Dr. Curtis E. Swift

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August 30, 2012
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SWIFT: Shedding light onto our stressed out maple trees

As I look around the valley I can tell at a glance which trees are silver maples or silver maple hybrids. These trees look like its fall with their burnt leaves. They started mid-summer with yellow leaves and went downhill from there. The trade name of the most common of these maples is Autumn Blaze, a tree once touted as great for this area.The Autumn Blaze maple has the botanical name of Acer x freemanii. The x between the genus and species names indicate this tree is a cross, in this case of the silver maple Acer saccharinum and the red maple Acer rubrum. The cultivar name is 'Jeffersred' thus the true name of this maple is Acer x fremanii 'Jeffersred' Autumn Blaze.Silver and red maples evolved in areas where the soil is acidic. Unlike trees that evolved in areas where the soil is alkaline, silver and red maples did not develop the ability to produce the compounds necessary for the absorption of iron and other elements trapped in alkaline soils.Alkaline soils are defined as those soils with a pH above 7. The problem with alkaline soils typically is not the lack of these nutrients in these soils but rather the inability of the plant to solubilize and absorb the proper form of elements such as iron and magnesium into their sap stream. In order to release these trapped elements, plants that evolved in alkaline soils trigger the release of phytochelates and hydrogen ions (H+) from their roots when a deficiency is noted. Phytochelates change insoluble iron into a soluble form while the hydrogen ions acidify the soil immediately around the root. Both of these processes make the iron more available for uptake. It is equally possible the first maple tree evolved in alkaline soils with the ability to accomplish these tasks but lost this ability when encountering more acidic soils. These reactions were not needed in acidic soils and thus were discarded. Plants whose roots lack the ability to produce these compounds in alkaline soils are subject to nutrient deficiencies. The problem with Autumn Blaze could also be due to graft incompatibility.I'm not sure what rootstock the Autumn Blaze is grafted onto, but it is most likely a seedling silver maple as these seedlings are cheap to produce. Silver maple as I indicated is not adapted for survival in alkaline soils. Nurserymen plant out hundreds of seeds collected from silver maples and when the seedlings are of the proper diameter, a bud from an Autumn Blaze is grafted into each seedling stem. The seedling is then cut off just above the grafted bud. This creates a tree that is Autumn Blaze on the top with a different maple for its root system. There are reports of these trees dying years after planting due to the tissue at the graft no longer being able to move water, nutrients, and food materials through the graft union.Even if this hybrid maple was propagated on its own roots, the tree would still most likely have problems with nutrient uptake and eventually suffer plant death, at least in soils with a pH above 7. It would still have the genes of its parents, genes that are not adapted to respond to nutrient deficiencies in alkaline soils. Grafting into the rootstock of a maple that evolved in alkaline soils, such as the Tartarian maple, Acer tartaricum, might solve the nutrient deficiencies of the Autumn Blaze maple but would most likely create a smaller tree.The silver and red maple evolved in areas where the soil pH is acidic. Our soils are basic, that is they have a high pH. These soils are also referred to as alkaline. Some people feel they can change the basic pH of these soil by adding acid and some viticulturists have applied acid to their grapevines through their drip system for six years without seeing any change in soil pH. In other words, the pH is still basic. This is due to the abundance of free calcium (Ca++) in our soils buffering our soils from pH change.Some people believe they have the answer to yellow leaves and tree decline - iron. There are several types of iron recommended by nurserymen to correct this problem. These are chelated iron products. A chelate is a compound that holds the iron ion, typically Fe++, until it comes in contact with a root where it releases the ion. While I'm simplifying the process, this is basically what happens. Not all chelates work in alkaline soils; they have varying degrees of ability to retain Fe++ depending on the pH of the soil. When applied to a high pH soil (i.e. alkaline) some chelates to include CDTA and DPTA release their iron to the soil and chelate a calcium ion. The iron ion released converts to a non-soluble form. Selecting the proper chelate is therefore critical. Even then applying an iron chelate may not work to correct the problems the Autumn Blaze maple has in alkaline soils. Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curt.Swift@alumni.colostate.edu, visit WesternSlopeGardening.org, or check out his blog at http://SwiftsGardeningBlog.blogspot.com.


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The Post Independent Updated Aug 30, 2012 05:36PM Published Aug 30, 2012 05:34PM Copyright 2012 The Post Independent. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.