Invasive Plants – Page 6

The Callery Pear tree is an invasive ornamental tree commonly planted throughout the United States. Scientifically known as Pyrus calleryana, it is native to China and grows quickly reaching heights of 15' in as little as five years. Easily recognizable by it's white flowers in Spring, or small brown pears in Autumn, it easily colonizes disturbed areas. Also known as the Bradford Pear tree, it was originally developed by the USDA in the 1960's as a sterile, ornamental street tree. It quickly gained popularity due to it's growth rate, shape, and above all else, showy white spring-flowers. By grafting onto root stocks it was thought it would be sterile, as pears need two genetically distinct trees to cross pollinate. Unfortunately sprouts would develop below the graft, and would produce genetically distinct flowers, which would allow for cross pollination. This resulted in pears being produced, which are eaten by passing birds, who then increase the spread of the tree. Every April on my drive home from work, most trees still haven't budded out. But at the same time I do see many other trees that are covered in white flowers. I see them in the edge of forests and wild areas. I see them in ditches and drainage areas, along fence lines. And I see them in nearly every subdivision. Every single neighborhood or development has these horrible invasive trees.... These white flowered weeds are commonly known as the "Bradford Pear" (Pyrus calleryana). They were introduced by landscape designers in the 1960's, and often used in urban landscaping.[1] Planting them along streets, and residential landscapers also sold them to new home buyers as a flowering tree. In this article: Why the Callery / Bradford Flowering Pear Tree is harmful How the Callery Pear tree became invasive and spread How it spreads Falling Limbs How to control the Callery Pear What you can do to help Native Alternatives to Callery Pear Trees Why the Callery / Bradford Flowering Pear Tree is harmful The Callery Pear Tree is very harmful to the environment, and potentially your property in the following ways: It displaces native trees and plants in the wild, and can form dense colonies A reduction in native trees lowers our biodiversity, as no insects are hosted on the Callery Pear tree.[3] The growth rate is faster than most native trees, thereby slowing or preventing their growth It leafs out before woodland wildflowers emerge from dormancy, robbing them of critical sunlight and preventing their growth. This can also harm certain pollinators that use these plants as their larval host plant.[3][4] The structure of the tree and brittleness of the wood make it prone to falling limbs that can damage people or property[5] Now I will go into more detail to each of these bullet points to expand upon them, and provide you the detailed reasoning as to why you should not use this tree in your landscaping. More naturalized Callery Pear Trees. This location is approximately 300 yards from the nearest subdivision. What is the difference between Callery Pear and Bradford Pear tree? The Callery Pear tree is the common name for "Pyrus calleryana". It is the straight species from China. The Bradford Pear tree is a cultivar of the Callery Pear tree. Bradford Pear and Callery Pear are often used interchangelably. How the Callery Pear has spread / become invasive Well, they are a flowering tree. A Callery Pear tree in full bloom looks gorgeous. It also looks a bit strange though, as pretty much all other trees have no leave or even buds while the Bradford Pear Tree is a tall bastion of white blooms. Close up of the flowers These trees can tolerate almost any environmental conditions and grow very fast, reaching 15' (5m) in just 5 years or so. With a max height of roughly 30' (10m). Also, they have a nice round or egg-like shape to them, which can be attractive for residential yards and landscaping. There was a second tree about 20 yards behind this one, happily growing in the woods. An invasive tree, the history and how a 'sterile' tree can spread The first Callery Pears were imported into California in the early 1900's. Seeds were brought in to determine if a suitable variety could be developed that would be resistant to Fire Blight, that was decimating the edible pear crop of California.[1] These seeds came from a variety of trees in China, giving it a large amount of genetic diversity. Some 'escaped' Callery or Bradford Pear Trees. These trees will grow faster than most native specimens. The Callery Pears grown from the seeds brought from China mostly became rootstock for edible pears. However, in 1952, at a USDA research station in Maryland, one of the original trees grew from seed was noted for it's shapeliness, foliage, and flowers. A Dr. John Creech grafted some scions onto rootstock to determine if they would make suitable ornamentals.[1] The initial trees were a hit with the public for their white flowers, and thus the 'Bradford Pear' cultivar was born. Now, pear trees need two genetically different specimens to successfully reproduce. And the grafted cultivars were genetically identical (above the graft anyway), which should make them sterile. So people planted these trees thinking they would not reproduce. Grafted varieties can still produce viable seed! Sometimes these trees sprout new shoots from the rootstock, and since these are emerging from below the graft, they will be genetically different from the upper portion of the tree, which means the small fruits would now contain viable seeds! I've seen these shoots with my own eyes on a tree that was present when I bought my current home. To further compound the problem, nurseries have been making their own cultivars for some time, and cross pollination regularly occurs. However, it has been shown (and I observe it each Spring) that the trees can cross-pollinate with other species. Meaning these small little pears that are produced in the fall, are actually viable seeds. Well, the birds will eventually eat them and spread the seeds in a natural way, leading to many new saplings in the Spring. Each of these tiny pea-sized pears likely contains a viable seed! Chuck Bargeron, University of Georgia, Bugwood.org Now, to further compound the problem, consider the following thought experiment...one that has probably happened many times. One can easily imagine a scenario where an entire neighborhood has one cultivar of the Callery Pear, but then one house decides they want a second tree, and if they purchase a different cultivar than everyone else......boom! Cross pollination occurs via bees, and thus viable seeds are produced. The sterile trees are no longer sterile! Since these are hybrids, the seeds will produce one of two things - the original Callery Pear tree from China, or what ever it cross pollinated with. The true Callery Pear from China is even worse than these 'Bradford' varieties. As the Chinese Native version produces thorns that can get up to 4" long, similar to a Honey Locust tree. These thorns can injure people, animals, and puncture tires. Additionally they can form dense thickets, and make land less desirable for people AND wildlife. Video Guide Below is a video I made documenting just how widespread this problem is in my (and many other people's) areas. https://youtu.be/PDmGPRRcQM8 An invader from another land The main reason the Callery Pear tree blooms before Oaks and Maples have leaves is because it isn't from North America. Since the Callery Pear tree is native to China, it has evolved in a completely different ecosystem. So it doesn't take many warm days after winter before the tree 'wakes' up from its Winter slumber and thinks it is time to put leaves out. Note that the Callery Pear Tree in this picture is in full bloom. While the native trees next to it have barely formed any buds. Hardy and Robust These trees are extremely adaptable. They can grow just fine in moist conditions, drought, sandy soil, clay - and a large range of ph levels. In fact the American botanist Frank Meyer, who originally brought seeds back from China was amazed at the variety of ecosystems it grew in.....dry slopes, standing water, barren mountain sides, and bamboo forests![1] Why is leafing out early a problem? The Bradford Pear tree going into full foliage so early in Spring causes problems for other native species. The shade created from the flowers, and then soon followed by the leaves takes the sunlight away from the other plants on the forest floor, and other trees that are smaller. This is a large issue for our native trees. Now, it doesn't effect native trees that are taller than the fast growing Bradford Pears, but it does reduce the growth rate of native saplings. More escaped Callery/Bradford Pear Trees. This location is about 200 yards from a relatively new subdivision. The longer that Oak, Maple, and other native trees are small saplings, the more prone they are to being browsed/eaten by deer. So, it becomes difficult for a piece of land to grow the native trees that have evolved within our ecosystem, when the Callery Pear can just out-compete them via it's fast growth rate and ability to shade out other saplings. Even though the deer get some nutrition from eating a young oak sapling. They would certainly get more food in the long run if that Oak sapling was able to grow to maturity and produce acorns! Too bad we can't make the deer understand that fact. Not just a problem for other native trees The sunlight 'stolen' by the earlier-than-native foliage of the Bradford Pear tree effects our woodland wildflower population. Virginia Bluebells, Spring Beauty, Trout Lily, Blood Root - all early Spring woodland wildflowers. These flowers are a valuable source of nectar to bees and other native pollinators. Early Spring woodland flowers emerge from dormancy and are generally blooming before trees have put out leaves. They have evolved to grow in this manner because they need to get the sunlight before the Oaks, Hickory, Maples and other trees put out leaves and shade the forest floor. Bradford Pear trees rob these plants of their necessary sunlight by putting out foliage before some of these plants even emerge. Dense colonies of the Callery Pear trees form on woodland edges, preventing native trees, and thus other woodland or native partial shade plants. So, it is a detriment to our native bees and butterflies that don't have many nectar options in the early Spring. Falling Limbs Callery / Bradford pear tree branch structure. Note the steep angle the limbs make, relative to the wide shape of the tree. Bradford Pear trees have a very steep branch structure. The wood is also noted for being quite brittle. This combination makes the limbs inherently weak and at risk for falling during high winds or storms. This can pose a serious threat to people and property. Some municipalities have banned or are removing it from city landscaping just to avoid any liability. Once the tree is full grown, the crotch of the branch/trunk has trouble supporting the weight of the limbs. Some states have listed it as an invasive species (South Carolina, Ohio, Alabama, Georgia, New Jersey, Maryland, North Carolina, and Pennsylvania). This is due to is prolific nature, as it has the ability to form thick stands of trees on the edge of natural forests - and thus preventing our normal, native plants from growing. But the falling limbs from mature trees can be a real issue. Imagine if a limb fell onto your car, or worse yet, a person! That is too steep of a price to pay just to have a flowering tree in your yard. Storm damage to a Bradford Pear Tree. The brittle nature of the wood of this tree, coupled with the branch structure make it prone to dropping limbs in high winds. Credit: Famartin [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)] How to control Callery Pear How to best control the Callery Pear tree depends on the size of the tree. Take time to assess the infestation or specimen tree you have before cutting it down. And, remember, if you hire someone to take down your tree - make sure they are insured. For small trees For small isolated saplings that have germinated from seed, you can control it by digging out the root ball with a spade. Then just leave the tree lying on the ground or dispose of it properly. For controlling larger specimen Callery Pear trees For larger trees, such as one that was planted many years ago as an ornamental, you will have to cut it down and poison the stump. I strongly recommend the cut & paint method. See here for my detailed guide on how to stop tree stumps from sprouting. But the general method is to cut the tree down with a chainsaw, then immediately paint the stump with Triclopyr or a glyphosate herbicide. I personally prefer triclopyr (sold as brush killer) as it has a much shorter half-life and doesn't linger in the soil for more than six months. But remember, large trees can easily damage homes if they fall down wrong, and cutting them down can be hazardous. If you are not confident you can perform this job safely, hire an insured tree removal company. Also, for mature trees, the roots will often have enough energy to sprout new saplings for several years. This isn't that big of a problem, as you can easily cut them with pruning shears a couple times a year. Eventually all the stored energy will be exhausted and the roots will die. For controlling large infestations of Callery Pear If Callery Pear trees are allowed to invade an abandoned lot, field, or pasture and grow unchecked for several years, you may need to employ a combination of methods. Removal of the large trees (2" diameter or more) with a chain saw will be necessary. However, repeated mowing can control smaller sprouts and re-sprouts, provided the tires can withstand any thorns that may be present. The original native species in China can have up to 4" long thorns/spikes, and new germination of hybrids can often revert to the mother species. What you can do to help The first step we can all do to help reduce the numbers of this invasive species is to stop planting them. If you live in Ohio, Pennsylvania, or South Carolina, then take comfort that it will be illegal to sell those trees after 2024. But for other states, you should write to your local state representatives, senators, and Department of Natural Resources to advocate for the banning the sale of this tree. Feel free to cite the references I list below as evidence, as there is a huge body of peer reviewed literature that has found the Callery Pear tree to be harmful to the environment. The next step you can do is to cut down and kill any trees you have growing on your property! Kill them, help your neighbors kill them, and try to educate people as to how damaging this tree is to the local environment. And finally, if someone is in love with trees that flower in Spring, suggest native flowering trees! There are many I list below that will flower beautifully in the Spring, and benefit the local wildlife! Native Alternatives to Callery Pear Trees There are many different native trees that are perfect for landscaping that produce lovely Spring Flowers. RELATED - Find our complete list of invasives, and native alternatives here. Eastern Redbud Tree The Eastern Redbud is a gorgeous pink flowering tree that gets 20-30' tall, with a 25' spread. This tree produces nectar for pollinators, just as the Bradford Pear does. But it will flower and leaf out a bit later, so not impacting woodland wildflowers. Also, there is a white variety commercially available if you are really wanting 'white' flowers. The Eastern Redbud Tree flowers about 1-2 weeks after the Pear Trees. Gorgeous pink/purple blooms are edible, and help pollinators. Serviceberry Service Berry is a white-flowering tree/shrub that can get about 20' tall. This tree flowers early in the Spring, and produces service berries! They are edible to both human and wildlife. So, it achieves the same goals of the Callery Pear, but without the negative environmental implications. Serviceberry makes an excellent alternative to the Callery Pear tree. Crabapple Crabapple (Malus angustifolia, Malus coronaria, or Malus ioensis). There are several crabapple trees native to North America. All of which flower beautiful white flowers in Spring. They generally reach 30-40' tall. Dogwood (Cornus sp) There are several native Dogwoods and numerous varieties available. These native trees bloom large, absolutely stunning cream-colored flowers in late Spring, usually 1-2 weeks after the Eastern Redbud. Growing 20' tall and adaptable, they can grow in most suburban yards. A magnificent specimen of Dogwood Caroline Silverbell The Caroline Silverbell tree (Halesia carolina) grows 30-40' tall in full sun, slightly acidic and well draining soil. Blooming later than other Spring trees, it will bloom drooping 'bell-shaped' white flowers. Chokecherry Chokecherry (Prunus virginiana) is native to most of North America that grows 20-30' tall and blooms white flowers in Spring. Hardy from USDA zones 4-7 and growing in full sun to partial sun, it thrives in most soil well-drained soil conditions from dry to medium moisture levels. Viburnum More of a shrub, Viburnum sp. is a shorter (10-20') tall shrub that grows well in full sun or partial shade. A species that grows particularly well near me is known as Blackhaw, viburnum prunifolium. I've seen this growing on the edge of forests and out in the open in full sun. Blooming beautiful clusters of white flowers in Spring, it also provides a fruit in Autumn that will attract birds. Blackhaw flowering in Spring. Find more about Invasive Species Here