GARDENING: Yellow maple leaves could indicate a treatable condition
Guest Gardening Columnist
TREE AUCTION & PLANT SALE, OCT. 5
If you are in the market for a tree, the Tri River Area Master Gardeners will be holding their annual Tree Auction and Plant Sale Saturday, Oct. 5, at the CSU Extension office at Mesa County Fairgrounds. Come enjoy the fun; the sale starts at 9 a.m. and the auction begins at 10. Proceeds benefit the Master Gardener program in Mesa, Delta, Montrose and Ouray counties.
Have you noticed while driving around town that certain trees are a pale yellow green, or even completely yellow? Look closer, and you will discover that in most cases the trees in question are maples. Silver maples are especially likely to display this condition called chlorosis.
These maples don’t really like our soils here in the valley. They typically have shallow roots which are at great risk from winter dehydration. This is especially true if the soil where they are planted wasn’t well prepared prior to planting, and if they are watered heavily during the spring and summer months. Both of those conditions contribute to shallow root growth and subsequent loss over the winter.
The best solution to the problem of chlorosis is to avoid planting susceptible trees in the first place, but if you already have these trees it’s too late for that. If the tree is in a lawn, having the turf core aerated in spring and fall, followed by a top dressing of a fine-textured compost at the rate of one-third of a cubic yard per thousand square feet, will gradually improve the soil and encourage deeper rooting. Water the area deeply but as infrequently as you can without stressing the lawn. Root growth requires oxygen as well as water, so the soil around the roots of the tree needs to be allowed to dry out partially between waterings.
Winter water is essential for these trees. Water once a month or so, on a day when the air temperature is above freezing, and water during the middle of the day. Cover the entire area from the trunk to the dripline, and farther out of you can. Yes, domestic water is expensive, but not as expensive as a new tree.
In the spring when the tree leafs out, apply a nitrogen fertilizer at the rate of one pound of actual nitrogen per thousand square feet. Actual nitrogen can be calculated from the number on the product, which is a percentage. For example, ammonium sulfate is 21-0-0, or 21% nitrogen, so five pounds would be needed to provide one pound of actual nitrogen. The combination of winter watering and a spring nitrogen application may take care of the problem without further treatments. It’s where we recommend people start when treating chlorotic trees.
Iron is often recommended for treating chlorosis. Although there is abundant iron in our soils, our high pH means that it is largely unavailable to our plants. Iron can be applied as a soil treatment, a foliar treatment, or with trunk injections by a skilled technician. Each method has its advantages and disadvantages. A soil application is the easiest and the least likely to harm the tree; a chelated iron with EDDHA has been shown to give excellent results as a soil application. Ferrous sulfate is less expensive but slower to take effect and results can vary. Soil treatments should be done in the fall or early spring. A foliar application of chelated iron is faster to take effect, but somewhat impractical for large trees. It may be done during the growing season but should be avoided in the heat of summer.
Trunk injections should be done by an experienced technician to avoid further damage to the tree. The most effective time is early in the spring during bud break; avoid treating in hot, dry, windy weather as this may cause a temporary blackening of the leaves. The tree should be well watered both before and after this treatment.
With any iron treatment, the results can be variable. It is very likely that some leaves will green up, some will remain chlorotic, and some may burn. The treatment that works best will depend on specific soil conditions in your location. Some experimentation may be necessary to determine what will work best for your trees.
Susan Rose is horticulture education specialist with the CSU Extension.
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