Have yellow trees? Low iron often the culprit

KENNEWICK -- Earlier in the season I noted that, once a hater of shrubs, perennials and ornamental grasses with naturally yellow to yellow-green leaves, I have started to like them for the color contrast they provide.

However, I still don't like yellow trees.

That's because many local trees with "yellow" leaves are supposed to be green. They have a problem.

Yellowing of leaves that should be green is called chlorosis. It's most often caused by a lack of iron in leaf tissues.

Iron is required for the formation of chlorophyll, the green pigment found in plants. Chlorophyll is essential for photosynthesis, which allows green plants to transform the sun's energy into energy for their growth.

Iron chlorosis is characterized by yellowing between the veins with the veins staying greener.

When severe, the veins also turn yellow, and brown tissues develop along leaf edges and between the veins. If the condition persists, branches start to die back, and eventually the tree dies.

If a lack of iron in plant tissues is the cause of chlorosis, it would seem logical that applying iron to the soil would be the solution. Unfortunately, it's not that simple. Our local soils contain plenty of iron, but it's not in a water-soluble form that roots can absorb. That's because most of our local soils are somewhat to very alkaline, with the average pH being higher than 8.0. In alkaline soils iron is in an insoluble form, making it unavailable to plants. That's why simple applications of iron fertilizer to the soil won't solve the problem.

Most of our local soils already are quite alkaline, but construction practices can exacerbate the situation by raising the pH even more. New construction usually involves soil excavation with the top most layer of soil being removed and hauled away during the build process.

The ideal would be to have that same soil brought back and placed on top of the subsoil that was left. This doesn't frequently happen and an owner is left with subsoil that can be significantly more alkaline. This also happens due to extensive excavation and soil movement when digging a basement or creating landscape features.

Add to all of this the concrete used in construction. After a foundation, driveway or walkway is poured, workers typically wash the truck off at the site. This wash is very alkaline and raises the pH of the soil in the area, making it even more alkaline. There are other factors that can increase the severity of the iron chorosis including excessive soil moisture, soil compaction, drought, cool soil temperatures early in the season, extremely high levels of phosphorus, irrigating with hard water, and use of black plastic as a mulch.

Certain types of trees and shrubs are very sensitive to alkaline soil and prone to chlorosis. This includes dogwood, silver maple, pin oak, red maple, dawn redwood, Amur maple, azalea, rhododendron, blueberries, raspberries and grapes.

Locally, we sometimes see chlorosis on sweetgum, magnolia, birch, tulip tree and cherry when the soil pH is very high.

I occasionally drive by a group of silver maples in Kennewick where I suspect excavation resulted in the trees being planted in very alkaline subsoil. These trees are becoming more and more yellow each year. What can be done to save them? In a future column I'll discuss approaches for solving chlorosis problems.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.