Are Callery Pear (Pyrus calleryana) Cultivars Invasive?
The Callery Pear is an ornamental tree species from China that hasrecently begun spreading throughout the United States with wild individuals appearing in disturbed sites. A common misconception is that certain cultivated varieties (called “cultivars”; Table 1) of this species, commonly purchased in nurseries and home improvement stores nationwide, are invasive. It is not the cultivar itself that is invasive per se, but rather the problem arises when two different cultivars cross-pollinate. Once this cross-pollination happens, fruits can be produced which are often consumed by starlings which in turn fly off to natural areas where they defecate the seeds (which then can germinate and form wild trees). Another way that wild plants can be produced is if a single Callery Pear tree is not cared for and the rootstock is allowed to sprout (this can also happen if a well-being person nicks a root while they are mowing). If the rootstock is allowed to flower, it can cross-pollinate the main portion of the tree and fruits can develop.
So which cultivars are the cause? This is exactly what my lab has been studying for the past few years. We have been conducting hand-pollinations between various cultivars and we also have a way to identify the parents of wild plants using their DNA. In short, the answer is that any cultivar that produces viable seed and/or ovules can serve as a parent of wild offspring. So the problem in not in any given cultivar, but rather in the fact that two or more cultivars may be planted together. This is different from several other cases where a given cultivar of one species may have invasive tendencies but not other cultivars of the same species.
For those of you who love your Callery Pear trees, there is still hope. First, trees can be treated with certain chemicals to reduce or even eliminate fruit production – but you have to do this every year. Second, horticulturalists are now working on a sterile cultivar of Callery Pear, which if truly sterile (this still needs to be tested), it could potentially reduce formation of wild populations.
So why should we care about this problem? There are many reasons, but one is that many cities and towns across the United States are now having to spend taxpayer money removing the wild pear trees from along roadways, parks, and residential areas where they are starting to appear. These wild trees outcompete other species (including many natives), interfere with succession, and also are not too pleasant to be around since a number of wild trees produce sharp thorns. Wild populations are historically a problem in the Southern and Eastern US where different cultivars of pears have been planted the longest, but here in Ohio we are now starting to see more wild populations appearing. The Callery Pear is one of the most popular urban street trees planted in the United States – what this means is that as more people plant these different cultivars, we will see more fruits produced, and more wild populations appear. Thus the issue of wild pear trees will be around with us for quite some time to come.
Table 1. List of cultivars of Pyrus calleryana, including the year each became commercially available, the site of origin where they were first developed, United States patent or trademark number, and the source of the cultivar, if known. Note that references with “SPI” refer to collection accession numbers from the United States Department of Agriculture (USDA).
|Cultivar||Approx. year of availability||Site of origin||U.S. patent (PP) or trademark (R)||Source|
|Bradford*||1960||Glenn Dale, MD||none||Chinese seed purchased in Nanking, China in 1919 (SPI 47261); original tree planted at the USDA station (Santamour and McArdle 1983).|
|Chanticleer®* (Cleveland Select, Stone Hill, Select, Glenn’s Form)||1965||Olmsted Falls, OH & Corvallis, OR||PP2489
|Original tree planted in Cleveland, OH was derived from commercial seed purchased in 1946; synonymous cultivars likely propagated from same street tree (Santamour and McArdle 1983).|
|Rancho*||1965||Olmsted Falls, OH||PP2092||Unknown|
|Avery Park||1970’s||Corvallis OR||none||Originated from a population of P. calleryana seedlings planted in Avery Park, Corvallis, OR|
|Chinese seed collected by Meyer; selected from P. calleryana seedlings in 1969|
|Redspirea||1975||South Brunswick township, NJ||PP3815||‘Bradford’** X unknown pollen parent|
|Valzam (Valiant®)||1975||Perry, OH||PP8050
|‘Cleveland Select’ X unknown pollen parent|
|Princessa||1976||Olmsted Falls, OH||none||Unknown|
|Whitehouse*||1977||Glenn Dale, MD||none||‘Bradford’ X unknown P. calleryana parent at USDA Station; original tree destroyed during clearing a field for cultivation.|
|Autumn Blaze*||1978||Corvallis, OR||PP4591||Parent originated from Chinese seed from Reimer’s 1917 or 1919 collection (possibly from SPI 45592, part of which was given to Reimer; Westwood 1980)|
|Trinity®* (XP-005)||1978||Portland, OR||PP4530
|Grant St. Yellow||abt. 1980||OR||none||Unknown|
|Capital*||1981||Washington D.C.||none||‘Bradford’ X unknown P. calleryana parent|
|Edgewood® (Edgedell)||1997||DuPage County, IL||PP10151
|P. calleryana X P. betulifolia(found growing in a cultivated area)|
|Veyna||abt 2004||Visalia, CA||PP15299||‘Aristocrat’ X P. kawakammiiunknown cultivar|
|Cambridge||abt 2003||Cambridge City, IN||applied||Unknown|
|Earlyred||unknown||Vincennes, IN||none||‘Bradford’ X unknown pollen parent|
|Jaczam (Jack®)||1999||Perry, OH||R2708807||Unknown|
|Jilzam (JillTM)||1990’s||Perry, OH||none||Unknown|
|Cleprizam (Cleveland Pride®)||1990||Perry, OH||R1683475||Unknown|
|Bursnozam (Burgandy SnowTM)||1990’s||Perry, OH||none||Unknown|
|Fronzam (FrontierTM)||1990’s||Perry, OH||none||Unknown|
|Gladzam (Galdiator®)||1993||Perry, OH||R2708808||Unknown|
|Mepozam (MetropolitanTM)||1990’s||Perry, OH||none||Unknown|
|New Bradford® (Holmford)||1996||Boring, OR||R2034326||Unknown|
* Valid cultivar name, as of 1983 (Santamour and McArdle 1983).
** Genetic tests do not support this cultivar as a potential parent (Culley and Hardiman, unpubl. data). Note: Cultivars of species other than Pyrus calleryana are not shown here. These include P. faurieri (e.g. ‘Korean Sun’) and P. betulaefolia (e.g., ‘PzazzTM’ and ‘Paradise’ [also known as ‘DancerTM’ and ‘Southworth’]).
References: Santamour FS, McArdle AJ. 1983. Checklist of cultivars of Callery Pear (Pyrus calleryana). Journal of Arboriculture 9:114-116. Westwood MN. 1980. ‘Autumn Blaze’ ornamental pear. HortScience 15:830-831.|