Strolling through the park yesterday morning I was struck by the bronze foliage of one of the remaining Zelkovas (Zelkova serrata, Japanese zelkova; Ulmaceae (elm family)) around the fountain. Bronze is a color typical of fall foliage; this tree is not exhibiting the dark green typical of summer.
The bronze (brown) color indicates leaf scorch. The entire canopy appears scorched.* Also, there is some dieback in the center of the canopy. What are the causes of scorch? From The Ohio State University "Disease Control in the Landscape" (Bulletin 614):
These diseases commonly result from winter damage as well as from poor root function coupled with high temperatures and moisture shortage. In some cases, bacterial infection of the plant is involved. Scorched leaves are brown around the edges and sometimes between the veins. Leaf yellowing and wilting may occur in late summer. Dieback and decline is often mistaken for Verticillium wilt. No wood streaking is present, however. Girdling roots may also cause these symptoms. See Chapter 7 for identification of Verticillium. Considerable leaf scorch and defoliation occurs in late summer, especially during dry seasons.When I looked into the tree well I noticed that someone (or an animal) had dug near the trunk and at the edge of the basin. Looking into the cleared soil next to the trunk I did not see the root flare. Planting a tree so that its root flare is at the soil surface is important -- for gas exchange, disease control, and to prevent girdling roots. Here is another extensive quote, from a Purdue University Extension Forestry &amp; Natural Resources factsheet entitled "Trees Need a Proper Start –Plant Them Right!" by Rita McKenzie and Harvey Holt:
If your objective in tree planting is to grow healthy, long-lived trees, then keeping the bark above ground is vital. Lenticels in the bark are necessary for the exchange of oxygen and carbon dioxide within the tree’s tissues. When tissue is below ground, gaseous exchange cannot take place, and trees become stressed and weakened. Also, bark that is underground on a deeply planted tree is constantly moist and subject to fungus, disease, and insects.
Roots need oxygen, which is more abundant in the top 18” of soil. A properly planted tree’s roots will move laterally in this top layer of soil. However, roots on a deeply planted tree will grow vertically toward the surface searching for oxygen. These roots can encircle the tree and become girdling roots that interfere with the tree as it grows in diameter. If a tree is planted at the proper depth, its roots will not usually hamper tree growth.
|Image: A tree with visible root flare, Rita McKenzie and Harvey Holt, Purdue Extension (source)|
If this Zelkova was planted correctly with its root flare at the soil surface, this is no longer the case. Mulch and soil have accumulated above the root flare and are contributing to reduced gas and water exchange.
Leaf scorch could be the result of saturated soil which also limits the uptake of gas and water.
|Image: London planetree reduced root zone, Washington Square Park|
Can this tree be saved? Watering will not turn the brown leaves green, this season, but Colorado State University Extension recommends winter watering as a recovery strategy:
A deep soaking once a month, when there is no snow cover, will help prevent root die-back due to dehydration. The roots of mature trees extend outward several times the height of the tree, and this entire area needs water. To water in winter, choose a day when the air temperature is above freezing and water early enough in the day for the water to soak in before the nighttime freeze. Water that freezes on the surface will cause root suffocation and make the problem worse.Will the roots of the Zelkovas that have been removed be assessed? A root assessment might provide an indication of the cause(s) of leaf scorch, dieback, and in general, death of the Zelkovas around the fountain.
|(taken on May 13, 2011)|
* I did not check the branches (are they dry and brittle? or limber?) nor did I check below the bark (is it green?) -- the Zelkova might be dead.
Update, October 8, 2012: The Zelkova that was planted to replace the one that died last summer appears to be dead (see below).
Below is a photo of a live, similarly-aged Zelkova, also growing around the fountain. The leaves look significantly different.
Note: this essay was originally posted on August 26, 2011.