There are lot of yellow leaves showing on the trees around town right now and it is not because of an early fall. The trees, mostly maples, are experiencing an iron deficiency, chlorosis, which results in less chlorophyll in the leaves. It is the chlorophyll that gives the leaves the green color so when amount of chlorophyll is reduced the leaves turn to a paler yellow green (sometimes reddish) and the veins within the leaf remain a dark green. Chlorophyll is essential to photosynthesis which is the energy production process that keeps plants alive and growing. So without enough chlorophyll the trees are at risk.
It is the higher pH of our alkaline soil that makes the iron already in our soils develop into a solid form that is unavailable to the plants’ roots. Iron chlorosis is further aggravated restricted air flow into the soil caused by plastic sheet mulching, soil compaction and water saturated soil. When the topsoil has been removed and plants are in the subsoil chlorosis is often more severe. The plants need to have an iron supplement in a form that they can readily and easily absorb, sticking nails into the soil or sprinkling iron shavings won’t work.
The most common form of effective iron supplementation is to use chelated iron as a soil drench. It is even more effective when the EDDHA molecule is included in the mixture. This is quite easy to use and yields quick results. However is not particularly long lasting (one season or a little less).
A longer lasting but more labor intensive and slower acting solution is using a combination of ferrous (iron) sulfate and elemental sulfur. This is applied to trees by drilling a series of holes around the tree and filling them with the mixture. Using a Tree and Shrub or Lawn fertilizer that contains iron and sulfur can increase the duration of both the treatments.
Foliar spray and trunk injections, especially on larger trees, are probably best left to an arborist. We carry a variety of the treatment products for soil and drenching application, with or without the EDHHA additive. The drenching products can be used as foliar sprays on smaller trees and shrubs. Timing of foliar spraying is important to avoid burning the leaves. Sprays act fast but have a short duration and have to be reapplied to the foliage as the tree grows.