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The Homeowners Column
Nutrient deficiencies can cause yellow leaves in trees and shrubs
July 14, 2016
State Master Gardener Coordinator
Designer plants come in all leaf colors from the deep purple leaves of 'Palace Purple' coral bells to the chartreuse yellow leaves of 'Margarita' sweet potato. However, not all leaves vary by design from the 'traditional' green.
Chlorosis describes the abnormal yellowing of plant leaves due to a lack of chlorophyll development or destruction of chlorophyll. As Travis Cleveland reported in a recent issue of University of Illinois Home, Yard & Garden Pest Newsletter (http://hyg.ipm.illinois.edu/) the yellowing is often due to a nutrient deficiency resulting from either a lack of nutrients or the plant's inability to absorb existing nutrients.
In trees and shrubs, the two most commonly deficient nutrients are iron and manganese. Chlorosis is often seen in our area on pin oak, sweetgum, maple (especially red, silver and their hybrids), birch, azalea, rhododendron and blueberry.
Iron and manganese chlorosis can produce similar symptoms according to Cleveland. However, iron deficiency causes the most intense symptoms on the newest leaves, while manganese deficiencies affect the older leaves. Often the leaf is yellow but the leaf veins remain green. In some cases, only one side of the tree may be affected.
As chlorosis progresses, the leaves may show brown speckling, become stunted and newer leaves may die. Branch tips and eventually entire branches may die. After several years of suffering even mature trees may die.
Different situations can cause chlorosis but according to Cleveland high pH soil in relation to the plant's needs is the primary cause. In most cases, the soil has plenty of micronutrients for plant growth; however, soils with pH above 6.5 bind some of the iron and manganese and render it unavailable to the roots. Plants such as pin oak have a high need for iron; therefore, insufficient iron is available in high pH soils.
Situations that exacerbate high pH soils include: soil compaction, poor drainage, root injury, flooding and drought. In addition, plants near limestone driveways or sidewalks can suffer chlorosis. Excessive applications of limestone, wood ash or fertilizers high in phosphorus can aggravate chlorosis.
The best solution is to avoid planting tree species susceptible to iron and manganese deficiencies on sites with elevated soil pH. Prevent chlorosis problems by correcting poor soil drainage and compaction and avoiding overwatering with excessive irrigation.
Management for chlorosis involves determining what element is missing, then trying to alleviate the cause of the deficiency. A soil test is a good place to start to identify nutrient availability and to check pH.
Several options for supplying nutrients are available according to Cleveland. Which option is used depends on the severity of the problem, tree age, soil pH and site restrictions. A certified arborist may be needed.
Acidifying high pH soils addresses the true cause of the problem, but is a challenging and perpetual task. Additionally, acidification may not be feasible for larger, established trees. Soils can be acidified using elemental sulfur (economical, slower acting) or iron sulfate (more expensive, faster acting).
Fertilize with an available form of iron or manganese, using one of the following techniques.
1. Foliar fertilization involves spraying the micronutrients directly onto the foliage. Often used for small trees and shrubs it offers quick but temporary results, and will need to be reapplied annually.
2. Chelated iron fertilizers can be worked into the soil surrounding the tree using a variety of techniques including incorporating the fertilizer into the top few inches of soil, applying it into evenly spaced, 12-15" deep holes, or the chelated iron can be dissolved in water and then pressure injected into the soil.
3. Various trunk injection technologies by professionals are available to treat deficiencies. All require small holes at the base the tree. Results may not be observed until the following growing season. Treatments generally last 2 or more years. While effective, some fear repeated applications may injure the tree; however, untreated trees also suffer damage.