Invasive Trees in Maryland: Princess Tree, Callery Pear & Tree-of-Heaven

Last month, my blog post was an introduction to invasive plants and today I want to share information on three invasive, deciduous trees found in Maryland. 

Springtime provides a breathtaking display of contrasting flowers in a wide array of colors, shapes, and sizes, which are found in herbaceous plants, as well as woody shrubs and trees. Unfortunately, some of these spring flowering trees are invasive and you need to be aware of their negative effects on ecosystems such as competition for resources including sunlight, soil nutrients, and space.

Q: What medium-sized invasive tree has white blooms in the early spring in Maryland?

invasive callery pear trees crowded along a road
Callery pear (Pyrus calleryana) in flower. Photo: Britt Slattery, US Fish and Wildlife Service, Bugwood.org

Callery Pear (Pyrus calleryana), sometimes referred to as ‘Bradford’ Pear, has several cultivars including ‘Chanticleer,’ ‘ Cleveland Select’, and ‘Autumn Blaze’, all of which will be in bloom around this time of year. The Callery pear was imported from Asia to Maryland with the hope of being able to help edible Bartlett pears, which were being threatened by a disease called fire blight. The plan was to cross these pears to gain disease resistance in the pear industry. Unfortunately, this experiment was unsuccessful in preventing fire blight, but these crosses seemed to have potential for the ornamental industry and were planted widely. Bloom time often coincides with our native serviceberry, which produces white flowers too.

The ‘Bradford’ cultivar was thought to be sterile; however, when new cultivars of Callery pear were developed, it was able to cross-pollinate with those and produce viable seeds. Birds and other wildlife eat the fruits, which results in spreading them to different areas. The seeds sprout and grow into dense thorny thickets, which are very difficult to control and result in crowding out native vegetation. Callery pear provides a stunning show of beautiful white blossoms; unfortunately, these trees are highly invasive, which has led a few states to ban all cultivars of Callery pear (Pennsylvania, Ohio, and South Carolina). This tree has an attractive V-shaped crotch that unfortunately will often split from high wind and snow weight damage. 

For more information, history, and control options, please visit the University of Maryland Extension webpages: Invasives in Your Woodland and Bradford Pear.

Q:  What invasive tree has purple blooms in early spring?

lavender flowers of princess tree

Princess Tree (Paulownia tomentosa). Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Paulownia (Paulownia tomentosa), also called princess tree or empress tree, has purple, pleasant-scented blooms that appear before the foliage in early spring. I saw this tree for the first time when I traveled to Howard County for training in 2012, as this is not a tree we have in Garrett County. Its striking, large, heart-shaped leaves automatically caught my attention and reminded me of the Northern catalpa tree, a great native tree that should not be mistaken for Paulownia. According to Plant Invaders of the Mid-Atlantic Areas, one empress tree is capable of producing 20 million seeds that mature to flowering in only 10 years! These trees prefer full sun but can grow on disturbed soils, creek banks, and even forested areas which gives them a big advantage over native species that often require more special environments to grow and thrive. Paulownia’s ability to sprout from adventitious buds on stems and roots allows it to survive fire, cutting, and bulldozing. It is, therefore, a very difficult and costly invasive plant to control, according to the Maryland Invasive Species Council Plant Invader of the Month listing. 

A great alternative to Paulownia is the native Eastern redbud, which produces early, purple/reddish-purple blooms. 

For additional information, history, and control options for Paulownia, visit the University of Maryland Extension web pages Invasives in your Woodland and Princess Tree. 

Q: What invasive tree looks similar to black walnut?

tree of heaven foliage looks very similar to that of black walnut and sumac
Tree-of-Heaven (Ailanthus altissima). Photo: Chuck Bargeron, University of Georgia, Bugwood.org

Tree-of-Heaven (Ailanthus altissima) was introduced in the late 1700’s and thrives in many soil conditions. It has an amazing ability to grow very quickly and can disturb building foundations and even pavement! It was widely planted as a street tree and thus is found far and wide in the Eastern United States. This tree is called by other names including China-sumac, stinking sumac, or varnish tree due to its strong unpleasant odor. Tree-of-Heaven is sometimes mistaken for sumac, hickory, or black walnut because of similarities in leaf shape; however, look for the glands on the bottom of each leaflet to confirm its identity. 

Just in case you needed one more reason to remove Tree-of-Heaven, it is a preferred food source for the new, exotic, invasive insect pest, spotted lanternfly.  

For more specific information, history, and control options, visit these web pages:

Keep an eye out this spring for non-native, invasive trees as there are many others that I did not address in this article. Take any opportunity to help educate friends or family about the negative impact of invasive plants on biodiversity.   

Resources for invasive plant identification: 

Resources for non-invasive plant suggestions: 

Check out the University of Maryland Extension website on Introduction to Invasive Plants for more information on how to reduce invasive plants.

Remember, the absolute best way to avoid invasive plants is to never plant or introduce them into your landscape! 

By Ashley Bodkins, Senior Agent Associate and Master Gardener Coordinator, Garrett County, Maryland. Read more posts by Ashley.

Plant shopping soon? Avoid buying invasive plants

It’s a sunny day in late February and that means I’m looking at seed catalogs and dreaming of new plants! Have you been plant shopping yet this year? Adding new plants and seeds to your garden creates new scents, textures, colors, and shapes and is the easiest way to increase biodiversity in your landscape!  

As you begin revitalizing your garden space this spring, I want to bring some attention to invasive plants, a category of plants that should strike fear and dread in your heart! Okay, maybe that’s a bit dramatic, but truly this is a topic that everyone needs to learn more about. 

Q. What is an invasive plant?

A. An invasive plant is a non-native, “alien” species that was introduced intentionally or by accident into the landscape and causes ecological and/or economic harm. These plants tend to be free from predators, parasites, and diseases that could help keep them in check. These plants reproduce rapidly with multiple methods (i.e. seeds, stolons, root cuttings, runners, etc.) and spread aggressively. They tend to be deer resistant or deer tolerant, a big reason why they are purchased and planted in landscapes. Below is a photo of purple loosestrife. Notice how it is creating a monoculture, a visual key that might mean the plant is “invasive.”

purple loosestrife flowers crowding a field
Invasive purple loosestrife (Lythrum salicaria).
Photo: Richard Gardner, Bugwood.org

Did you know that some invasives are still for sale at nurseries, greenhouses, and in mail-order catalogs?   

It’s true. Many of the plants on “watch lists” are still readily available to purchase. Japanese barberry is an invasive plant that is a very popular landscape plant still being widely planted today; however, research shows that black legged ticks have been found in areas with invasive barberry thickets because these non-native, invasive forest shrub thickets create ideal microclimates.   

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Look out for wintercreeper

wintercreeper vines climbing up trees in a forested area
Wintercreeper euonymus (Euonymus fortunei) overrunning a woodland floor and climbing tree trunks. Photo: Ryan Armbrust, Kansas Forest Service, Bugwood.org.

Q:  I realize English ivy is widespread in Maryland, but there are some evergreen vines clinging to trunks that look a bit different than typical ivy leaves. Are they native, or should they too be removed?

A:  An evergreen climber I see covering tree trunks in parks which might be confused for English ivy at a distance is wintercreeper (sometimes written winter creeper; Euonymus fortunei). This non-native invasive acts like English ivy in that it’s a groundcover when no support to climb is present, and a clinging vine when trunks or walls are available.

This species also has negative impacts on the trees and our ecosystem and should be removed if growing on your own property. Parks manage invasives as best they can, but with limited resources, we can do our part by not cultivating the species likely to spread into them, even if we don’t live immediately next to parklands. Weed Warrior volunteer programs exist, such as in Montgomery County, if you wish to be trained in invasive plant ID and to help with their removal on public lands. While wintercreeper has been banned for sale by the Maryland Department of Agriculture since 2018 as a Tier 1 invasive plant, established plants in the region can still mature enough to produce fruit (berries) that wildlife then inadvertently spreads into natural areas. Long a popular landscaping groundcover due to those vibrantly green leaves (and the variegated forms for their color), I always recommend removal and replacement with alternatives, preferably a medley of native species instead.

As with English ivy, it’s safest for the tree to simply sever the climbing wintercreeper stems’ connection with the roots in the ground and let them slough off the trunk on their own as they dry out and disintegrate. Even though they attach via root-like structures, those aren’t functional roots and no moisture or nutrients are absorbed by them. Pull up, smother (deny them light), or spray any running stems covering the ground. As with any tenacious weed, eradicating an established patch of this species may take time and repeated efforts at removal before finally being successful. Be vigilant, because birds could always re-introduce it in a future year. (Invasive plants…the gift that keeps on giving.)

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

Have a plant or insect question? The University of Maryland Extension has answers! Send your questions and photos to Ask ExtensionOur horticulturists are available to answer your questions online, year-round.

Nasty weed, Jack-in-the-pulpit look-a-like: Pinellia ternata – Featured Video

Wish this enemy on no one because at the professional gardens they discard the contaminated weedy soil. Or they have resorted to sifting the soil to remove the corms. It is a bad weed. And solarization does not outlast the corms’ durability!

By Joyce Browning, Harford County Master Gardener. Follow on Harford County Master Gardeners on Facebook

My Pachysandra is Dying, What Can I Plant in Its Place?

landscape in partial shadeResident seeks groundcover options to replace Pachysandra. Photo: University of Maryland Extension / Ask an Expert

Q: My Pachysandra is Dying, What Can I Plant in Its Place?

A patch of Japanese Pachysandra in my yard was formerly healthy but in the past three years, it has died back. I would like to plant deer-resistant plants or groundcover in its place. Can you recommend some perennials I can try? This area gets filtered sun most of the day.

Answer: Volutella is a common fungal disease of Japanese Pachysandra that attacks both the leaves and stems and causes dieback symptoms. It is most severe in overgrown plantings and is often associated with scale (insect) infestations. This may have contributed to the decline of your plants.

It is a good idea to consider different options for this space. In addition to its susceptibility to Volutella dieback, Japanese Pachysandra has escaped garden cultivation and is now invasive in some natural areas of Maryland. We no longer recommend planting it. In the forest understory, it outcompetes native plants such as our spring ephemeral wildflowers and the wildlife (insects, birds) they support.

japanese pachysandra in a forested area
Invasive Pachysandra terminalis covers a large area of forested ground in Howard, County, Maryland. Photo: C. Carignan

There are other groundcover choices that are unique, beautiful, non-invasive, and adapted to Maryland’s growing conditions. For a partially shaded, moist area, try a combination of ferns – Christmas fern (Polystichum acrostichoides) and marginal woodfern (Dryopteris marginalis), sedges – blue wood sedges (Carex glaucodea or Carex laxiculmus) and Pennsylvania sedge (Carex pensylvanica), Canadian ginger (Asarum canadense), golden groundsel (Packera aurea), foam flower (Tiarella cordifolia), and creeping phlox (Phlox stolonifera). Some of these plants also support pollinators.

native plants garden
Groundcover plants were in bloom in April in the native plants garden at Camden Yards, Baltimore. From front to back: Phlox, Foamflower, Golden Groundsel. Photo: C. Carignan

Additional Resources

Planting Groundcovers | Home & Garden Information Center

Twelve Easy Native Plants for Shade | Montgomery County Department of Environmental Protection

Groundcovers | Plant NOVA Natives

By Christa K. Carignan, Maryland Certified Professional Horticulturist, Coordinator, University of Maryland Extension Home & Garden Information Center

Have a plant or insect question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask an Expert.

Q&A: Is this giant hogweed or poison hemlock?

poison hemlock
Poison hemlock can be mistaken for giant hogweed

Q: I think I might have giant hogweed on my property, or maybe it is poison hemlock. How can I tell for sure?

A: Giant hogweed (Heracleum mantegazzianum) was found recently in Clarke County, Virginia, and it has raised awareness and concern about the plant – and rightfully so. The plant produces toxic sap that can cause very severe skin inflammation. We have received a lot of questions about it lately.

poison hemlock
Poison hemlock Photo: E. Nibali

What you have here is NOT giant hogweed. It is poison hemlock (Conium maculatum), which is much more common. The ferny foliage makes it possible to distinguish it from giant hogweed.

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