Category Archives: Trees

A Beautiful Success Story

Kentucky yellowwood flowers, Schenley Park, 20 May 2019 (photo by Kate St. John)

This flowering tree is a native North American but was so rare that few people ever saw it until botanists fell in love with it.

Originally found in small patches from Arkansas to Kentucky and Tennessee, the Kentucky yellowwood’s (Cladrastis kentukea) beautiful flowers, mid-story height, and tolerance for full sun in urban settings makes it the perfect ornamental.

Original range of the Kentucky yellowwood tree (map from Wikimedia Commons)

Planted in eastern North America for over 200 years, it became naturalized in scattered locations from Ohio to Massachusetts. Allegheny County is one of the few new places where Kentucky yellowwood grows wild.

On Sunday in Schenley Park, our group was awed by the profusion of vanilla-scented flowers at the Visitors Center. We didn’t recognize the species so I went exploring yesterday and found it both cultivated and wild.

Kentucky yellowwood at the Schenley Park Visitors Center, 20 May 2019 (photo by Kate St. John)

Here are some other cool facts about Kentucky yellowwood:

  • It’s called yellowwood because “a clear yellow dye is obtained from the heartwood.”
  • It is the only Cladrastis native to North America.
  • The flowers are attractive to bees. Narratives say the tree is attractive to birds.
  • Flowering varies from year to year with heavy blooming every 2-3 years, particularly after a long hot summer. 2019 is a big year for Kentucky yellowwood in Pittsburgh.
  • Every description says the tree flowers in June, but blooming started here in mid May — two+ weeks early, probably due to climate change.

Once I started looking I found the tree in many out of the way places in Schenley Park, probably growing wild. Kentucky yellowwood is a beautiful success story in Pittsburgh.

They Fold Their Leaves

Black locust, young leaves (photo from Wikimedia Commons)

Now that black locusts (Robinia pseudoacacia) are leafing out in Pittsburgh we can watch their leaves do exercises.

According to Wikipedia black locust leaves fold together at night and during wet weather, a trait of the Legume family called nyctinasty. I’ve often seen nyctinasty in clovers but have never noticed it in black locusts because I haven’t been paying attention. This month I plan to take a look.

It will be a good week to get close to black locusts. They’re blooming now with a sweet grape-like scent. See photos and read more about them in last year’s article: The Sweet Smell of Trees.

Tiny Hemlock Pest Has Hatched

Hemlock woolly adelgid nymphs (photo by Elizabeth Benton, University of Georgia,

A week ago I received a message from the USA National Phenology Network that hemlock woolly adelgids would hatch very soon in Pittsburgh and the southern Appalachians. This is worrisome because the nymphs are the active phase of this forest pest.

Hemlock woolly adelgid hatch announcement from USA NPN

Originally from Japan, hemlock woolly adelgids (Adelges tsugae) kill eastern hemlocks in 4-20 years by locking on where the needle meets the stem and sucking the lifeblood out of the tree (closeup at top).

The adults are sedentary, attached to a tree. The nymphs, however, are tiny and mobile. They blow on the wind and hitchhike on clothes, equipment, birds and animals. They spread very easily just after they’ve hatched.

The message above says “You should see active nymphs” but you won’t. At 1/100th of an inch they’re smaller than a grain of sand, almost microscopic. And yet, their effect is devastating.

Hemlock woolly adelgids have already killed up to 80% of the hemlocks on parts of the Blue Ridge Parkway and in Shenandoah National Park. They are eating their way through the Great Smoky Mountains, shown below, and they’re killing hemlocks in Pennsylvania.

Tree death from Hemlock woolly adelgid in the Great Smoky Mountains (photo by Ignazio Graziosi, University of Kentucky,

We won’t know how far they’ve spread this spring until they reveal their presence next fall when the females deposit woolly egg sacs on the undersides of hemlock branches.

Hemlock woolly adelgid eggs covered in white “wool” (photo by Steven Katovich, USDA Forest Service,

Right now the nymphs are spreading.

Read more about hemlock woolly adelgids at the USA National Phenology Network.

(photos from Bugwood’s, message and map from USANPN. Click on the captions to see the originals)

Not A Nice Flowering Tree

Callery pear gone wild (photo by Richard Gardner,

Yesterday I noticed white flowering trees on the hills and swales near Robinson Town Centre (I-376 West). “How nice,” I thought, “Who planted those trees in the empty places?” No one. They’re invasive.

April is the perfect time to see the invasive extent of callery pears (Pyrus calleryana) because they bloom before our native white-flowering trees: chokecherry, downy serviceberry (shadbush), and hawthorn.

Originally imported from China in the early 1900s as root stock for pear orchards, USDA bred them as landscape trees in the 1950s and came up with a winning cultivar, the thorn-less sterile “Bradford pear.”

From 1960 to the 1990s callery pears were wildly popular as street trees in suburbia. They’re pretty in early spring, colorful in fall, and they grow well in the full sun and disturbed soil found in new subdivisions. The Bradford cultivar is also brittle so commercial plant breeders created other cultivars. That’s when the genie came out of the bottle.

In a single cultivar population the fruits are sterile but if two different cultivars are planted near each other, or even grafted together, insects cross-pollinate them and the trees produce fertile fruit. Birds eat the fruit and disperse the seeds. The trees escape to the wild.

Callery pears grow anywhere. A patch can start with a single tree that becomes a thicket in several years. Dense thickets push out all native species. To make matters worse, the wild trees can have 3-inch thorns! The best field-scale control measure is to brush-hog and then mow every year. They still come up!

Callery pears take over disturbed soil (photo by Britt Slattery, US Fish and Wildlife Service,

Callery pears now grow wild from Texas to New York and Massachusetts. They’re listed as invasive in eight states including our own: Pennsylvania, Maryland, Virginia, South Carolina, Georgia, Alabama, Tennessee, and Illinois.

You’ll see them this month on open hillsides and fields along the interstate, near shopping centers, and at the edges of subdivisions.

Don’t plant callery (Bradford) pears. They are not nice flowering trees.

(photos by Richard Gardner and Britt Slattery via; click on the captions to see the originals)

Red Maples Are Complicated

Male red maple flowers fallen from the tree, 10 April 2019 (photo by Kate St. John)

This week the hillsides turned faintly red as red maples (Acer rubrum) bloomed across southwestern Pennsylvania. The city’s maples bloom sooner than the suburbs so I’ve had a preview of what’s to come.

In Schenley Park the ground under some red maples is carpeted with fallen flowers (above) while others retain flowers that are setting seed (below).

Female red maple flowers on the tree, developing samaras, 10 April 2019 (photo by Kate St. John)

That’s because red maples are sexually complicated. They are polygamodioecious which means some trees have only male flowers, some have only female, and some have both (i.e. hermaphroditic). And they can even switch back and forth:

Under the proper conditions, the tree can sometimes switch from male to female, male to hermaphroditic, and hermaphroditic to female.

Wikipedia Acer Rubrum

Watch your local red maples to see what they’re up to. The one in my backyard dropped its flowers a few days ago. This year it’s a male. 😉

p.s. For more on maple phenology, read Chuck Tague’s blog post: Maples In Spring: A Study in Diversity.

(photos by Kate St. John)

Buds About To Burst

Yellow buckeye buds about to burst, 1 April 2019 (photo by Kate St. John)

Yesterday I found these buds about to burst in Schenley Park.

Yellow buckeyes (Aesculus flava) are one of the first trees to leaf out in the spring, unfurling their dramatic palmate leaves. They’re such a welcome splash of green that I photograph them nearly every year. This is the first time I noticed the bud at this stage. I didn’t expect it to be red.

Over the years my buckeye photographs have documented the vagaries of spring in Schenley Park. In cooler years — such as 2015 — the buds weren’t this far along in mid-April. Here’s a closed bud on 15 April 2015.

Closed bud on 15 April 2015 (photo by Kate St. John)

In hot years — such as 2012 — the buds opened weeks ahead of schedule. This buckeye was completely leafed on 19 March 2012.

Yellow buckeye tree 19 March 2012 (photo by Kate St. John)

This year appears to be a “normal” spring … whatever that means these days.

(photos by Kate St. John)

The Trees Are Snowing

Drift of fluffy seeds from London plane trees, 17 March 2019 (photo by Kate St. John)

Last week I found fluffy drifts on the sidewalks in my neighborhood. They’re the airborne seeds of London plane trees (Platanus × acerifolia), planted in Pittsburgh in the late 1800s because they’re tolerant of air pollution.

Like their parents — oriental plane tree (Platanus orientalis) + American sycamore (Platanus occidentalis) — London plane trees retain their dangling seed balls over the winter.

London plane tree fruit in autumn (photo from Wikimedia Commons)

Then in the first warmth and light of spring the seed balls disintegrate and the seeds blow away. You’ll find drifts near American sycamores, too.

London plane tree fruit disintegrating (photo from Wikimedia Commons)

In March the trees are snowing.

(seed drift photo by Kate St. John; remaining photos from Wikimedia Commons, click on the images to see the original)

Weeping Willows Hint of Spring

Weeping willow turning yellow, Schenley Park, 15 Feb 2019 (photo by Kate St. John)

Weeping willows (Salix babylonica) are popular landscape trees that were brought here from Asia. They’re easy to notice at this time of year because their drooping stems turn yellow in very early spring. From a distance you see a splash of yellow.

Imported species, especially those from Europe, grew up in a steady climate with few spring surprises so they’re quick to bloom in the spring and late to drop their leaves in autumn.

Meanwhile our native trees are still brown, conservative about producing tender shoots because they know that volatile spring weather can bring a killing frost in April.

Are you tired of winter? Watch for the weeping willow’s hint of spring.

(photo by Kate St. John)

Sap Freezing

Frozen sap of red oak, 10 Feb 2019 (photo by Kate St. John)

The weather this month has been up and down like a yo-yo: A low of 6oF on February 2, highs in the 50s and 60s for six days, then a low of 14oF on February 9. During those warm days the sap started running in the trees. I wouldn’t have noticed except …

On February 10 during a walk in Schenley Park I found flash-frozen sap on the damaged trees. At top, a fallen red oak made a red-orange waterfall. Below, a small amount of sap in a fungi-encrusted tree dripped like orange ribbons.

Sap runs and freezes inside healthy trees, too. We just can’t see it.

(photos by Kate St. John)

The Shapes of Trees: Tuliptree

Tuliptree in winter, Schenley Park (photo by Kate St. John)

The Shapes of Trees continues today with the tuliptree or yellow-poplar, Liriodendron tulipifera.

Tuliptrees are the tallest eastern hardwood, 70 to 190 feet tall at maturity.  They’re characterized by straight branch-less trunks for most of their height because they jettison their lower branches as they grow.  The crowns have arching branches and up-swept twigs(*). On mature trees some branches are bent or zigzag, but they are never as gnarly as black locusts. 

The photos below show their typical shape as seen from the side and below.  Notice that the trunk is straight and limbless.

Tuliptrees in winter (photos from 1609187 by John Ruter, Univ. GA, UGA1118275 by Brian Lockhart, USDA Forest Service)

Younger trees have tapered tops like candle flames.

Young tuliptree with characteristic candle flame shape (photo by Kate St. John)

In early winter tuliptrees are still dotted with clustered samaras that look like pale wooden flowers, but you have to use binoculars to see them. 

If you’re lucky to find a twig at eye level you can easily identify the tree by its large smooth end bud, shaped like a duck’s bill.

Tuliptree twig with duckbill bud (photo by Kate St. John)

But the mature trees have no low hanging twigs so you’ll have to use other clues.

Learn how to identify tuliptrees in winter and see photos of the bark, buds and samaras in this vintage article: Winter Trees: Tuliptree.

(*) Quoted from The Sibley Guide to Trees, page 100.

(first photo and bud photo by Kate St. John. Tuliptrees in winter from 1609187 by John Ruter, Univ. GA, UGA1118275 by Brian Lockhart, USDA Forest Service. Click on these links to see the originals)