Colorado—the word means “red” in Spanish. And Colorado’s soils are often reddish, due to the abundance of oxidized iron. Here in Colorado Springs, Garden of the Gods (right) attracts visitors with bright orange sandstone monoliths. Further north, Red Rocks Park and Amphitheatre is part of the same formation. Our well water has so much iron in it that our white laundry turned pink—we had to install an iron-removal component to our water system.
So, with all this iron present in our soils, why do so many plants here suffer from a deficiency?
Along with the pretty landscape, large sections of Colorado also have calcareous soils—soils with high levels of calcium carbonate, or limestone. All that calcium raises the pH to the point (over 7.5) that all that iron present is unavailable to plants.
Some species are more likely to develop chlorosis than others. Maples, otherwise well adapted to our climate, suffer greatly. From apples and arborvitae to spiraea and sweetgum, popular plants that thrive in other parts of the country have difficulty in our limey soils. Even natives are affected—aspen, cottonwood, spruce, douglas-fir, junipers, and pine can all exhibit chlorosis.
The easiest fix is just to avoid growing these species. But what if you want an apple tree or grapevine? What if your favorite fruit is raspberries and you want to grow your own? And how can we live in Colorado and now include conifers in our landscape? All these species have a high susceptibility to iron chlorosis—can we still grow them here?
If high pH is the cause of the problem, why not just lower it? Adding sulfur will do the trick, but you’d have to add so much sulfur to buffer the calcium in our soils that it’s not a practical solution. We have to look elsewhere.
Soil pH isn’t the only factor that contributes to chlorosis. Pretty much anything that stresses the plant will tip it over the edge, and it will begin to develop the tell-tale yellow leaves with green veining. Over-watering, soil compaction, overcrowding, winter injury, too much humus (compost) in the soil, high salt levels (such as results from the use of packaged dehydrated manures), too much fertilizer, and even high soil temperatures can all be part of the problem.
If your plant shows signs of chlorosis, the first thing is to address the above list. Are you watering too much? Perhaps your sprinklers need adjusting. Is the soil compacted from too much foot traffic? Try creating a permanent path, then mulching the area around it to let the soil recover. If the affected tree is planted in a lawn, aeration may help.
Competition can be reduced by first pulling all the weeds and then, if necessary, eliminating some of the more desirable plantings. Winter injury can be avoided by wrapping the bark of young trees, or painting them white to reflect the winter sun.
In mid-summer, soil temperatures can soar, injuring roots just under the surface. A cooling three to four inch layer of mulch will shade the ground.
It’s hard to believe that there’s such a thing as having too much organic matter in our soil. But years of following the popular gardening mantra of adding a yearly layer of compost and working it into the ground can result in humus levels far above the optimal 5%. Over-fertilization can also cause problems. It’s best to have your soil tested so you aren’t “flying blind” when it comes to amendments and nutrient levels.
If none of these fixes restores your plant to health, try adding iron. Most garden centers sell chelated iron, a form that the plant can immediately use. The best time to apply chelated iron is now, before summer’s heat. Plants should respond relatively quickly. If they don’t, they may be deficient in another element as well—manganese deficiency another common issue in Colorado. Iron applications may only last a couple of years, then need repeating.
Happily, with some effort, we can grow chlorosis-susceptible plants in our yards.