Hydrangeas are flowering shrubs native to Asia and America. They can grow from three meters up to 30 meters tall, depending on the species. They have big, beautiful blooms that look like pom-poms and come in various colors. These plants can also suffer discoloration and yellowing leaves for various reasons, some of which may impact their health and longevity.
Hydrangea leaves turning yellow: What are the reasons?
1. It could be due to leaf scorch
Leaf scorch is characterized by yellowing and wilting leaves. The ones most affected are those on the top and sides of the plant that get the most sunlight. They may also have brown tips and edges. To correct this, move the plants to a shaded area to shield them from the hot sun. While some varieties can tolerate full sun, most hydrangeas require afternoon shade. Also be sure to water the plants more than usual on hot days.
2. It could be due to leaf spot
Another reason for hydrangea leaves turning yellow could be Cercospora leaf spot, caused by the fungal pathogen Cercospora hydrangea. This disease is prevalent during warm and rainy weather. It is not a life-threatening disease but could take a toll on the plants due to unattractive symptoms such as purple spots that turn gray, and yellowing leaves that may also turn brown and fall off.
To fix the problem, remove and destroy the fallen leaves. Water the plants at the base, and not from overhead. Fungicides are not needed unless the plants have been affected by the disease more than once already. If so, treat the affected plants with a fungicide that contains chlorothalonil.
Follow the instructions carefully and spray only during springtime, making sure that the fungicide covers all of the foliage except for the flowers and undersides. Reapply the fungicide once a week until the plants have healed.
3. It could be due to iron chlorosis
Chlorosis is a reduction or absence of the normal green color of leaves. This is typically due to iron deficiency in lime-rich soil, lack of light, or disease. If you notice that your hydrangeas’ leaves are turning yellow but the veins remain green, it is most likely iron chlorosis.
To treat the problem, add chelated iron to the soil. Combine one and a quarter ounces of liquid chelated iron with one gallon of water, as a general rule. The amount of solution will depend on the size of your plant: if your plant is one to three feet tall, use at least six ounces, and if it is three to six feet tall, use 12 ounces. Pour the solution directly into the soil beneath the plant and water as you normally would.
If the symptoms still show, the soil’s pH could be too high. You can try reapplying the iron soil drench, or try to lower the soil’s pH using pine bark mulch.
Another option is to apply sulfur to the soil. Use 12 pounds of sulfur soil acidifier per 100 square feet. Spread the sulfur solution to the dripline and water the plants as you normally would.
4. It could be due to nitrogen deficiency
Another culprit of yellowing leaves among hydrangeas is nitrogen deficiency. The cure for this problem is simple, as you just need to apply ammonium sulfate to resolve it. Apply one-half to one pound per 100 square feet. Scatter evenly around the plants and water them as you normally would.
5. It could be due to overwatering or underwatering
Overwatering is a common cause of yellowing leaves in plants, including hydrangeas. If you are using pots with no drainage holes, the leaves may turn droopy and yellow. Because the pots are not draining well, the water stays stagnant around the roots, aggravating the problem. The plants also tend to have stunted growth, and are more prone to fungal diseases.
Similarly, underwatering can also cause leaves to turn yellow and could lead to wilting, droopy plants. Forgetting to water your plants or being unable to do so when you are away could lead to leaf discoloration if not addressed promptly. Treat the problem by making sure your potting mix is well-draining, and that the root zone stays moist without being too wet. Water the plants at least once a week.
Hydrangea leaves turning yellow is a cause for alarm among plant growers. It indicates a problem that could endanger your plant’s life and should be resolved promptly. Common reasons for this problem include overwatering or underwatering, iron chlorosis, other nutrient deficiencies, leaf spot or sunburn.
Image: istockphoto.com / CatherineL-Prod