Geranium leaves that begin to turn yellow are noticeable because of their scalloped edges and larger size. A number of factors contribute to the yellowing of leaves on a geranium plant; identify the cause so proper treatment can be applied to the plant so it will continue growing.
Too little water causes the leaves of a geranium to become yellow; drought conditions in particular can become a problem for these plants. As the geranium loses water, the edges and tips of the geranium’s leaves will turn yellow and brown first; eventually, the geranium plant will enter into a dormant state to keep itself alive until there is adequate water. Water geraniums at least once a week to keep the plants healthy and thriving.
Geraniums require nutrients like nitrogen and iron to grow healthy, green leaves; a deficiency of these nutrients will cause the plants to put out weak, yellow leaves or experience yellowing at the edges of the leaves. Serious nutrient deficiencies affect the ability of a geranium to produce healthy flowers during the growing season. Add fertilizer or compost to the garden around the base of the geranium to supply the plant with the nutrients it needs.
Anthracnose is a fungal disease that affects a wide range of garden plants, from trees to flowers; it thrives in excessively moist soil or standing water and causes the leaves of geraniums to turn yellow, starting at the edges. This is followed by rapid wilt and eventually the death of the geranium. Apply fungicides to the geranium and pinch off or prune severely damaged portions of the plant to slow the spread of the disease.
Aphids, spider mites and other insect pests like caterpillars feed on the leaves of geranium plants, leading to yellow edges first. Many insects feed along the edge of the leaf, which leaves holes in the plant that dry up and turn yellow. Apply a pesticide, insecticidal soap or horticultural oil to the geranium to control insect problems and keep the geranium’s leaves healthy. Follow all manufacturer’s instructions when applying pesticides and other chemicals to plants.