Why are the leaves on my tree turning yellow?
Chlorosis, a foliage condition where the leaf veins remain green but the surrounding blade tissue turns pale green or yellow, is a common occurrence among certain tree species on the Northern Plains. The tree species we typically see these symptoms appearing on at this time of year are Amur maple (Acer tataricum subsp ginnala), eastern pin oak (Quercus palustris), red maple (Acer rubrum), northern red oak (Quercus rubra), river birch (Betula nigra), and silver maple (Acer saccharinum). The reason for the chlorotic foliage is not a fungus but usually the lack of iron in the foliage. Iron is the most common microelement missing from the foliage and is the problem for oaks and usually river birch but the lack of manganese can also create chlorotic foliage in maple leaves, particularly Amur and silver maples, turning yellow.
The lack of iron or manganese is not due to the soils containing inadequate amounts of these microelements but from our alkaline soils rendering them into a form not available to the tree. Any soil with a pH greater than 7.2, and that includes many of the soils in our communities, can result in these tree species mentioned earlier turning almost a golden yellow by mid-summer because the soil form of iron or manganese is not accessible. Severely affected leaves can begin falling by the end of July leaving the tree almost bare by autumn and the problem returns each year. Since the problem is not the lack of iron or manganese merely adding these microelements to the soil may not help nor will pounding nails in the tree, as 1) this is a poor way of getting iron in the tree and 2) most nails do not contain a lot of extractable iron. The solution is to either spray the foliage with an iron or manganese containing solution, implanting or injecting iron or manganese capsules into the trunk, fertilizing with a chelated form of iron or manganese or reducing the soil pH so the microelements already in the soil become available.
Spraying the foliage with a ferrous or manganese sulfate will provide a quick “green up” of the foliage but only if the application is made just after the foliage fully expands. If done late in the season the leaves may not remain green very long. The application is only a temporary fix even when timed properly as any new foliage that expands after the application will often become yellow later in the summer. Misapplication of these microelement sprays can also damage foliage and stain concrete pavement or stucco homes so read and follow label directions.
Implanting iron or manganese in the trunk can provide a green-up within a few weeks of the application and the benefits may last a year or two. This is generally a much better system than attempting to spray the foliage. There are implants that are available for homeowner use, such as Medicaps, but these are rarely carried in local garden centers and department stores. Some of the larger chemical supply stores and some larger garden centers may carry them. The products are easy to apply but the directions should be carefully followed to avoid any unnecessary injury to the tree.
There are also injectable FE or MN products that are highly effective in treating the symptoms and are also long-lasting. However, these materials are applied by commercial tree companies. The companies that offer injection treatments have the knowledge to determine which microelement is missing, either iron or manganese (and occasionally both) and the injection equipment to apply the product while minimizing injury to the tree.
Chelated forms of iron and manganese can be applied to the soil and these applications provide benefits for usually a year or two but it may take several months before the leaves lose their chlorotic appearance. The chelating agent keeps the iron or manganese in a form available to the tree but not all chelating agents are effective in our slightly to moderately alkaline soils. The best chelating agent for our soils is EDDHA and this one should be on the label. Chelated iron and manganese is available at many garden centers and farm supply stores but you still may have to do some looking.
Altering the pH so that the iron and manganese in the soil is available is the best solution but is not easily done. The alkaline soils in our region are well-buffered meaning the pH is not easily lowered or will stay lowered for very long. However it is worth a try and the most common acidifying agent is elemental sulfur (sold as organic soil acidifier). This can be easily purchased in the garden section of many building supply stores.
Since the chlorosis problem occurs more often on trees that have a poor root environment, improving the soils by mulching may improve root performance by providing a cooler, moist environment. The best mulch is shredded pine bark mulch to a depth of 2 inches out for at least several feet (but try to leave about 6 inches around the trunk mulch free to discourage rodents). This is not being done to lower the pH, as it will not have much of an effect, but just to provide better conditions for root growth. Mulching alone rarely solves the chlorosis problem.
Obviously the best solution is not to plant red oak, river birch, Amur maple or silver maple on soils with a pH above 7.2 and for pin oak and red oak, 6.8. I always recommend a soil test before selecting a tree to plant – better to avoid the problem to begin with rather than have to treat it. There are numerous publications including the new book Trees, Fruit, Nut, Ornamental, Shade and Windbreak Trees for the Northern Plains that list every species that will grow here by their alkaline tolerance. If your soil test says the pH is very high, 7.8 or more the list of suitable trees becomes small but there still are choices. The most alkaline tolerant trees for the Northern Plains are:
Boxelder (Acer negundo), ginkgo (Ginkgo biloba), honeylocust (Gleditsia triacanthos), mulberry (Morus alba), plains cottonwood (Populus deltoides subsp. monilifera) and American elm (Ulmus americana).
These are not perfect trees but they are among our toughest in regards to soils!
A Question from a Brookings Gardener
By David Graper, SDSU Extension Horticulture Specialist
How can I get rid of the hundreds of maple seedlings growing in the mulch around spruce trees? Is there a spray that I can use? What about the seedlings growing in the lawn?
The continued wet spring weather has made it a great year for seeds of many different kinds of trees to germinate and form colonies of little tree seedlings. You will mostly see them in mulched areas or along buildings where the seed collected and has now germinated. They can also grow in lawn areas. If your mowing height is relatively short, they will be mowed off which after a few mowing should kill them. If you have a taller mowing height, it will take a while longer but they should die out as well.
The seedlings can be controlled by pulling, tilling or by using most broadleaf herbicide products in mulched areas or in lawns. A non-selective herbicide containing glyphosate will also work in areas where you want to kill everything or it can be used to do spot spraying around existing trees and shrubs, being careful to not get it on exposed green tissue of desirable plants.