Under normal healthy conditions, hydrangeas have large green leaves. However, different factors may cause leaves to suddenly turn yellow. This may be caused by over watering of the plant or unhealthy soil conditions.
When established leaves on a hydrangea begin to turn yellow, nitrogen deficiency may be the cause. This can occur if the soil has a pH level that is too high. Adding a fertiliser that includes nitrogen, but does not include phosphorus, can solve this problem.
Hydrangeas need a lot of watering, but it is possible to over water them. If leaves begin turning yellow, cut back on watering to see if the condition improves. The type of water used can also be the culprit of yellowing leaves. If new leaves are sprouting yellow, the water may be too highly alkaline.
Iron chlorosis is a common cause of new leaves on hydrangeas turning yellow. As the pH level goes up, the amount of iron in the soil is less available. This can be corrected by adding iron supplements to the soil.