Why Are My Trees Turning Yellow?

A chlorotic maple - note the green veins

Yellowing leaves is a symptom in trees called chlorosis. This is a situation in which the pigments that make the leaf appear green are not getting expressed as they usually would — this is why fall color appears red or yellow.

Limited chlorophyll pigments can be caused by a number of factors, but the most common in our area is nutrient deficiencies. The nutrients that make up the chlorophyll, which is where photosynthesis takes place, are limited. If we look at the building blocks of chlorophyll we see that iron or (Fe) is a big one. In fact iron is the backbone of chlorophyll similar to how it is the backbone to Heme in your red blood cells. If the tree can not get enough iron, the leaves will turn yellow and it will be less productive. Specifically, the veins will be green and the spaces in between look yellow. 

Nutrients can be mobile in the tree or immobile. The tree will move nutrients around in the tree depending on where it is best used, most of the time this is in the newest and most productive leaves. So know which leaves are chlorotic (old or new) can give you an insight into which nutrient might be limiting. 

So, why can’t the tree get enough iron? There is in fact copious amounts of iron in our soils. Our slick rock is red with it. The problem is the soil is holding the iron too tightly. This is because of an ionic attraction between the negatively charged soil and the positively charged iron. The more negative the soil (alkaline) the more difficult it is to pull positively charged compounds like iron and manganese out of it. 

Our native trees have come up with ways to bypass this problem. They are able to capture certain particles in the rhizosphere by chelating them. I think of chelation as putting something in a pill capsule. It allows the charged particle to separate from the soil and get absorbed into the root. Trees that come from different soil types like the east coast do not have this ability. Therefore, they become nutrient deficient. 

There are many more reasons trees can become chlorotic. To start, there are other nutrients that can become limited in the plant. Each nutrient has a unique expression when it is limited. Nitrogen can be one of them. This is fairly uncommon in our area, but it should be noted that adding nitrogen often greens up grass and trees.

Another reason trees can be chlorotic is vascular disease. I often see trees with one or more branches that have a fungal disease will also have chlorotic leaves on those branches. If I see a native tree with chlorosis I usually am suspicious of vascular disease. 

Another reason is root rot can sometimes cause a tree to have chlorotic leaves. The roots are not able to function properly and take up the nutrients necessary to the plant. Therefore, the plant becomes chlorotic. 

Some trees are actually selected for their genetic predisposition for yellow leaves. A great example of that is the Sunburst Honey Locust. This tree is not unhealthy. It was bred for this trait. 

So what can you do about it?

If you are seeing yellowing on your trees you might have an iron or manganese deficiency. Or it may be one of the other issues we discussed. 

First, it is probably best to get the opinion of a certified arborist. Chelated iron is expensive and you don’t want to throw money on the ground. 

Second, if it is an iron deficiency, you can get “chelated” iron from Cliffrose nursery. Follow the label for mixing and dosing and apply it to the soil around your trees up to four times a year. 

Third, mulch your trees and water them. Adding organic matter in the soil can slowly change the soil profile. This is a very long term solution and may not be totally helpful for extreme cases. 

Fourth, You can also have us apply iron at a high rate using a soil injector. The soil injector bypasses the grass roots and distributes the iron to your trees’ roots effectively. We may also decide to throw in some nitrogen to aid in uptake. 

Finally, if you have extreme chlorosis we can do a trunk injection. We literally drill holes into the trunk and inject iron and manganese straight into the vessels of your trees. This is guaranteed to green up your trees to a verdant green, and it will keep it that way for up to three years. For those three years, the tree will be healthier since it is more packed with chlorophyll, and it will look like the tree you originally wanted.