To determine what ails the leaves of your geranium, start with a close look at the flowers. What is commonly called geranium might not be a hardy or true geranium, but a zonal geranium or regal geranium, annuals in the Pelargonium genus. The five petals of geranium flowers are similar whereas the two upper petals of pelargoniums are different from the three lower petals. Knowing which variety of plant you have matters because yellowing and dieback of pelargonium leaves is part of the natural aging process. Yellow older leaves at the bottom of the plant, which are often concealed by abundant younger leaves, are nothing to worry about.
If your plant is a true geranium, finding the cause of yellowing is crucial so you can take appropriate steps to remedy it.
Yellowing leaves on hardy geraniums are a sign of too much water. They are well adapted to dry conditions and only need watering when the top 1 inch of soil is dry to the touch. To prevent overwatering, make sure to plant in well-draining garden soil, or in containers with large drainage holes.
Because geraniums are drought-tolerant, yellow leaves from underwatering are less likely. It usually only occurs in extended dry periods. In underwatered plants, leaf edges and tips turn yellow first. Rather than waiting for that to happen, water when the top 1 inch of soil is feels dry. Water the plant slowly and deeply so water reaches the entire root ball.
Lack of Sunlight
Most geranium varieties need full sun to partial shade. In a hot climate, full sun in the morning and partial shade in the afternoon provides the best light conditions. In a location with too much shade, the leaves turn yellow. If you cannot move the plant to a sunnier spot, prune surrounding plants to give the geranium more light.
Hardy geraniums are perennials and as they approach dormancy in the fall, their leaves turn yellow, which is part of their natural life cycle. In spring, after they produce new growth, late cold snaps can also cause discolored leaves, especially in extended cool and wet weather. As it gets warmer, yellow leaves are soon replaced by new healthy foliage.
Lack of Nutrients
Geraniums are not heavy feeders except when planted in poor soil. Apply a balanced slow-release fertilizer once a year at the beginning of the growing season in the spring. Yellowing leaves might hint to a deficiency in micronutrients, first and foremost magnesium which plays a crucial role in photosynthesis and is responsible for the green pigment of leaves. Other micronutrients that can cause yellowing leaves are sulfur, zinc, and iron deficiencies. Apply a balanced fertilizer that includes magnesium. If the foliage remains discolored, do a detailed soil test (available through your local extension Office) before adding a more concentrated form of magnesium.
Yellow leaves can be a sign of bacterial blight. Look for water-soaked sunken spots on the leaves, 1/8 to 1/4 inch in diameter, which then develop into V-shaped yellow areas. As the disease progresses, all the leaves on a branch wilt, turn yellow, and die. At an advanced stage, the stems turn brown or black. The disease occurs often in warm, wet weather, or when the foliage remains wet for a long time due to insufficient air circulation. Bbacterial blight is highly infectious and remains in plant debris in soil for up to a year. Remove the entire Infected plant and dispose of it in the trash.
Southern bacterial wilt is another serious problem, which causes leaves to wilt and yellow. Unlike in bacterial blight, the leaves do not show any spots. The wilting and yellowing leaves are more evident in warmer weather. It is important to get rid of any infected plants promptly, as southern bacterial wilt not only affects geraniums but also ornamentals and vegetables in the nightshade family.
Yellow leaves can also be a sign of herbicide damage. This can happen through accidental overspray or from drift from adjacent properties. If you suspect a spray as the cause of yellowing leaves, water the soil thoroughly to wash out excess. This can help prevent the roots from absorbing an amount that could cause the plant to die.
- Geranium > Leaves > Blotches or spots on leaves. University of Minnesota Extension.
Bacterial Blight of Geranium. Penn State Extension.
Southern Bacterial Wilt. University of Massachusetts Amherst.