Yellowing leaves with green veins may be a sign that they need more iron. You may also be seeing signs of cold weather. This time of year I get some of the older leaves yellowing and dropping, but come spring, they bloom like crazy and are just fine.

{{gwi:386596}}Yellow leaves with green veins is a well understood condition called chlorosis. However, it is so well understood that it has many causes. Although it indicates an iron deficiency, the most common cause is lack of acidity and powdered sulfur is a remedy. A soil pH test will verify this. Powdered sulfur is slow acting but long acting which is a good combination. Do not use Aluminum Sulfate. They use it on hydrangeas but it will kill rhododendrons and azaleas.
The desired pH is 5.5. 3 Tablespoons of powdered sulfur per square yard will lower the pH by about 1/2 a point.

The other causes of chlorosis are:

Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency or too much phosphorus in the soil. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways. Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron that provides the same results. Follow the label recommendations for mixing and applying chelated iron. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer.