Rose of Sharon (Hibiscus syriacus), a species of hibiscus that's also called shrub althea, is a deciduous plant that produces cup-shaped flowers. Colors of the rose of Sharon flowers may be red, pink, blue, purple or white. Unlike many early-flowering shrubs, it blooms in mid- to late summer or early fall as a perennial in U.S. Department of Agriculture plant hardiness zones 5 through 8.
The Missouri Botanical Garden categorizes rose of Sharon as a low-maintenance plant that prefers a sunny location and well-drained soil. If the leaves of your rose of Sharon are yellowing, consider certain causes to correct the problem.
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Overwatering May Be the Culprit
Rose of Sharon is a plant that needs careful watering. It likes moist soil, but not soggy soil, which may cause its leaves to turn yellow and drop off the plant. If your soil is heavy clay or experiences other drainage problems, allow the soil to dry slightly between waterings. Incorporating soil amendments, such as compost or well-aged animal manures, helps to loosen and aerate the soil, which also keeps it draining well.
Rose of Sharon Leaf Spot
Though rose of Sharon generally is a plant with few problems, it can suffer from leaf spot. Leaf spot can be caused by either a fungus or bacteria, which manifests as spots on the leaves of the plant. It can lead to yellowing of the leaves and pustules on the undersides of the leaves. Treatment includes removing the leaves and destroying them and treating the shrub with a foliar fungicide, according to the Texas A&M AgriLife Extension.
Aphids on Rose of Sharon
An infestation of aphids can also cause yellowing of leaves on rose of Sharon shrubs. These tiny insects suck the juices from the leaves causing foliage to pucker and yellow. They can seriously undermine the health of plants if untreated.
Aphids can often be controlled simply by spraying them with water from your garden hose to remove them from your plants. For persistent problems, spray the shrub with insecticidal soap according to label directions until the aphid population decreases.
Whiteflies on Rose of Sharon
Whiteflies are another garden pest that can cause yellowing of leaves in plants. These insects suck the juices from plants causing a decline in health and discoloration of leaves. Set out sticky traps to capture the insects only if they pose a real threat because the traps will also capture beneficial insects.
Chlorosis Causes Yellow Leaves
Chlorosis is a nutritional condition of plants that occurs when there is an insufficient amount of iron in the soil for the plant's use or even when sufficient iron exists in soil but the soil pH renders the iron unavailable for uptake through the roots. This deficiency will cause yellowing of leaves in your rose of Sharon.
Diagnose the problem with a soil test, which your local cooperative extension service will perform based on a soil sample you submit. You may need to apply chelated iron, available at garden centers and plant nurseries, to the soil as directed on package instructions to correct this problem.