Category Archives: Winter Weeds & Trees

The Trees With Leaves Are…

Yellow leaves and bare trees, Schenley Park, 23 Nov 2020 (photo by Kate St. John)

24 November 2020

By now all the leaves have fallen in the Pittsburgh area. Or have they? There are still a few trees with bright yellow leaves in Schenley Park — Norway maples.

As their name implies Norway maples (Acer platanoides) were imported from Europe where their native range extends further north than Pittsburgh.  Our short November days are the same length as those they experience in October back home.  The sun will be up for 9 hours and 39 minutes today, 24 November, in western Pennsylvania.  That’s the day length on 21 October in Oslo, Norway.

Right now our native trees are bare or retain just a few yellow leaves at the very top (tuliptrees) or dried brown leaves overall (oaks and beeches).

Because non-native plants are out of synch with our seasons late November is the best time of year to see them in the landscape.

The trees with leaves are aliens!

Fun fact: Pittsburgh’s latitude is very far south of Scandinavia. Did you know we are on the same latitude as Madrid, Spain?

Quiz: What North American city is nearly the same latitude as London, England? The answer is surprising.

(photo by Kate St. John)

The Trees Are Snowing

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

28 March 2019

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)

The Shapes Of Trees: Ailanthus

Ailanthus in winter (photo by Kate St. John)

The Shapes of Trees continues today with an Asian import.

Some Pittsburghers call this tree a sumac but it’s a case of mistaken identity. Sumacs are in the genus Rhus. This tree is an Ailanthus, specifically Ailanthus altissima. Its common names include Tree-of-Heaven as it is called in China, and “Tree From Hell” because it’s so invasive in North America.

Ailanthus has a unique shape in winter with sparse, thick, crooked branches in an open crown. They’re always reaching up.

The branches look crooked from a distance because the twigs are stout and blunt and the buds are large and alternate. Each bud changes the angle of the twig. A twig can be as fat as your finger.

Ailanthus altissima twig (photo by Kate St. John)

Ailanthus grows easily in waste places. David Sibley writes in his Guide to Trees,

Trees have been known to sprout from roots 150 feet away from the original trunk and grow over 12 feet in a year.

The Sibley Guide to Trees, page 353

Read more in this vintage post, Winter Trees: Ailanthus.

(photos 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)

The Shapes of Trees: Black Locust

Silhouettes of two black locusts in Schenley Park (photo by Kate St. John)

Now that the leaves have fallen we can see the skeletal shapes of trees.  Did you know it’s possible to identify them by shape?

Seven years ago I wrote a series on identifying trees by their buds and bark but I didn’t mention shape except for this gnarly twisted tree, the black locust.

Black locusts (Robinia pseudoacacia) are easy to find in Pittsburgh because we’re within their native range and they grow well in disturbed soil.  They’re often described as brittle and invasive but that’s because of what we did to them. 

Native range of black locust (map from Wikimedia Commons)

Because their hard wood is very durable and prized for furniture, flooring and fence posts, we planted black locusts around the world.  Outside their range the trees became invasive.  Meanwhile their natural enemy, the locust borer (Megacyllene robiniae), followed them wherever they went.

Locust borer larvae drill holes in the heartwood and weaken the tree.  Before 1900 the bugs were in balance but that year the locust borer population exploded everywhere. Since then black locusts are usually infected and brittle so the trees rarely reach full size before they blow down. 

Black locust damaged by locust borer (photo by James Solomon, USDA Forest Service, via

But they do retain their distinctive shape.  Here’s another example. 

Gnarled black locust tree (photo by Kate St. John)

On Throw Back Thursday, learn more cool facts about black locusts and how to identify them in this vintage article: Winter Trees: Black Locust.  Then take a walk outdoors to find their distinctive gnarly shapes. 

In the next few months I’ll add more Shapes of Trees as I encounter them outdoors.

(photos by Kate St. John; map from Wikimedia Commons. click on the caption to see the original)

Tiny Opals

Hackberry fruits (photo by Paul Wray, Iowa State University,

This month I read Lab Girl by Hope Jahren and learned that she made an amazing discovery in 1998 during research for her doctoral dissertation:  Inside common hackberry fruit is a small hard pit with a lattice made of opal.

Hackberry fruits, pictured at top, are drupes similar to cherries and peaches with fleshy fruit surrounding a central pit.  The fruit is thin and the pits are large so we rarely eat hackberries but birds love them.

The pits in cherries and peaches are made of wood (or something like it) but hackberry pits are made of stone: calcium carbonate inside a lattice framework. When Hope Jahren used Xray diffraction on the crushed lattice material its composition came up “opal.” 

When I found this out I searched for the pits under hackberry trees in Schenley Park. At this time of year the fleshy purple fruit is gone, only the white pits remain.  Here’s what I found, one whole, one opened. The exterior is a network of tiny raised lines. 

Opal is in these hackberry pits (photo by Kate St. John)

The pits don’t look like opal and probably never will.  You’d have to use acid to remove the calcium carbonate (the white stuff of seashells) and then examine the remaining latticework under a microscope.  There’s a tiny bit of opal in there.

And so I wonder: How does a tree put opal in its drupes?  I don’t know, but here are the raw materials:

[The rock] Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually opal is formed. 

From Opals Down Under

Trees take up water that contains dissolved minerals including the building blocks of opal.

Miraculously, the hackberry tree pulls out what it needs and makes an opal latticework inside its drupes.

Learn how to identify hackberry trees in winter at Winter Trees; Hackberry.  Then search the leaf litter for tiny opals.

For further reading see : Hackberry: A Gem of a Weed

(photo credits: hackberry fruits by Paul Wray, Iowa State University,, hackberry pits by Kate St. John)

Why Not To Clear Your Garden This Fall

Goldenrod gall with a woodpecker hole (photo from Wikimedia Commons)

I’ll bet you have a gardening project planned this weekend or next. Here’s some time-saving advice:  Don’t clear your garden in the fall.

Why not?

  • Seeds on the old plants provide winter food for birds and animals.
  • Insects overwinter on plants in egg masses, cocoons and galls.  Birds eat those insects. 
  • The brush provides shelter for the birds.
  • You won’t have to mulch.
  • You’ll enjoy watching birds among the old plants.

The photo at top shows that an old goldenrod gall contained food for a woodpecker. He hammered a hole to get the bug.

On Throw Back Thursday, read more about this time saving plan in a 2010 article: Why Not to Clear Your Garden

p.s. The only downside I can think of is this: It’s hard to plant bulbs when the old stuff is in the way. 

(photo from Wikimedia Commons; click on the caption to see the original)


Flowering cherry tree in snow, 4 Jan 2016 at Carnegie Museum in Pittsburgh (photo by Kate St. John)
Flowering cherry tree in snow, 4 January 2016 in Pittsburgh (photo by Kate St. John)

After a month of warm weather, these cherry trees were fooled into blooming in early January at Carnegie Museum.

Then last Monday the temperature dropped into the single digits and hit everything that couldn’t get out of its way.  Nothing could protect those delicate pink flowers.

Unlike plants, birds can get out of the way and some of them decided to leave this week.  In my neighborhood, there were many American robins in December but most of them have left since the cold snap.  Did your robins leave, too?

Meanwhile, don’t be fooled by today’s warmth.  Here’s a graph of Pittsburgh’s actual and predicted morning low temperatures for the first two weeks of January.

Graph of morning low temperatures in Pittsburgh, PA, actual+forecast for January 1-14, 2016 as of 1/9/2016 (graph uses NWS data)
Actual+forecast morning low temperatures in Pittsburgh, PA, January 1-14, 2016 (graph uses National Weather Service data as of 1/9/16)

It’s a yo-yo.


(photos by Kate St. John)