Overwatering Is as Bad as Under Watering
Overwatering plants is an extremely common problem, and one of the signs is yellow leaves (when they should be green). The roots of plants not tolerant of waterlogged soil won’t be able to breathe, and they die of a lack of oxygen. After roots drown in this manner, you’ll see the drying up of the leaves because the debilitated roots won’t be able to use the remaining water.
Drainage Is Critical
You may not be overwatering your rose of Sharon, but if your soil isn’t well-drained, the roots could be resting in waterlogged soil. If you suspect you have poorly-draining soil, your best recourse would be to transplant the rose of Sharon plants to an area where the soil is well-drained. You can prepare the site in the summer (mix peat moss into the soil to improve drainage) and transplant the root ball in autumn once the weather cools off.
Know the Signs of Transplant Shock
Rose of Sharon usually adjusts well to moving, but it can still go into shock within two years of being transplanted. Signs of transplant shock include yellowing leaves, new leaves that grow poorly, flower buds that dry out and fail to develop, drooping foliage, and stunted growth. When you see a plant starting to decline, your first impulse might be to reach for fertilizer (outside of its regular schedule) but don’t. Overfertilization could make the situation worse and kill the plant. To prevent transplant shock, ensure the planting hole is two to three times bigger than the root ball, giving the roots soft soil to grow into. When refilling the new spot, use soil from the old planting site and avoid using amended soils, which can be too much change too quickly. Transplantation is best done in late fall, winter, or early spring. Avoid doing it on hot summer days.
Achieve a Balance in the Soil
Deficiencies in the soil can lead to yellowing or chlorosis, a common condition that frequently causes the yellowing of shrubs. Chlorosis is caused by insufficient iron in the soil and can be fixed by applying iron chelate to the soil. Chlorosis can also form when sufficient iron exists in soil, but the soil pH renders the iron unavailable for uptake through the roots. If it’s iron chlorosis, it usually shows up on new growth first. A second clue is if the veins on the yellow leaves remain green while the rest of the leaf turns yellow. To know for sure, you will need a soil test, which your local cooperative extension service can perform based on your submitted soil sample. Another issue that causes yellow is inadequate or wrong fertilization. Rose of Sharon leaves turn yellow when they do not have enough nitrogen for proper growth. Too much fertilizer can also cause yellowing or scorching, burning the roots and damaging the plant. Apply fertilizer only to moist soil, then water well to distribute the food evenly.
Insect Activity Can Ruin a Plant
Rose of Sharon is a relatively pest-resistant plant, but aphids and whiteflies can cause yellowing problems when the insects suck the juices from the plant. These and other sap-sucking pests are easily controlled by regular applications of strong water sprays, insecticidal soap, or horticultural oil.
Fungal or Bacterial Infection Can Affect Leaves
Rose of Sharon isn’t susceptible to many diseases but can develop leaf spot. Leaf spot looks like spots or blotches on the plant’s leaves that eventually turn yellow. Leaf spot crops up when the plant is overwhelmed by Cercospora fungus or bacteria. If the plant also has leaf rust, it will develop pustules growing on the undersides of the leaves. To treat leaf spot, remove and destroy the affected leaves. Apply a broad-spectrum or copper-containing fungicide to the rest of the shrub.
Why Only Some Plants are Dying
Are some of your plants thriving while others have yellowing leaves and are dying out? What seems random may not be. Some plants are more resilient than others. The rose of Sharon plants that are doing well may have gotten off to better starts as seedlings and are simply a bit more hardy than the dying plants.
