Yellow leaves on maple tree and some aspen
I have some type of maple tree in my back yard that has had yellow leaves the last few summers. They used to be green. I also have one aspen that is somewhat yellow with it's leaves. I heard they need iron and we put in iron stakes last year. It seemed to help two of the three aspen I have but not the maple nor the third aspen. Should I do more iron stakes? Is there something else I should do?
Larimer County Colorado trees and shrubs
It sounds like your trees may have iron chlorosis, a common ailment of certain tree species in Colorado due to high pH, wet soils, compacted soils or salty soils. Most commonly, it's because of high pH. We have plenty of iron in the soils, but at high pH (above 7.0), it becomes unavailable to plants. Then the leaves begin to yellow and the veins of the leaf tend to stay green. It can cause dieback/death over time if not treated.
If you decide to treat, it's important to use the correct iron chelate. The most common one available for use is EDTA. This will not work in our soils. Instead, you need to find a chelate that is EDDMA or EDDMHA. These tend to be more expensive, but work far better than EDTA. The iron spikes (if I had to guess) are likely EDTA.
There are three ways to correct iron chlorosis...foliar sprays, trunk injections or soil drenches. All three are explained in greater detail in this CSU Extension Garden Note: http://www.ext.colostate.edu/mg/gardennotes/223.html