Sunday, December 16, 2007

Elm envy

The devastation wrought by Dutch elm disease in the middle of the last century left Americans largely bereft of our favorite street tree. We lost no time in trying to find substitutes that could reproduce the look of American elm and resist Dutch elm disease. Japanese Zelkova, Zelkova serrata, was seized upon as a tree that could replace American elms along roadways.

Zelkova was little known until a few decades ago. It is rare to encounter a mature one. In the last few decades the tree has become a very popular elm substitute. The tree shares the American elm's vase-like habit, but Zelkova's vase is typically squatter. The habit is also stiffer. American elm limbs undulate as they ascend. Not so the limbs of Zelkova, which lacks the elm's sinuous grace.

The tree has a handsome and distinctive bark. The young bark has very prominent lenticels(1), horizontal striations, that are reminiscent of the bark of some cherry trees. Older bark is rough and peeling.

The unusual bark and vase-like habit make this species easily recognizable even when it is leafless.

One peculiarity of Zelkova that has the potential to cause trouble as the tree matures is the density of its branching pattern. An entire forest of limbs emerges from the trunk a short distance above ground level. The Central Avenue tree pictured above illustrates the tremendous branching density. The Putnam Avenue Zelkova pictured below shows how numerous limbs typically originate in a bunch.

One wonders what is going to happen to all those limbs when they get large. You would have to bet that some will be crowded out and will drop to the street. Professor Edward. F. Gilman of the University of Florida (formerly of Rutgers) has drawn attention to this problem. In a presentation at the New Jersey Shade Tree Federation meeting in 2006, Professor Gilman spoke of Zelkova as a special case among shade trees in that it needs careful pruning for the first two or three decades of its life in order to develop a healthy form. Is he being overly pessimistic? Hard to know. There just aren't that many mature Zelkovas around on which to base a judgment. If he is right, the enormous vogue that Zelkova now enjoys is going to lead to large maintenance expenses for the municipalities planting them in great numbers. The only mature Zelkova I know is at 9 Remington Avenue in Edison (off Park Avenue). That tree has a diameter at breast height of about three feet and is quite a beautiful specimen. Pruning history? I haven't a clue.

Another American elm substitute that is growing in popularity, but still a rarity, is lacebark elm, Ulmus parvifolia. Like Zelkova, lacebark elm is resistant to Dutch elm disease. Kathryn Uhrich and Jeff Holmes recently planted two lacebark elms in the curbside strip in front of their house at 1441 Evergreen Avenue. They are the only lacebark elms I know in Plainfield. The peeling bark of this species is exceptionally beautiful. These two elms are an exciting addition to Plainfield's urban forest.

Siberian elm, Ulmus pumila, is yet another elm with resistance to Dutch elm disease. There is an example of this species near the Muhlenberg Hospital Park Avenue parking lot. The first tree on your right as you turn off of Park Avenue onto Laramie is a mature Siberian elm.

Michael Dirr's Manual of Woody Landscape Plants, the Bible for many gardeners, calls this species "one of...the world's worst trees" because of its susceptibility to damage by wind and insects.(2) I think Dirr is exaggerating. I have seen many attractive Siberian elms (not in Plainfield, where it is uncommon), and I find the coarse, chaotic bark quite attractive.

Finally, some people are still planting American elms. Dutch elm disease-resistant varieties include 'Princeton', 'Liberty', and others.
(1) Lenticels are loosely bundled groups of cells that permit gas exchange, allowing the tree to "breathe" through its bark.

(2) Manual of Woody Landscape Plants, fifth edition, Michael A. Dirr, Stipes Publishing 1998, p. 1048.

Copyright Gregory Palermo


Anonymous said...

Hi Greg,

Could you define "habit" as it pertains to trees.

I thoroughly enjoy your blog.


Gregory Palermo said...

Thank you.
Definition of "habit" (as lifted from Michael Dirr's Manual of Woody Landscape Plants): "...form or outline of the plant..."

Cynthia said...

I have just been appointed to the Environmental Commission in Englewood, NJ. Two of the trees on our "approved planting" list are Zelkova and Sugar Maple, both of which you mention in your blog.

After reading the info here, I'm not sure these trees are the best selection. Can you suggest a good resource for appropriate NJ street trees?


Gregory Palermo said...

I think that both Zelkova and sugar maple are good NJ street trees. Zelkova might need more pruning help than some other species. I would be careful about using sugar maple where it would be exposed to road salts and a great deal of pollution.
Trees for New Jersey Streets published by the New Jersey Tree Federation is a useful publication. It does not give much attention to liabilities of the various tree species.
The University of Conneticut plant database mentions liabilities for each species:
Michael Dirr's Manual of Woody Landscape Plants discusses landscape value and problems for each species.
Arthur Plotnik's The Urban Tree Book is a good and entertaining source of information.