(Editor's Note: This article was originally published on March 5, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Many of us remember or have been involved in the ‘Bradford' pear debacle. Once touted as the perfect street and landscape tree, this once popular tree has fallen from grace and is an acknowledged scourge in many parts of the country. See several comments penned by PlantFiles contributors. Deb Magnes wrote about the pitfalls of Bradford Pears in this 2007 article.
Vigorous growth, weak wood, and poor branch structure caused the trees to break apart in wind and ice events. Many people found the odor of the blossoms offensive, and the fruits made a mess in the landscape when they fell. In addition, people noticed that few of the trees lasted beyond 15 to 20 years. As if that were not enough, thorny escapees unlike the parent plant began to proliferate and spread into native areas, creating monocultures and displacing native species. While the first ‘Bradford' pears were sterile, the introduction of several other cultivars allowed cross-pollination to take place, which had disastrous results.
Similar stories have been told of other trees that have come highly recommended. Consider the Chinese pistache (Pistacia chinensis), which is recommended by the University of Florida and was named a Texas Superstar. Known to be a long-lived, winter hardy shade tree with spectacular red to red-orange fall color and outstanding heat and drought tolerance, it seemed to be an excellent landscape choice for Texas, Florida, and the South.
Edward L. McWilliams, Professor of Horticulture at Texas A&M University, points out that a significant lag phase sometimes exists between the time a tree is introduced and the time it becomes invasive. He says that the female Chinese pistache trees do not produce large quantities of seeds until they have been established for fifteen or twenty years. He cites several places near the campus where seedlings have come up, and notes that they are especially numerous under birds' roosting sites. Residents of Austin, Texas, are now cautioned to avoid the tree because of its invasiveness.
The Chinese elm (Ulmus parvifolia) is sometimes recommended as a non-invasive substitute for the Bradford pear. Yet documentation from TAMU indicates that even this popular tree is in the early stages of naturalization. Seedlings commonly occur in irrigated landscapes and in other favorable, damp places. In addition, it has an aggressive root system which absorbs water, nutrients, and space. Will it be our next exotic invasive tree?
Should this serve as a warning to Florida and other southern states? Assuredly, we are not to assume that just because a plant is invasive in one part of the country that it is invasive everywhere. But shouldn't we exercise caution? Experiences with the Bradford pear and others cited above alert us to the dangers of choosing exotic trees.
The danger can sometimes be avoided by selecting native species. Some flowering trees that are frequently recommended as a replacement for the Bradford pear and other imported species include Eastern redbud (Cercis canadensis), flowering dogwood (Cornus florida), sparkleberry (Vaccinium arboreum), red buckeye (Aesculus pavia), serviceberry (Amelanchier canadensis), Carolina silverbell (Halesia carolina) and fringe tree (Chionanthus virginicus).
Here again, just because a tree is native does not mean that it is not invasive. One specific example is the cherry laurel (Prunus caroliniana). Although this tree received mixed reviews on PlantFiles, my experience with it has been very negative. I know that I have pulled up hundreds of seedlings in my landscape. In spite of my efforts, trees come up underneath azaleas and other shrubs and are unnoticed until their canopy grows taller than the shrubs. Then getting them out is almost impossible. Cutting them down and painting the stumps with stump killer initially slows them down, but determined sprouts pop up around it. Constant vigilance is necessary for me to keep my landscape from becoming a cherry laurel jungle.
I know the arguments for the cherry laurel. White flowers bloom in spring and attract bees and pollinating insects. Trees are drought, salt, and urban tolerant. The canopy provides a dense, evergreen screen, and birds are attracted by the fruits. But please, spare me the pleasures and ease of growth of this weedy tree. I wish my neighbors would cut down their mature cherry laurels and save me all the hours of hard work that I must spend removing them from my landscape.
The lesson we can learn is to find out all we can about a tree before we include it in our landscapes. Native trees are usually preferable, but even they have faults that we might choose to avoid. Careful consideration should be given to any plant included in the landscape, but especially to the largest plants that make up the backbones of our gardens. We will like our gardens much better if we do not have to contend with invasive plants.
Thanks to agl and ulrich for their pictures of Chinese elm and Chinese pistache.