It's that time of year when iron chlorosis has started to show up again as the yellowing of leaves with a darker green color immediately around the veins in a leaf. This usually shows up on the younger leaves first. This yellowing is particularly noticeable on pin oaks and sweet gums, but may be seen on other species.

The cause is the lack of available iron for the plant. Manganese is another element that can cause these symptoms. There can be tons of iron in the soil, but if the soil pH is not acid enough the plant cannot take the iron up. Possible solutions include: altering the soil pH with either nitrogen or sulfur (be careful since it may take a truckload to alter the soil around a large tree), spraying leaves every 2-4 weeks with a foliar iron compound, or implanting iron tablets in the trunk which would last from 2-4 years.

Injury from iron chlorosis is the eventual decline of the plant, and it may lead to plant death over a period of time. Also, don't expect treatments to green leaves up immediately when applied now. In fact, many times the implanted iron tablets don't show green leaves until the following year when the sap begins to rise. Many people have been using the iron tablets in the dormant period of the tree and have had good results.