Overwatering
Yellowing leaves on hardy geraniums are a sign of too much water.
To prevent overwatering, ensure to plant in well-draining garden soil, or in containers with large drainage holes.
Underwatering
Because geraniums are drought-tolerant, yellow leaves from underwatering are less likely.
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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.
Early fall foliage of a wild geranium (Geranium maculatum)Glenda Christina / Design Pics / Getty /Images
Water the plant slowly and deeply so water reaches the entire root ball.
In a location with too much shade, the leaves turn yellow.
As it gets warmer, yellow leaves are soon replaced by new healthy foliage.
Hardy geranium leaves after frostThomasaurus / Design Pics / Getty Images
Apply abalanced slow-release fertilizeronce a year at the beginning of the growing season in the spring.
Other micronutrients that can cause yellowing leaves are sulfur, zinc, and iron deficiencies.
Apply a balanced fertilizer that includes magnesium.
Diseases
Yellow leaves can be a sign of bacterial blight.
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.
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.
Herbicide Injury
Yellow leaves can also be a sign of herbicide damage.
This can happen through accidental overspray or from drift from adjacent properties.
This can help prevent the roots from absorbing an amount that could cause the plant to die.
Bacterial Blight of Geranium.
University of Massachusetts Amherst.
