Saturday, February 03, 2007

Pyrus calleryana, aka Bradford pear: Ornamental Excellence or Invasive Infamy

The Bradford pear, Pyrus calleryana, found on many invasive species sites, won an award as “Urban Tree of the Year for 2005”.

An invasive moment of stunned silence now follows.

In suburban Maryland, the ubiquitous tree lights the roadscapes in a blaze of white spurring spring gardeners to action. Many have no idea what the tree is, and a few people, who garden on a part-time basis, may think it is a native dogwood. Home builders rush to plant the tree in the spring as a sure enticement to new home sales. In an interesting ironic twist, horticultural service providers, nurseries and landscapers, both offer to sell and plant the tree, and to remove it when it splits and falls apart during a future storm. The callery pear should be given an award as the poster species for short term desires versus long term effects.

In an earlier posting, I wrote about the disconnect, which seems to occur regularly, between public agencies and their service suggestions. On the one hand, landscape manuals list the flowering pear as a legitimate planting choice, while on the other hand, natural resource departments struggle to remove the same tree. The horticulture industry is more than willing to both supply the tree and remove as the market demands.

The following information on this species is taken from the Delaware River Invasive Plant Partnership: ”The species Callery pear is native to China; in 1918 seed was brought to the United States for potential use as rootstock for cultivated pears. Of the initial batch of 100 pounds of seed that was planted at the Plant Introduction Station at Glen Dale, Maryland, one vigorous, non-spiny seedling was selected and named ‘Bradford’. The 'Bradford' callery pear proved to be an attractive landscape specimen with a neat growth form, attractive flowers and foliage, and no pests. Furthermore ‘Bradford’ was not self-pollinating and thus no fruit or seeds were produced. The landscape industry popularized it and before long it was being planted in urban and suburban settings from parking lots and streets to home landscapes. In 1982 the National Landscape Association voted 'Bradford' callery pear the second most popular tree in America.

However, with time other callery pear cultivars were developed and introduced into the nursery trade. With several cultivars in circulation, cross-pollination could take place and the trees began to produce fruits and seeds.

The spread of callery pear along roadsides, rights-of-way, and in successional old fields was first noticed in southern Maryland and around Washington, DC. In Pennsylvania naturalized populations are known in Bucks and Montgomery Counties. Naturalized populations generally exhibit characteristics of the species including wide-spreading branches and thorniness. Fruit size may vary from ¼ inch to nearly 1 inch in diameter.” [Ann F. Rhoads and Timothy A. Block, Morris Arboretum, University of Pennsylvania]

The point of sale information provided to the public usually does not mention the species’ rabbit-like tendencies. “If it's a street tree you want - or even a specimen for your landscape - the best all-purpose callery pear is probably ‘Chanticleer,' which was introduced in 1965. A fairly narrow, upright tree, 'Chanticleer' matures to a pyramidal or oval form up to 35 feet in height but usually spreading less than 20 feet. The glossy green leaves make it attractive even when the flowering is over, and in fall the leaves progress through shades of red, yellow and orange before reaching their ultimate burgundy color. 'Chanticleer' has performed well as far north as Minneapolis (Zone 4). Proper pruning when the tree is young can eliminate most problems caused by brittle wood.” [Flower & Garden Magazine, Feb-March, 1996 by Becke Davis]

Even our universities get into the act. “Chanticleer pear (Pyrus calleryana ‘Chanticleer’) The 'Chanticleer' Pear is an upright-pyramidal tree that is much narrower than other ornamental pears. This tree makes a valuable addition to the landscape and is a good choice where lateral space to spread is limited. It has attractive flowers, foliage and fall color. 'Chanticleer' is less susceptible to early freezes than other Pears. It will grow up to 40 feet high and 15 feet wide. The 'Chanticleer' Pear is very adaptable to many different soils and it tolerates drought, heat and pollution. Plant in full sun. Prune in winter or early spring. Hardy to zone 4. Because of its shape, the crown is less prone to branch breakage with heavy winter snow. This variety of callery pear is resistant to fireblight” [© CSU/Denver County Cooperative Extension Master Gardener 1999-2007]

It should not come as too big a surprise then that the industry grows and sells this tree, nor should too many be amazed that the public’s first reaction when informed that callery pears are potentially disruptive of natural areas, be one of incredulity; after all the people who know are singing the praises of the tree. To make things even more interesting, and in keeping with good marketing strategies, there are many cultivars from which the discerning gardener may choose. Among the offerings: 'Select', 'Cleveland Select', 'Stone Hill.', 'Redspire', ‘Capital’, ‘Aristocrat’, ‘Trinity’, ‘Whitehouse’ and ’Autumn Blaze’ (developed in Oregon in the 1980’s). The horticulture industry prides itself on being able to offer a tree for every purpose or need, tall, short, narrow or wide.

Meanwhile, on the other side of the garden gate, a withering condemnation of the tree continues to grow. Yet another award is from the United States Forest Service, “Weed of the Week” - Ecological Impacts: Callery Pear is often found growing in the company of many other nonnative plants and competes with both the native and nonnative species.
This tree has a tendency to split, fall apart or uproot under wind glaze and snow events.”
[Produced by the USDA Forest Service, Forest Health Staff, Newtown Square, PA. WOW 09-26-05]

To ad to the confusion, recommendations contain advice which seems to suggest more exotic ornamentals as substitutes for the troubled flowering pear. From Alabama Cooperative Extension comes the following suggested alternatives: “Chinese Elms are an excellent choice, although consumers should be careful not to confuse these with Siberian Elms, which are often passed off as a poor substitute for Chinese Elms," Musgrove says.
To ensure you’re buying the right elm, Musgrove says consumers should look for "small, reddish-brown, pointed leaf buds and beautiful cinnamon patches of bark." Siberian Elms, by contrast, have round, black leaf buds. In addition to being long-lived, Chinese Elms, are resistant to Dutch elm disease and do not have the pest and dieback problems associated with Siberian Elms. Other fast-growing alternatives include the Chinese Pistache and Japanese Zelkova. Crab apples and Japanese cherries are also good alternative flowering ornamental trees, although homeowners should remember fruit can cause branch stress on some cultivars. Red maples also are a good choice.” [Alabama Coopeerative Extension System, Auburn May 10th]

Demonstrating that there are alternatives can lead one down the road of unintended consequences. The above list includes only one non invasive species. Part of the problem is that until a species is widely and repeatedly distributed, it may not be currently demonstrably invasive, and yet there are warning signs to be found.

“Zelkova serrata is regarded as an invasive weed by many municipalities, though it is not as aggressive as Chinese Elm. There may be local restrictions, regarding its availability in nurseries, in some areas.” [Copyright 2006-2007 The Knowledge of Bonsai Forums]

“Chinese Elm, Ulmus parvifolia is an ornamental tree in urban areas planted for tough
durability, interesting bark and yellowish to reddish purple fall foliage as well as being
resistant to Dutch elm disease and air pollution. It has escaped intended plantings to
invade native plant communities. The aggressive root system absorbs water, nutrient, and space.” [Produced by the USDA Forest Service, Forest Health Staff, Newtown Square, PA. WOW 04-18-05]

“Chinese pistache, Pistacia chinensis is only moderately invasive. However, every rural population of this tree studied eventually escaped from cultivation. The continued widespread planting of Chinese pistache coupled with the high rate of escape of the tree indicates that Chinese pistache will become widely naturalized on upland sites in Texas. The term cultivasion is used to describe this highly predictable horticulturally accelerated invasion.” [E. L. McWilliams, Horticultural Science, Texas A&M University]

Crabapples and some Japanese cherries have shown invasive tendencies also, but the good news here is the recommendation to plant the native red maple, Acer rubrum.

With flowering pears, the conflicting streams of information become readily apparent. The end user, the public, is understandably in a confused position when experts seem to disagree. Because the challenge of invasive species is more complex and does not readily lend itself to simple discrete choices, and , because the conversation includes research in progress and traditions in place, as well as problems of event time line horizons, the public tends to find itself captive to shifting aggregations of wisdom.

A link to a paper by the comment author is provided 4/3/08:


Anonymous said...

See my paper on the species as an invasive: Vincent, M.A. 2005. On the spread and current distribution of Pyrus calleryana in the United States. Castanea 70: 20-31.

Michael A. Vincent

Melinda Young Stuart said...

See my posts on Flickr--I've just discovered there is an InvasiveSpecies Group on Flickr.


Anonymous said...

I have one Bradford Pear Tree on my one acre lot, and even thoughtrees for sale