trees and shrubs

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

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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.

Q&A: What can I do about rust fungus on my juniper?

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  A rust gall on juniper with spore “horns” just starting to emerge. As soon as it rains after this point, the “horns” turn gelatinous and bright orange. Photo: M. Talabac


Q:  I heard rust fungus can infect junipers but they aren’t worth spraying to treat. Is there anything else I can do to reduce the fungal spread from them to my other trees?

A:  If you see and can reach fungal galls on the branches, clip them off. Some rust fungi (though not all) create a gall on their juniper hosts. Plant galls are tumor-like in that they’re clusters of malformed tissue, often in response to a pest or (in this case) an infection. When the weather starts to be consistently mild in spring and we receive enough rainfall, the galls where the fungus is spending the winter will begin to exude their spores.

These rust spores blow on the wind or wind-driven rain to vulnerable host plants like various members of the rose family. In our gardens, this includes apples and pears, hawthorn, serviceberry, quince, and crabapple. Fungus spores are extremely tiny, so how will you know what to look for? If the gall is producing orange goo, you’re missing the window since spores are already being dispersed. Ideally, remove all visible galls before this point, while they’re still hard and dry. Now is an excellent time to inspect junipers on your property for galls. Trim them off with hand pruners and toss them in the trash (don’t compost).

This is not a foolproof method for eliminating the risk of rust infection on other plants this year, but it certainly could help reduce the disease pressure. Fortunately, infections like cedar-apple rust, while an aesthetic nuisance from time to time, generally don’t cause serious damage to all hosts, though they can be more serious for some, like apple.

Find photos and more information about Rust Diseases of Trees on the Home and Garden Information Center website.

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 Extension. Our horticulturists are available to answer your questions online, year-round.

Plant shopping soon? Avoid buying invasive plants

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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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Invite spring indoors with forced flowering branches

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Cherry branches are ideal for forcing into bloom indoors.  

If the muted greys and browns of winter have you down, beat the blues by forcing some cut branches into flower. 

It’s easy, gets you outside, decorates your home, and gives you the pleasure of watching buds open and flowers unfurl.

All you need is a sharp pair of hand pruners and some spring-flowering tree and shrub branches to cut and bring indoors.

Fortunately, this is a great time to prune most deciduous trees and shrubs, so this is a twofer. You’re pruning your plants and gathering branches to force into bloom.

Branches are ripe for harvest in February. Plants have had the six weeks of dormancy they need and are starting to plump their buds in anticipation of spring.

And no, you’re not harming these trees and shrubs by cutting them. You’re just nudging their branches into flower a few weeks earlier. 

Cut branches on a milder day when the buds are softer and better able to make the transition to warmer indoor temperatures.

Snip branches one- to two-feet long for forcing. Branches pencil-thick or thinner are best.

Look for branches loaded with flower buds. These are the fat, round ones. Leaf buds are smaller and pointed. When in doubt, slice a bud open. Do you see tiny leaves or flower parts?

Sorry, I couldn’t resist sneaking in a botany lesson.  

As you’re cutting, remember your pruning basics. Make sharp cuts at an angle so water runs away from the cut. Prune out crossed branches or ones that grow inward. 

Thin dense areas for air circulation. And step back often to make sure you’re creating a good shape and not cutting away more than a third of the plant.  

Gather the branches you’ve cut and bring them inside. Recut the stems and put them in a bucket of warm water in a cooler room. Let them drink overnight, then arrange them in vases.

Some experts cut stems underwater, add floral preservative, or put stems directly into vases. Regardless of your technique, change the water at least weekly to keep bacteria at bay. 

Place your finished design away from fireplaces and other heat sources. To mimic spring’s temperatures, display your branches in a cooler room out of direct sunlight.

But certainly, put some of your branches somewhere where you can have the pleasure of watching the buds pop open to reveal their spirit-lifting blooms.

Did I say blooms? You need not limit yourself to bold flowers. Pussy willows and other plants that form catkins – fuzzy or thin, dangling clusters of tiny flowers – look wonderful indoors. 

Cut branches of pussy willow. Photo: C. Carignan

Is it possible to walk by a vase of furry pussy willow catkins and not pet them? I think not. 

Some spring-blooming trees and shrubs that force well are cherry, forsythia, redbud, quince, serviceberry, red maple, witch hazel, magnolia, crabapple, pear, and plum.

For catkins, take cuttings from beech, birch, filbert, and willow. If you want a longer show, cut branches every few weeks.  

Make spring come early. Gather branches for forcing and enjoy the show. 

By Annette Cormany, Principal Agent Associate and Master Gardener Coordinator, Washington County, University of Maryland Extension. This article was previously published by Herald-Mail Media. Read more by Annette.

Pruning 101: The basics for success

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This is a great time of year to prune most deciduous trees and shrubs so let’s cover some tips and techniques.

What is pruning? Pure and simple, it’s removing the undesirable parts of plants.  

Good pruning improves plant health. It gets rid of dead and diseased parts and improves air circulation, shape, and appearance. It can also restrict growth, stimulate flowering and fruiting, and rejuvenate older plants.  

February to early March is the ideal time to prune many trees and shrubs because they are dormant. The cuts you make will add vigor without trauma. 

You need only a few tools. Start with hand pruners to clip small twigs and branches. Add a pair of loppers to cut larger branches. For tight spots, it’s hard to beat a folding pruning saw.

using hand pruners to prune a tree
Proper pruning improves plant health.
Tools such as this lopper can cut branches up to 3 inches thick.

No matter what tools you choose, keep them sharp and clean.

Leave to the pros – licensed tree experts or certified arborists – the pruning of large trees or work that involves climbing or cutting near power lines.   

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Fruit in your future? Start with small fruits, not tree fruits

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Small fruits give you lots to eat,
Tree fruits often spell defeat.

I am 100% pro-fruit! I would love to see more fruit plants of all types grown across Maryland. But it saddens me to see gardeners become frustrated and disenchanted with fruit growing because their first attempt was with apples or peaches.

A National Gardening Association survey showed that 41% of U.S. households grew edibles in 2021, a 24% increase since the start of the COVID pandemic. Many new vegetable gardeners naturally see fruits as their “next frontier.” Most vegetable crops are annual plants while all fruit plants are perennials, living year-to-year in the same garden space for years and requiring year-round attention. You need to up your game for fruit growing.

My advice for the fruit-curious gardener is to start off with some of the small fruits that are well-adapted to Maryland’s climate and soils. Strawberry, blueberry, raspberry, blackberry, grape, and currants get their generic “small fruits” name because the plants and their fruits are small relative to tree fruits. Apple, European pear, peach, cherry, plum, and apricot are all popular tree fruits in the Rosaceae (rose) family. They also grow well in Maryland and many people plant or inherit them without fully understanding their requirements and challenges. As a result, we get a ton of tree fruit problem questions every growing season through Ask Extension.

blackberries ripening
‘Shawnee’ blackberry plant fruiting
blueberries ripening
Blueberry fruit ripening (Photos from Ask Extension)

If you had your heart set on planting apple and peach trees this year, please put down the mail-order catalog or close the browser window showing an everyday gardener picking bushels of fruit from a pristine apple tree and consider this:

  • Small fruits are less expensive to buy and maintain and take up less garden space. Even dwarf apple fruit trees can take up 75 sq. ft. of space. 
  • Small fruit plants are easier to incorporate into a home landscape. They are also easier to prune and manage and to remove if they don’t work out or eventually succumb to old age or disease. (Mature grape plants with their massive root systems are the exception). Removal of a fruit tree can be costly.
  • It’s easy to overcrowd a part of your yard with fruit trees by planting them too close to each other, to structures, or to other trees in the landscape. Shading leads to poor growth, pest and disease issues, and low yields.
  • Fruit trees need to be trained and pruned in a careful and timely manner, especially in the first 3-4 years. Small fruits tend to be more forgiving regarding training and pruning. 
  • Yes, small fruits have plenty of potential pests and diseases but they can be grown organically with very good success. Some problems can be tolerated, like the fuzzy gray mold fungus that attacks strawberry, raspberry, and blackberry fruits in wet weather. Others can be prevented or managed through good gardening practices (like proper spacing and pruning) or applying an organic pesticide (like spraying lime sulfur in early spring to reduce disease pressure).
  • Tree fruits, conversely, have more insect pests and diseases that are more difficult to prevent and manage without synthetic pesticides. Trees must be monitored more closely for signs and symptoms of problems. Even if you spray effective pesticides at the correct time, you can end up with poor control if your sprayer is not capable of covering the entire tree including the tops and bottoms of the leaves. Multiple applications are usually needed to control the major pests and diseases.
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Trees & shrubs for pollinators

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For some reason, I feel that every time I think about what to plant for pollinators, the list of plants that comes to me is one full of herbaceous ones… however, it is odd that this is the case, because it’s not like our region lacks larger plants (e.g., trees, large shrubs) that are both fully able to support pollinators while also supporting other biodiversity and even contributing to flood and rain management! And because if we’re interested in going the large(r)-plant path, we need a bit of planning, in today’s post I would like to present some native shrubs and trees that are great resources for our pollinators. This way, you can start planning where to get them for planting in late winter to early spring.

Why consider trees and shrubs for pollinators?

Large perennial plants such as trees and shrubs have many characteristics that make them very attractive to any pollinator-friendly person in our region. Indeed, while there are many of these plants that act as wonderful food resources for many pollinators (both adult and larval stages), these larger plants represent long(er)-term and abundant resources that can serve different aspects of our ecosystem: they provide shelter and food for birds, they can assist in managing stormwater runoff, retain soil, reduce surface temperatures by their shading abilities, and provide structural complexity to our landscapes. Trees especially are a key component of creating climate-resilient landscapes. In fact, one of Maryland’s climate change mitigation goals is to grow 5 million more trees by 2031!

Planting trees is not necessarily expensive

From a financial perspective, although these plants may be costlier to obtain than the smaller herbaceous ones, there is a multitude of incentives, state vouchers, and programs that strongly reduce or sometimes completely cover the costs of obtaining them. In Maryland, for example, the state provides incentives through the Maryland Department of Natural Resources (discounts to be used at nurseries; all details here), the PG County Rain Check program or the TreeMontgomery program, city incentives, and free tree plantings (e.g., see College Park’s here). In all of these programs, a lot of trees native to our region are covered. If you would like to participate in any of these programs, make sure to check the specific tree lists covered by each (also, see this list of recommended native trees for the state of Maryland). Note that these programs I mentioned here are just a few of the many that exist; if you’re interested in this, make sure to check your city, county, and state resources!

What to plant?

I hope by now I have at least made you curious about the idea of choosing trees and shrubs for pollinators. Below, I made a very small selection of a couple of plants that appear in the native lists, and that are great for pollinators. Let’s take a look at them.

Tulip tree (Liriodendron tulipifera)

Tulip trees are a great native plant that can serve as a great pollinator resource. This tree is in the same family as Magnolia trees. It can reach a large size and it displays stunning yellow and orange flowers. This tree grows fast and is large (considered the tallest native tree in the eastern USA, along with sweetgums), so it can be a good choice for large spaces where a canopy is wanted relatively quickly. The flowers produce a lot of nectar, which attracts a massive number of pollinators. This makes it kind of fun to stand under the tree on warmer days during the blooming time: the buzzing coming from the tree is pretty impressive. Here are some more details on the conditions preferred by this tree.

the yellow and orange flower of a tulip tree
Tulip trees are among the tallest trees in the eastern USA and have wonderful resources for pollinators that they carry in their stunning flowers. Photo: W. Cutler CC.

American linden or basswood (Tilia Americana)

This tree can reach relatively large sizes, and when it grows to full size it has a very rounded canopy. I personally love this tree, because of the fact that I feel it’s a “social” tree: one can sit with friends under its shade on hot summer days, and just enjoy the life it hosts and the cool breeze it forms under it. Once the season is coming to an end, this tree’s leaves turn a lovely yellow. The flowers of this tree are small and not very colorful, but they are extremely fragrant and full of nectar, which makes them a great magnet for pollinators. You can learn more about the requirements of this tree at Virginia Tech Dendrology.

white flowers blooming on American linden tree
The American linden has discrete flowers that are very attractive to pollinators. Photo: A. Zharkikh CC.

Hawthorns (Crataegus phaenopyrum and C. viridis)

These are mid-size trees that also sustain a variety of fauna through their flowers, fruits, and bird nest-friendly thorny branches. Their flowers are white, have a typical Rose-family structure (like those of cherry trees), and are attractive to bees, syrphids, butterflies, and hummingbirds. Besides being great for fauna, the two species do well in urban environments, because they both tolerate a wide variety of conditions. Here at pollenlibrary.com and on the Lady Bird Johnson Wildlife Center website you can learn more about each of these species.

White flowers of a hawthorn tree in bloom
Hawthorns have lovely white flower clusters. Photo: F. D. Richards CC.

Fringe tree (Chionanthus virginicus)

This small tree/tall shrub is a great addition to green spaces, and is ideal for hedgerows or just as a stand-alone plant. I am always surprised by the super cool shape of the flowers of this plant, which have very elongated petals that create long white fringes. These flowers attract bees and other pollinators, which come to collect some of the nectar that is produced. This plant is usually dioecious, meaning that one individual plant harbors either male or female flowers, but rarely both. This plant is ideal for areas that receive a lot of sun, because it is under those conditions that it will do best (although it can do fine in less-sunny areas as well). You can learn more about this plant by visiting this website.

white flowers of the fringe tree
The whimsical flowers of fringe trees are not just attractive to us, but also to many pollinators! Photo: 阿橋 HQ CC.

Serviceberry (Amelanchier laevis)

Although all of them can provide very good resources for pollinators, I picked this one to showcase because this species grows relatively fast and does not get too large. This is indeed a larger shrub that has beautiful white flowers, and later on, delicious small berries. Because of all this good stuff, the flowers are visited by many insects, and the fruits are favorites of birds (so you’ll have to win over them if you want to get at the fruits! 😉 ). Here and on the University of Maryland Extension website, you can learn a bit more about this cool plant.

Serviceberries make everybody happy: pollinators in the spring with their flowers, and birds and humans in the summer with their berries! Photo: Henna K. CC.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!