that bother Knockout roses. Inspect the undersides of leaves and stems to see if these pests are causing your roses harm. While commercial insecticides do exist, many growers suggest removing insects by hand and maintaining good plant hygiene instead of chemical solutions.

Soil

• Nutrient deficiencies in the soil also cause yellowing. Nitrogen deficiencies cause leaves to become yellow, since nitrogen is the nutrient that plants use during rapid growth. If the leaves are yellow but the veins within the leaves remain green, this points to an iron deficiency instead. Before fertilizing, have your soil tested at a garden center to see what nutrients you need -- over fertilizing is as harmful as lack of nutrients.

Air

• While Knockout roses are resistant to many diseases, certain strains still affect the plant. Rose black spot is found in areas with high humidity and temperatures. This disease causes leaves to turn yellow and fall off of your roses. The key to avoiding black spot is good plant management. Plant Knockout roses at least 3 feet apart to allow for good air circulation, preventing close environments where black spot-causing moisture becomes trapped.