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Thursday, April 25, 2013

Inedible Fruit as Food for Thought This Arbor Day

This post was supposed to be for Earth Day on Monday (April 22).  But a weekend of fishing and gardening prevented me from writing it prior to Monday, and my day job prevented me from writing it on Monday.  Fortunately, I have a chance at redemption, because Arbor Day is on Friday, April 26.  And this piece really is more appropriate for Arbor Day than Earth Day.

Hopefully lots of trees will be planted all around the country this weekend in observation of Arbor Day.  What kind of trees?  I hope everyone considers planting trees native to their own area, because natives are already suited to the climate and typically offer some degree of benefit to wildlife.  But really, the better question to ask is:  What kind of trees should we NOT plant. I’m glad you asked.

Throughout the eastern the United States, you can easily spot Callery pear (Pyrus calleryana) trees this time of year.  Here in the mid-Atlantic states, they are now just starting to go from their full, brilliant-white bloom to leafing out.  Look for trees with upward spreading branches that are entirely in bloom in white, and you’ll probably find Callery pear trees.  In autumn, their leaves turn various pleasant shades of red.  Sounds like a really nice tree, right?  Maybe too much of a good thing.

Many communities have allowed and even encouraged Callery pear cultivars to be planted as street trees.  Planting a monoculture of any one tree species is bad, because a species-specific blight could wipe out entire neighborhoods of those trees in just a couple of years.
The Callery pear tree is native to Vietnam and China. First introduced to the United States in the early 20th century for experimentation to develop blight resistance in the common pear, the Callery did not become popular ornamentally until the 1950s.  As its ornamental value grew, various cultivated varieties were developed.  Probably the most popular cultivar of the past 50 years has been the Bradford pear.

Cultivars in the U.S. originated from China and are different genotypes. Some genotypes require cross-pollination from another genotype to set seed, but others can self-pollinate. Different genotypes growing within about 300 feet of each other can cross-pollinate and produce fruit with viable seeds. The Bradford pear cultivar is one of several varieties of Callery pear that are capable of spreading and being invasive.

The fruits of Callerys and their cultivars are not edible by humans, being only about one centimeter in diameter.  But birds readily eat these small fruits and disperse the seeds in their droppings, and fertile seeds will establish anywhere they are dropped.  Seeds that germinate can grow rapidly in disturbed soils such as along roadsides and the margins of cultivated fields and woodlots. The plants that spread in natural areas are not true cultivars. They are sexually reproducing populations consisting of multiple genotypes that recombine every generation to produce more viable seed and perpetuate the invasion of this non-native pear tree.

Callery pear cultivars that have formed a thick patch in a former

cultivated field.
Once established, Callery pear trees can form dense thickets that push out native trees that cannot tolerate the Callery’s shade or compete with them for water, soil, and space.  A single Callery or fertile cultivar can spread quickly via seed suckers and form a formidable patch in less than 10 years.  Its success as an invasive species is aided by a general lack of natural controls like insects and diseases.  Callery pear trees are reported to have escaped cultivation and are out-competing native trees in at least 25 states.

    Size:  mature height is 30-50 ft. tall and 20-30 ft. wide; young trees may be thorny.
    Leaves: deciduous, alternate, simple, broad-ovate to ovate, 1½-3 in. long; shiny dark green and leathery, small round-toothed margins; scarlet, mahogany, purple hues in fall.
    Flowers, fruits and seeds: flowers in early spring before the leaves, white with five petals, about 1 in. across; fruits mature in fall and are small, hard, brown, and almost woody.
    Spreads: by seeds that are dispersed to new locations by birds that eat the fruits.


Simply do not plant the Callery pear or any of its cultivars such as the Bradford.  Lobby your municipal planning commission to prohibit Callery pear cultivars from inclusion in land development plans.

Cut down medium or large trees and treat their stumps with a systemic herbicide like glyphosate (better known as Roundup).  If you find them coming up in your yard or even in the wild, you can simply pull the seedlings and shallow-rooted saplings when the soil is moist. 

If you succeed in pulling the seedlings or small trees out of the ground, remember that these trees and other invasive plants will take advantage of any disturbed soils to germinate their seeds.  So instead of leaving a patch of disturbed soil to germinate new invasive plant seeds, consider having some sort of native plant(s) ready to immediately plant in place of the former pear invader.
Callery pears that have escaped cultivation (meaning they probably sprouted from seeds disbursed by birds).  These trees are on the edge of a former cultivated field.

 Native Alternatives

There are several native trees that are excellent substitutes for Callery pears.  They include: serviceberry (Amelanchier species), hawthorne (Crataegus sp.), redbud (Cercis canadensis), Dogwood (Cornus florida) and crabapple (Malus coronaria).

Bottom line is this -- by all means, go out and plant several trees this weekend to celebrate Arbor Day.  Just select your trees with regard to the environment.  Natives are all-around winning selections.  But if you go with a non-native cultivar, please do your homework to make sure you are not buying a cultivar that is likely to “escape cultivation.”  Happy planting!