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Perplexed by yellowing leaves on trees, plants and shrubs?

Posted on: August 7th, 2017

Outdoor plants which appear to be stressed are common along Colorado’s front range during the summer months. From plants and turf, to shrubs and trees – they are all affected by something called chlorosis (yellowing leaves).

Chlorosis is a term that describes a lack of chlorophyll (green pigment) in the leaf.  Chlorophyll is the energy production engine in the plant.  Along Colorado’s front range, the most common causes of chlorosis include alkaline soils (high pH) and hot summer temperatures.


Alkaline soils (high pH) account for much of the chlorosis issues in Colorado.  High pH soils are negatively charged.  Iron particles are positively charged.  As you may remember from chemistry class – opposites attract.  Therefore, iron binds to highly alkaline soils.

While ample iron exists in our soils, it is chemically bound to the soil particles and less available to the plants.  Plants that are chlorotic from a lack of iron may be green early in the spring, but become chlorotic as iron is depleted.


Excessive heat exacerbates chlorosis.

In the soil, extremely warm temperatures may slow root growth – especially on shallow rooted trees and shrubs.  In the canopy, foliage is cooled by evaporating water on the leaf surface.  However, as air temperatures continue to rise, this cooling system may not be able to keep up.

When heat becomes extreme or the plant becomes drought stressed, the foliage will essentially overheat.  At this point, chlorophyll production slows and the leaves become yellow (scorched) and will eventually turn brown.

What can you do about yellowing leaves? 

Introduce iron treatments into your landscape.

Treatments include foliar, trunk injection and soil treatments.

Foliar (spray) iron treatments may be done any time plants are in leaf.  The foliar treatments are short lived requiring multiple applications per year.

Trunk treatments involve putting iron into small holes drilled into the tree. Results are often favorable, but the procedure is invasive for the tree.

Soil treatments for trees and shrubs are best completed in spring for the current year or fall for the following growing season.  Make sure to use a chelated iron (chelation keeps the iron from being bound to just the soil and available for the plant as well).


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