Weidner: Have you noticed the yellowing trees?

It’s still a touch too early for fall color, but some of our landscape trees are certainly changing and it’s not a good thing. While this change in color at this time of year isn’t something homeowners like to see, it is unfortunately rather prevalent. The yellowing is generally referred to as chlorosis. Trees in Colorado often deal with iron chlorosis, but can also be susceptible to zinc, manganese, and nitrogen deficiencies, which can cause similar effects.

Let’s start with iron chlorosis, though. An easy-to-spot symptom is yellowing leaves with green veins. These indicate an iron deficiency in the leaf tissues, especially in newer leaves toward the branch tips. Leaves can be smaller than usual and can even curl, dry up, and fall. Effects of iron chlorosis can include decreased plant growth, vigor, and stress tolerance, plus increased susceptibility to disease and insect infestations.

So, why is there an iron deficiency to begin with? We first need to think about pH because it plays a huge part in our soils and what can grow well in them. Soil pH is a measurement of the acidity or alkalinity of a soil. While 7.0 is neutral, anything below it is acidic and anything above is alkaline (basic). A soil’s pH is mostly a result of the parent material of the soil and of the climate. Here in Colorado, our soils are alkaline with a pH of 7.0 to 8.3 because of the high calcium carbonate content, known as free lime. We have a generous amount present because of rock weathering and limited rainfall. Other parts of the world with higher rainfall amounts have more acidic soil because the rainwater leaches ions that contribute to alkalinity out of the soil profile.

This free lime is what affects the availability of some of our plant nutrients, chemically tying up the nutrients, especially iron, in forms that are more difficult for plants to absorb. Plants that have evolved under these kinds of soil conditions often develop mechanisms to deal with this issue. That is why it’s important to select plants that are adapted to your soil pH when possible.

It is this situation that causes iron chlorosis in many of our maples and aspens, resulting in their leaves turning a sickly, washed-out color sometimes even verging into yellow. They are not able to extract enough iron in the chemical form they need. Our red soils will tell you that they are rich in iron, but that iron simply isn’t as chemically available to those plants’ particular absorption abilities.

While many trees can suffer from iron chlorosis, particular species are more susceptible, including most maples, aspens, birches, and some oaks. The best way to avoid chlorotic trees is to never plant them. There are plenty of trees out there, even particular maples, that are well-suited to our soils and climate, so opt for those. While aspens are native to Colorado, they are not happy at our elevation and with our plains climate. Save yourself the heartache and avoid planting them in the first place. Check out the Front Range Tree Recommendation List online for an extensive list of trees that have been vetted and rated for their landscape potential.

What to do if you’ve already got trees in the ground? You can diligently apply iron chelate, iron sulfate, or ferrous sulfate treatments to their root zones that will offset the chlorosis, but treatments must be repeated multiple times a year to keep the trees looking good and the effectiveness and type will depend on your soil’s particular pH.

You will never be able to adjust the pH of your soil. Yes, that is a hard truth and one that many don’t want to face, but with the amount of free lime in our soils, it is impossible to apply chemical fixes that often work in other parts of the country in the astonishingly high amounts that would be needed to see a permanent change here in Colorado. I’m sorry to share this bad news, but it’s better that we all know and accept rather than throw money, time, and effort at a lost cause!

Other helpful measures to avoid iron chlorosis or reduce its symptoms include avoiding compacting the soil, being sure not to plant new trees too deep or without fixing their girdling roots prior to planting, reducing nearby plant competition, being careful not to over-fertilize, which can increase salts in the soil, and ensuring that you are neither over- nor underwatering. All of these efforts are ensuring good root health and as much nutrient availability as possible.

Also, keep an eye out for the possibility of zinc and manganese deficiencies, which result in similar leaf symptoms as iron chlorosis but instead of showing up on young, new growth, they tend to first appear on older, interior leaves.

Nitrogen deficiency will show a uniform yellowing of the entire leaf, including the veins. It presents in the older leaves first. There are products out there and cultural practices that can help with these issues as well, but knowing just what soil conditions you are dealing with is essential to getting the most out of your treatments.

For a homeowner, a soil test gives a useful baseline on pH, soil salts, phosphorus, potassium, and free lime content plus other important nutrients. Colorado State University’s SPUR campus in Denver hosts our soil testing lab and we encourage gardeners from across the state to take advantage of their services so that they can get the details about their particular soil that will help them best manage and improve it over time.

Feel free to visit their website at agsci.colostate.edu/soiltestinglab to explore the different tests, their pricing, and instructions on how to submit a sample.

*Information sourced from CSU Garden Notes publications.

Amanda Weidner is the horticulture specialist at the Pueblo County CSU-Extension Office. She can be reached at 719-583-6581 and weidnera@pueblocounty.us.

This article originally appeared on The Pueblo Chieftain: Weidner: Have you noticed the yellowing trees?