Category Archives: Trees

Just a Hop Makes The Difference

Two trees called Ironwood: American hornbeam (left), American hophornbeam (right) (photos by Kate St. John)

26 July 2021

Two trees in the Birch family (Betulaceae) are common in the Pittsburgh area but I’ve struggled with what to call them because they have the same names.

Both are called Ironwood because their wood is hard, close-grained, and very strong. Ironwood is a poor name choice, though. About 160 species around the world are called “ironwood”.

Their scientific names are different but their default common names are very similar: American hornbeam (Carpinus caroliniana) and American hophornbeam (Ostrya virginiana). Hornbeam refers to their hard, strong wood: horn (hard, bony structure) + beam (Old English for tree). Hop is the only difference.

Fortunately they are easy to tell apart in the field at any time of year. In the photo at top:

  • The bark of American hornbeam looks like sinewy muscles (top left).
  • American hophornbeam bark peels in narrow parallel strips (top right).

Both trees produce fruit enclosed in an involucre, a whorl or rosette of bracts surrounding the inflorescence. This is where “hop” comes in.

Both species with fruit and bark paired.

American hornbeam or blue beech, fruit and trunk (photos by Kate St. John)
American hophornbeam, fruit and trunk (photos by Kate St. John)

They differ by a hop.

p.s. Because of their similar names I sometimes call “hornbeam” by another common name: blue beech. More confusion!

(photos by Kate St. John)

What Is It About Box Elder?

Box elder planted in a yard (photo by Tom DeGomez, University of Arizona,

13 July 2021

In Pittsburgh we hardly think about box elder (Acer negundo). It’s a native tree that grows by the river. No one plants it. It’s not a “bad” tree. So I was puzzled by this 1950’s story from my mother.

I never hear of box elder that I don’t think of your grandfather.  He never had a bad word to say about anyone.  He was a man of integrity and the absolute worst thing I ever heard him say was [this] about a member of the town council: “He was the kind of man who would plant box elder.”

— 1950’s family anecdote from my mother

My grandfather lived in a village in suburban Chicago in the heart of the Midwest where box elder is considered bad, ugly, weedy and invasive. Wikipedia provides this insight on how it got a bad reputation:

“After World War II, box elder’s rapid growth made it a popular landscaping tree in suburban housing developments despite its poor form, vulnerability to storm damage, and tendency to attract large numbers of box elder bugs. … It can quickly colonize both cultivated and uncultivated areas. … It grows around houses and in hedges, as well as on disturbed ground and vacant lots.”

In Wisconsin box elder is so disdained that the Urban Ecology Center wrote a blog in defense of it: Native Tree Spotlight: In Defense of Box Elder.

Box elder isn’t invasive in Pittsburgh so I had to go look for it on its home turf at Duck Hollow. There I found that as a shade tree it can look pretty good. This one is two box elder trunks intertwined.

A large double-trunk box elder shading the bike trail at Duck Hollow, 11 July 2021 (photo by Kate St. John)

However some of them die back leaving ugly bare branches at the top.

Box elder dying back, Duck Hollow, 11 July 2021 (photo by Kate St. John)

And if you cut box elder or chop it down it grows suckers from every crevice.

Midwesterners agree that box elder is bad. Why don’t Pittsburghers have this aversion? I think it’s because we are on the eastern edge of box elder’s range, we never planted it as a street tree, and it isn’t particularly invasive here.

Box elder range map (image from Wikimedia Commons)

What is invasive here? Japanese knotweed! Originally planted as an ornamental, we don’t think it’s pretty anymore.

Japanese knotweed has taken over large portions Duck Hollow, 11 July 2021 (photo by Kate St. John)

If my grandfather had lived in Pittsburgh perhaps he would have said “He was the kind of man who would plant Japanese knotweed.”

Aha. Now I get it.

p.s. Later this summer box elder bugs will appear though not in huge numbers at Duck Hollow.

Box elder bug (photo by John English)

(photo at top by Tom DeGomez, University of Arizona,, remaining photos by Kate St. John and John English)

The Honey Scent of Trees

American basswood in bloom, Schenley Park, 12 June 2021 (photo by Kate St. John)

21 June 2021

The air smells honey-sweet in Pittsburgh this month because the basswoods (lindens) are in bloom.

American basswood (Tilia americana) is a native tree in eastern North America with close relatives across the Northern Hemisphere. In Europe Tilia species are called linden trees. In the British Isles they are “lime trees” though not at all related to lime fruit. (Aha! Lindens are the “avenue of lime trees” in English novels.)

Basswoods grow naturally in the forest (left below) but are often used in landscaping. You can recognize them by their large size and distinctive teardrop shape (right below).

(photos by Vern Wilkins, Indiana University and Richard Webb via

In June you can find them by smell, which is how I discovered the basswood at top in Schenley Park on 12 June.

Bees find them, too, so here’s a word to the wise. Never ever spray pesticide on linden (basswood) trees in bloom. A pesticide company did that in Oregon in June 2013 and killed 50,000 bees, the largest native bee kill on record.

p.s. Sometimes a linden tree drops “sap” on everything beneath it. But it isn’t sap. It’s excretion from an infestation of linden aphids (Eucallipterus tiliae) that are sucking the Tilia leaves. The Toronto Master Gardeners website explains what to do. Again, do not spray the tree while it’s in bloom!

(photos by Kate St. John, plus Vern Wilkins and Richard Webb via

Bee Tree Broke In The Storm

17 June 2021

We are usually unaware of wild honeybee hives high in the forest and that was certainly true of this one near the Westinghouse Shelter in Schenley Park. The bee tree broke during last Sunday’s storm and just missed hitting the shelter. At noon on Tuesday I found the tree cordoned off by Public Works as they waited for the bees to be removed.

The massive hive was in a hollow 20+ feet up in a red oak. When a northwest gust hit the tree it broke at its weakest point and split the hive. Most of the hive remained in the upper section with a few empty honeycombs in the dangling piece.

Rather than step closer I zoomed my cellphone camera to show the bees covering the hive (center of photo) and more honeycombs at top right in the hollow.

Wild honeybee hive in a fallen oak, 15 June 2021 (photo by Kate St. John)

When I passed through at 1:30pm, beekeeper and DPW Schenley Park worker Kevin Wilford was carefully moving the hive to a bee transport box. He attached the white box to the tree to encourage the bees to go in it after he moved the hive. However, the hive was so deep that he could not reach it without more tools. The process took longer than I had time to watch but Kevin gave me a taste of it, a small piece of honeycomb laden with honey. Mmmmmm good! and sticky!

Beekeeper & DPW worker Kevin Wilford begins to move the bees, 15 June 2021 (photo by Kate St. John)

By Friday the beehive will be on a scenic hill above Hazelwood, the damaged tree will be gone, and the Westinghouse Shelter will be ready for use.

UPDATE on 24 JUNE 2021: As I passed by the bee tree today I could see that most of the hive was still in place. The bees are very deep inside the hollow so the tree is still down.

Bee tree still has bees as of 24 June 2021 (photo by Kate St. John)

UPDATE on 1 JULY 2021: The end of the bee tree. It is gone except for a very tall stump.

Bee tree is gone, Schenley Park, 1 July 2021 (photo by Kate St. John)

(photos by Kate St. John)

After The Storm

Sunset after the storm, 13 June 2021 (photo by Kate St. John)

Tuesday 15 June 2021

At 6pm on Sunday evening a violent thunderstorm blew through Pittsburgh with powerful wind gusts, hail and heavy rain.

Dave DiCello photographed the storm from the West End as it approached Oakland. The VA Hospital and the Cathedral of Learning are to the right of the lightning bolt.

Meanwhile my husband and I watched from our 6th floor apartment as a wind gust picked up the patio umbrella from the high-rise roof next door and blew it, Mary Poppins-like, until it crashed into our building. Then we saw no more as rain and hail battered our windows for half an hour, first from the north, then the east.

The tempest left behind flooding, downed trees, power outages, and a rainbow.

Rainbow after the storm, 13 June 2021 (photo by Kate St. John)

Yesterday morning I surveyed the damage after the cleanup had already begun. In a short walk I found trees down at Frick Fine Arts, Carnegie Library and Museum, and two small breaks on South Craig Street.

At Schenley Park the valley around Panther Hollow Lake was spared but the lake itself was full of flood water. This is by design. A flow control gate at the outlet holds back freshwater so that storms will not flood The Run.

Panther Hollow Lake holds back floodwaters from the storm, 14 June 2021 (photo by Kate St. John)

This morning the power was still out in parts of Squirrel Hill as I drove home from the grocery store.

My husband and I were fortunate. Our power never failed and that flying umbrella hit the wall below us and caused no damage.

p.s. The young Pitt peregrines are flying so well that they are hard to find. I saw both adults plus two of four juveniles on my Monday morning walk.

(photos by Kate St. John)

Attacks Trees From Underground

9 May 2021

Have you ever seen these long black ropes draped on a fallen log? They were hidden under the bark before the tree died, and they’re the reason the tree died. These are mycelial cords or rhizomorphs of Armillaria, a genus of fungi that ultimately kills trees. It attacks the trees from underground.

Armillaria consists of 10 species which are easiest to identify by their mushrooms, the reproductive stage of the fungus. Honey mushrooms appear near the base of an infected tree but the spores rarely cause infection in other trees.

Fruiting bodies of Armillaria solidipes, Cook Forest, PA (photo from Wikimedia Commons)

Instead, Armillaria spreads by the rhizomorphs shown at top which travel only eight inches below the soil surface, advancing about 3.3 ft (1 m) per year. As they make contact with another tree they invade the roots and then the trunk. If a tree is already infected it will spread the fungus via root grafts.

Underground spread of armillaria disease (illustration from Wikimedia Commons, cropped)

Armillaria spreads so far and lives so long that a single Armillaria ostoyae in the Malheur National Forest in Oregon was found to be 2,400 years old and the largest living organism on earth.

As the infection takes hold, the fungus invades more deeply via white mycelium sheets that damage the roots or girdle the tree. Here a fallen black cherry reveals its cause of death.

Black cherry toppled near its base due to Armillaria, as seen by white sheets inside the wood, Schenley Park April 2017 (photo by Kate St. John)

Schenley Park is riddled with Armillaria but we have no hint that a tree is invaded until it topples, sometimes at the roots.

Trees are so stoic. No matter what attacks them, they just have to stand there and take it.

(photos by Kate St. John and from Wikimedia Commons)

Trees That Bleed Orange

Yellow-bellied sapsucker, 2011 (photo by Steve Gosser)

Yellow-bellied sapsuckers (Sphyrapicus varius) leave behind evidence of their visit when they pass through Pittsburgh on migration. In their search for food sapsuckers drill horizontal rows of holes on tree trunks and large branches, then lick the leaking sap and eat the insects attracted to it.

The trees cope as best they can with this unusual damage. Shagbark hickories (Carya ovata) grow puckered bark around the holes.

Shagbark hickory in Frick Park, horizontal rows are holes drilled by yellow-bellied sapsuckers (photo by Kate St. John)

Tuliptrees (Liriodendron) display clean holes.

This spring for the first time I’ve noticed trees bleeding orange sap after sapsuckers visit.

My best guess is that these are sweet birch (Betula lenta), also called black birch or cherry birch. Nothing says their sap turns orange, but a few facts tip the species scales for me.

  • New sweet birch leaves resemble cherry leaves. (I saw this yesterday)
  • The bark of older trees is dark and looks like plates.
  • Sap rises in sweet birch trees later and faster than it does in maples. (true)
  • We tap sweet birch trees and collect the sap to make birch beer. (Why is birch beer red?)
  • Deer don’t eat sweet birches so they’re more common now in Pennsylvania’s forests.

Have you seen any trees that bleed orange sap?

Do you happen to know if these are sweet birches?

(sapsucker photo by Steve Gosser, tree photos by Kate St. John)

UPDATE! Horticulturalist Sara Showers points out: “This looks like a phenomenon called “slime flux,” when leaking sap – a sugary food source! – is colonized by yeasts and other fungi. Here’s an article from Cornell about the phenomenon:

Meanwhile I’m becoming more convinced that the tree species is sweet birch based on its leaves and bark and abundant sap. No, the sap is not orange.

Spring Green

Spring green among the trees, Frick Park, 8 April 2021 (photo by Kate St. John)

10 April 2021

This week Pittsburgh’s sugar maples are clothed in spring green flowers while the oaks remain bare. Most trees bloom long before leaf out so their leaves won’t block the pollinators. These flowers take full advantage of the wind.

Sugar maple flowers, Schenley Park, 9 April 2021 (photo by Kate St. John)

Did your allergies kick in this week? The trees are throwing off lots of pollen with little rain to lay the dust.

Insect-pollinated flowers will follow soon. On 3 April pawpaw flowers (Asimina triloba) were still tiny buds in Schenley Park but by the time they bloom the stems will be long and flexible. The dark maroon fetid-smelling flowers will hang like bells to attract flies and beetles. Click here to see a pawpaw flower.

Pawpaw flower bud, Schenley Park, 3 April 2021 (photo by Kate St. John)

Eastern redbud flowers (Cercis canadensis) had not opened in Schenley as of 7 April, but they showed promise.

Redbuds, Schenley Park, 7 April 2021 (photo by Kate St. John)

Spring cress (Cardamine bulbosa) was blooming at Raccoon Wildflower Reserve on Easter Day.

Spring cress, Raccoon Wildflower Reserve, 4 April 2021 (photo by Kate St. John)

And Virginia bluebells (Mertensia virginica) were open in Schenley Park on 9 April.

Virginia bluebells, Schenley Park, 9 April 2021 (photo by Kate St. John)

This winter I noticed that when moss grows up the base of saplings it looks like leggings on the trees. At Raccoon Wildflower Reserve I found an entire group of saplings wearing mossy leggings. Click here to see the whole group. (Anyone know what this mossy phenomenon is?)

Mossy “leggings” on saplings, Raccoon Creek Wildflower Reserve, 4 April 2021 (photo by Kate St. John)

Spring green will continue in the coming weeks as tiny leaves pop open and more trees bloom.

(photos by Kate St. John)

Sap Rising and Other Signs of Spring

Sap dripping from a fallen oak, Schenley Park, 5 March 2021 (photo by Kate St. John)

6 March 2021

Flowers are pushing up their leaves and sap is rising in the trees.

On Friday the sap was rising so fast in this fallen red oak that it was dripping to the ground. The oak keeled over last year in Schenley Park leaving just one root in the ground. That root is still doing its job.

Sap rising in a fallen oak, Schenley Park, 5 March 2021 (photo by Kate St. John)

With temperatures this week above freezing during the day and below freezing at night I imagine it’s still maple sugaring time in the Laurel Highlands. Sugaring ends when the buds open. They haven’t opened yet in Schenley Park (below).

Sugar maple buds are not open, Schenley Park, 5 March 2021 (photo by Kate St. John)

Other trees have swelling buds.

Trees buds are swelling, Schenley Park, 5 March 2021 (photo by Kate St. John)

Below, silver maple (Acer saccharinum) buds were about to burst when a squirrel gnawed the stems and they fell to the ground. Do squirrels nip off the buds to get to the sap?

Silver maple buds attempted to open, Schenley Park, 5 March 2021 (photo by Kate St. John)

Meanwhile daffodil leaves are turning green in Schenley Park.

Daffodil leaves, Schenley Park, 5 March 2021 (photo by Kate St. John)

Tulip leaves emerged on a busy street in Oakland where the deer can’t get them. 🙂

Tulip leaves, Craig Street, 2 March 2021 (photo by Kate St. John)

And there is mud.

The snow melted all at once last weekend and the Monongahela River rose high, flooding the Mon Wharf. On Wednesday 3 March it was sunny and 60 degrees, a great time for a walk … but not here!

Monongahela River receding from flood stage, Pittsburgh South Side, 3 March 2021 (photo by Kate St. John)

When the river receded it left behind leafy debris and deep chocolate-pudding-like mud at South Side Riverfront Park. I tried to walk down there but I gave up before getting muddy. Others were not so careful. You can see deep footprints in the shade in the photo above (bottom left).

Closeup of debris and mud as Mon River recedes, 3 March 2021 (photo by Kate St. John)

It’s Mud Season in Pittsburgh!

(photos by Kate St. John)

Living Fossils in Pittsburgh

Base of a dawn redwood in front of Phipps Conservatory, Jan 2021 (photo by Kate St. John)

This month Tree Pittsburgh is featuring the dawn redwood as their Tree of the Month so I walked to Phipps Conservatory to see four of the living fossils. On the way I found a fifth near the Cathedral of Learning.

Dawn redwood on Pitt’s campus next to the Cathedral of Learning, Heinz Chapel in the background (photo by Kate St. John)

Endangered in the wild, the dawn redwood (Metasequoia glyptostroboides) is the only survivor of the genus Metasequoia from the age of the dinosaurs. It was thought to be extinct until a single living tree was discovered in 1941 in the village of Moudao in Hubei province, China.

The discovery happened in the nick of time. The tree would have gone extinct by now were it not for local protection and a seed-collecting expedition in 1947 that distributed seeds to ornamental gardens and arboretums around the world. Joe Stavish tells the story in Tree Pittsburgh’s video.

Tree of the Month: Dawn Redwood from Tree Pittsburgh on Vimeo.

Across the lawn the dawn redwoods at Phipps smile to other living fossils in Pittsburgh — the ginkgos that line Schenley Drive.

(photos by Kate St. John)