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The Garden Plot for October 02, 2009
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Fertilizing Trees”
We get lots of questions about when to fertilize landscape trees. In the absence of a soil test, shoot growth is the best indicator of the need for additional fertilizer. If new shoot growth at the tips of branches is more than of 6 inches, then fertilization is probably not necessary. If shoot growth is less than 2 inches, then fertilizer applications may help the growth of your tree. Leaf color is another indicator of the need for fertilizer. Yellow or “off-color” leaves may indicate the need for fertilization. A final indicator of the need for fertilizer is the history of the landscape. Trees in fertilized lawn areas rarely need supplemental fertilizer. Trees growing in naturalized areas where little or no mowing takes place and leaves are not collected usually will not need regular fertilizing. In lawn and naturalized areas, supplemental fertilizer should only be applied to trees if shoot growth is less than 2 inches, or if a soil test reveals a specific nutrient deficiency.
If the only indicator of the need for fertilization is slow shoot growth, then a high nitrogen fertilizer should be applied. If, however, the leaves of the tree are yellowing, or there is some other indication of a nutrient deficiency, then it is best to take a soil test from around the root zone of the tree. Leaf yellowing may be due to another nutrient deficiency. The most common cause of leaf yellowing in Montana is a lack of iron. Iron deficiencies are common due to high soil pH (above 7.0) rather than a lack of iron in the soil. Because different trees do best at different pH levels, soils should be checked for pH before planting. If you know that your soil has a pH of 7.0 or higher, be sure to choose tree species that can take up iron under high pH conditions. If a sensitive tree has already been planted on a high pH site, pH can be reduced by amending the soil with elemental sulfur and using a fertilizer high in ammonium nitrogen, such as ammonium sulfate. Reducing the pH of a soil may take many years to correct. Some tree and shrub species that can tolerate higher pH include: Silver Maple, Paper Birch, Hackberry, Red osier Dogwood, Bur Oak, Linden, Arborvitae, Rocky Mountain Juniper, Pines (Limber, Lodgepole, Mugo, Ponderosa, and Scotch), and Black Hills and Colorado Blue Spruce.
When is the best time to fertilize trees? Most trees put out a single flush of growth in the spring followed by slower growth throughout the summer and fall. Because of this single flush of growth, nutrients should be available to the tree as this growth is about to occur in the spring. The best time to apply fertilizer, especially nitrogen fertilizer, is from when soil temperatures rise consistently above 45 F, to just as trees start growing in early May. On sandy soils, applications should be split, half in early spring and half in mid- to late May.
If trees show yellowing leaves or extremely slow growth, then fertilizer can be applied until the end of June, or in the fall after foliage has begun to color and drop. After July 1, trees should not be fertilized since new growth stimulated by the fertilizer may not have sufficient time to harden off before winter.
For more information on fertilizing trees and shrubs, contact your local county extension agent.
Helen Atthowe's new short program of gardening tips
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