Geranium leaves yellow when they receive too much or too little water. Yellowing leaves may also mean that the plant is not receiving enough nitrogen. Other times, the color may change when weather becomes cooler.
When geraniums receive too much water, the leaves yellow toward the bottom portion of the plants. If the leaves have yellow edges, alternately, the plants received too little water.
Geraniums should be watered when the soil completely dries, but the leaves should not become wilted. When the bulk of a geranium plant's leaves are yellow, this may indicate a nutrition problem. To correct this, gardeners should fertilize the geraniums every month with a 10-20-10 water-soluble fertilizer.