William Hamilton, Lombardy poplars, and the landscape of cemeteries

From Erica Maust, of the Woodlands Historic Mansion, Cemetery, and Landscape:

“In 1784, William Hamilton introduced the Lombardy poplar to North America on his Philadelphia estate, The Woodlands. In 1788, a visitor to The Woodlands wrote that Hamilton’s walks were “planted on each side with the most beautiful & curious flowers & shrubs. They are in some parts enclosed with the Lombardy poplar except here & there openings are left to give you a view of some fine trees or beautiful prospect beyond…”

Grove Street Cemetery in New Haven, Connecticut was the first private, non-profit cemetery in the world. Organized in 1796 as “the New Burying Ground in New Haven,” it was one of the earliest cemeteries to have a planned layout with privately owned family lots, named streets and avenues, and arrangements of ornamental plantings. The original 1796 planting scheme of the cemetery featured regular rows of–you guessed it!–Lombardy poplars (the very same tree Hamilton introduced to North America 12 years earlier), along with a poplar grove and meadow at the rear of the cemetery.

44 years later, in 1840, Hamilton’s very own landscaped Woodlands became a planned, rural cemetery, preserving his landscape and horticultural pursuits.”

For more, see here:


And for more about William Hamilton and the Lombardy poplar:



A different zelkova

Throughout Philadelphia, you’ll see the Japanese zelkova planted as a street tree, or planted in yards and parks, all over the place.  If you go over by Norris Square in North Philadelphia, just south of the Square, along Howard Street, there’s a row of enormous zelkovas.  Or if you’re in Chestnut Hill, at the triangle of green bounded by Ardleigh, Winston, and East Willow Grove, you’ll also see some large zelkovas.  And just in general, if you go around the city, looking at trees, you’ll see plenty of Japanese zelkovas – they have dark grayish bark, somewhat rough, with an orangey tone underneath, that you can see as the bark above it peels away.  Their leaves are toothed, and come to a bit of a point at the end – they’re shaped somewhat like the leaf on the Breyer’s ice cream logo.  Their branches spread out somewhat, and their canopy makes a nice rounded shape.  They’re tough trees that can live well in the city, and they also grow in an attractive habit – they look nice and they grow well, and so, they’re planted all over the place.  The Japanese zelkova (Zelkova serrata) is an ubiquitous street tree in Philadelphia.

But there is another zelkova in Philadelphia, the Caucasian zelkova (Zelkova carpinifolia) that is rarely, if ever seen.  In fact, the only one that I’m aware of in Philadelphia, and perhaps the only one in the state, and quite likely the only one in the Delaware Valley, is at the Woodlands Cemetery in west Philadelphia, and it looks like this:

Zelkova carpinifolia, Woodlands Cemetery, Philadelphia, PA
Photo courtesy Jessica Baumert

It truly is magnificent – with its upwards reaching branches (that is, its fastigiate growth habit), it looks like a wild upended broom in the wintertime.  Lush foliage in the spring fills its canopy with green that lasts through to the autumn, and in the winter, when it loses its leaves, the fastigiate habit makes for a wonderful structural element in the landscape.  And its gray silvery bark, without the orangey-red underbark of the Japanese zelkova, gives it a subtly rich texture throughout the year.  The branches also fuse with each other, making for an appearance like a strangler fig in the tropics, with the branching and anastomosing strands tangling into wonderful knots.

This fusion is an interesting characteristic, and not wholly uncommon in plants.  Plants are different from animals, in that they have far fewer moving parts than we do.  For animals, cells move with respect to each other – they move around the body, they shift as organs develop, they die, they grow, they can even travel quite far from where they were initiated, for example in the case of blood cells.  An animal cell did not necessarily grow up next to its nearest neighbor, they are moving around so much, and so they need to have a pretty sophisticated system of cell-cell recognition, to make sure that the cell nestled up against them isn’t a parasite, like a bacterium or fungus, as some of their neighbors may be.  And so, we (animals, that is) have immune systems that are pretty good at differentiating self from non-self.  This is good for reducing parasite load, however, it isn’t so good if you want to make a graft – hence the need for immune suppressing drugs when organ transplant surgery is completed.

Plants, however, grow by stacking one cell atop another as they divide and grow, and these cells are cemented together the one against the other as they do this – they grew up next to their neighbors and they’ll live next to them for the entirety of their lives, and after they die, they’ll be stuck there still.  And so, there has been less evolutionary pressure in plants for a tight recognition of self versus non-self – proximity does that job for them.  This isn’t to say that plants don’t have immune systems – they do.  But they are not as tightly controlled as ours, and this means that individual plants can fuse with each other much more readily than animals can.  And this is something that we (humans, that is) take great opportunity with, everytime someone grafts a tree, or a rose.  There is a downside to this, however, too, because it also means that diseases can flow from one plant to another, if a parasitized individual has fused with a non-parasitized one – this is what happened with the elms of North America, as Dutch elm disease arrived.  The trees’ roots had fused under ground, as they grew the one against the other along streets and avenues of the towns of America, and if one tree got sick, then rapidly all the trees in a row got sick, as the causative agent of this disease, a parasitic fungus, traveled along those fused roots.

And so, as with many things, there is a plus and a minus, advantages and disadvantages, to our respective immune systems, and how they spring from our differing developmental protocols, and how they then in turn affect that development as we grow.

And due to that development that allows for that fusion, the rummaging branches of the Caucasian zelkova have a fairyland apperance – like a mazed tangle of upright hair, knotting together, and then unknotting again, reaching Rapunzel like, but towards the sky.

When I first saw this tree at the Woodlands, the one that is pictured above, I’d thought it was very old, this enormous trunk.  It is very very large, and I’d been told that the Caucasian zelkova, this species, was at the Woodlands in William Hamilton’s time.  (William Hamilton was the Philadelphia gentleman and plant enthusiast who owned and cultivated the Woodlands up until the earliest part of the 19th century – he died in 1813, and there is a rich heritage of horticulture and natural history at the Woodlands that continues to this day)  And so from the first time I saw it I assumed that this trunk was somewhere around 200 years old.

This seemed to be supported by other evidence.  In an article I read, that I was pointed to by Joel Fry, Curator at Bartram’s Garden, this tree is discussed.  In 1876, in The Gardener’s Monthly, Eli K. Price, who was the commissioner of Fairmount Park, writes about this zelkova, and mentions it as having been here in Hamilton’s time.  And he and others had been thoroughly impressed with this arborescent spectacle – Price writes, in that same article, about a visit by Charles Sprague Sargent (a Harvard botanist) to Philadelphia, for the celebration of America’s centennial, at which time Sargent visited the Woodlands and marveled at this tree.  Price writes:

“These trees will be cared for and preserved in the Woodlands. What is more important is, that they should be secured to our country by propagation. If seed should appear next Fall, they will be gathered. In the meantime grafting should be attempted. Mr. Sargent is trying it at Cambridge, on English elms. I invite gardeners to get cuttings and try their success.”

Price does not mention if Sargent was successful with those grafts, nor if the legion of invited gardeners were successful with their cuttings, to propagate this tree, or even if they tried at all.

And so, as I looked at this wonderful tree and wondered why something so spectacular wasn’t growing in yards and parks throughout Philadelphia, like its cousin Zelkova serrata does, I thought I understood – this was, I believed, a very slow growing tree, and perhaps difficult to propagate, and it would take a very long time for it to get to this magnificent shape and structure, and so there were pretty good reasons for it not to have been commonly planted, and this, I thought, was why we don’t see it very often around here.

But I was wrong.  Yes, the Caucasian zelkova was at the Woodlands at Hamilton’s time.  This tree, this kind of zelkova, was brought into Europe from the Middle East in the 1780s by Andre Michaux, who had gone to Persia (what we would now call Iran) and collected it and brought it back to France.  At about this same time, William Hamilton, owner of the Woodlands at that time, was in England, visiting from Philadelphia, collecting plants and sending them back home – those plants would not have included the Caucasian zelkova, since it would have only freshly been introduced into Europe.  We’re not sure exactly how it got to the US, and to Hamilton’s estate.  But it may well have been via Michaux – he certainly was at the Woodlands, and he ran nurseries up in Hackensack (in New Jersey) and down in Charleston (in South Carolina) through which he might have imported this tree, and so while we can’t say for sure, we can make a rough approximation that Andre Michaux may well have been the fellow to have brought this tree to us, when it arrived here in the earliest part of the 19th century, or the latest part of the 18th.

But this large trunk now at the Woodlands, the one pictured above, was not from that and then, at least not directly.  As was pointed out to me by Joel Fry, John Harshberger wrote an article on the Woodlands in 1921, in The Garden Magazine, where he mentions these zelkovas:

“Outside of the remarkable Ginkgos, the rarest and largest trees of “The Woodlands,” are four remaining specimens of Zelkova crenata [note: this is an older name for this tree], native of the Caucasus regions. This species was originally planted in two rows forming an avenue of approach to the house. The single remaining tree of the west row near the stable was alive on June 24th, 1916, but is now dead. It measures 14 ft. 8 in. in circumference. In the eastern row, all of the three trees are now dead. These trees measure respectively 12 ft. 6 in.; 12 ft.; and 11 ft. in circumference. They are about 50 tall. Two young sprout trees have appeared between the second and third, which are already 10 ft. tall and promise to become lusty specimens.”

Though we know when that last one died back, it’s not clear when the others died, though we do know that the four mentioned above were here in 1905, as they are mentioned in Benjamin H. Smith’s 1905 “Some Letters from William Hamilton, of the Woodlands, to His Private Secretary” (The Pennsylvania Magazine of History and Biography, Vol. 29, No. 1 (1905), pp. 70-78):

“Only one specimen of the Ginkgo, now the oldest tree of that species in America, still remains in the vicinity of the old mansion ; near by are four large trees of Zelkova crenata, from the Caucasus, now in their old age, and these, with a few ancient English hawthorns, alone remain to attest the ancient glory of the gardens and grounds at The Woodlands.”

There is a photo of one of those original trees, currently in the Samuel N. Baxter collection at Bartram’s Garden (it was shown to me by Joel Fry), with a photograph from the 16th of April, 1920, of Mr. Baxter (he was the chief arborist of the Fairmount Park system at the time) gazing up at at a large trunked (13′ 4″ in diameter!) fastigiate tree – an original Woodlands Zelkova carpinifolia, “just before tree died”, as a label on the back of the photo tells us.  This provides further supporting evidence that those original Caucasian zelkovas are no longer there.

And so, by 90 years ago, the originals were gone.  This one we see now is a root sucker off them, one of the ones mentioned in Harshberger’s article from 1921, and it’s not more then 90 years old.  And it’s enormous, and so it’s not so slow a grower.  And so that is not why it is not planted commonly around here.

But perhaps it can’t be propagated well, and maybe that’s why we don’t see it.  Well, if we visit the McLean Library of the Pennsylvania Horticultural Society and look in catalogs from Meehan’s Nursery, a large one in Germantown, here in Philadelphia, we see that they had this tree for sale in the late 19th and early 20th century.  While it’s not in their 1858 catalog, it is in the 1880 catalog, under the name of Zelkova crenata – under a listing for plants that were “1 to 2 ft”, they went for 50 cents a piece, which wasn’t cheap at that time.  They didn’t sell them in bulk either, only by the piece, which suggests that they didn’t have a lot of them, or at least didn’t think they’d sell a lot of them.  The latter theory is borne out in the 1893 and 1895 Meehan’s catalogs, in which Zelkova crenata is noted as being “Rare in cultivation”, and in the 1896-1897 catalog, in which it’s either upgraded or downgraded, I don’t know which, to being “…not common in cultivation”.  Starting in the 1897-1898 catalog, it starts being sold under a different name, Planera richardii, and it was sold under that name for a few more years.  However, by the 1911 trade catalog, it isn’t there, and in the 1911 retail catalog, Planera is only listed in the back, and it is not described, nor are individual species listed – they are only listed in a footnote as being “in stock”.  The plant is not in the later catalogs – not in the 1916, nor 1917, nor 1923-1924.  Its few decades of being sold by Meehan’s Nursery had passed.  But it had been sold.

This tells us that it can be propagated, and the tree at the Woodlands shows us how well it can grow, how strong it can grow in a city, and just how beautiful it can be.  It’s a marvelous tree that can be propagated well enough to have been in the horticultural trade for a few decades, and it’s a tree that can grow quickly and live for a pretty long time, somewhere around a century, or a bit more, in a cemetery in the midst of one of the largest cities of North America (Philadelphia, that is).

So why isn’t it planted more often?  Why do we see it so rarely?  The only other one that I’m aware of anywhere near to here is one near the Capitol building, on the National Mall in Washington, DC.  Why aren’t there more of them planted in the cities and suburbs of the mid-Atlantic states?

I don’t know.  Its relative, the Japanese zelkova, is an extraordinarily common street tree in Philadelphia, but the Caucasian zelkova is not.   But this is a tree that could be planted, could be propagated for sale and distribution, could be grown in parks and yards throughout Philadelphia – it’s a tree that we don’t see much of, but we could see more of.  We just have to propagate it, plant it, and let it grow.

To read about the importance of cemeteries to urban land management, see here:


To read about some more natural history and open areas in Philadelphia, including cemeteries – see here:

Some Plants and Insects of the Woodlands

The trees of Monument Cemetery

Oakland Cemetery

Wissinoming, including Mt. Carmel cemetery

Cedar Grove

Hunting Park

Feral landscaping (Woodlands and Mt. Moriah cemeteries)

And to read about some other trees, see here:

American elms

“Penn treaty” elms

The saucer magnolia

American Chestnut

The Callery pear

Paulownia tomentosa, the empress tree

London planes and American sycamores

The sophora

Fringe tree

Feral landscaping

If you step into a cultivated landscape, like a cemetery for instance, and take a look around and see some trees, a general assumption, or a common one at least, is that those trees are there intentionally, that is, that it was decided by someone, or someones, to place them where they are.  For a tree to last from year to year it takes some amount of care, or at the very least it takes not cutting it down or pulling it up – and so if a tree is growing in a place that is landscaped, and the effort is being put into maintaining it, or at least it isn’t being removed, it would seem likely that it would have been placed there with some kind of thought – after all, who would take care of a tree if it hadn’t been meant to be there?  If they hadn’t meant for it to be there, wouldn’t they have gotten rid of it?  It just seems like common sense to assume that, in a cultivated landscape, if a tree is there, that it was put there by a person, that it was placed there by someone, that it was planted with intention.

This assumption, however, is wrong.  Last year, in July, I was out at Mt. Moriah Cemetery, in southwest Philadelphia.  Mt. Moriah was established in the 1850s and was designed as a “rural cemetery”, with an ornate entrance and winding roads going through it, like Laurel Hill in North Philadelphia, or Mt. Auburn in Cambridge, Massachusetts.  However, distinct from those rural cemeteries, Mt. Moriah was (and is) populated primarily with the middle class and the working class – in contrast to the more wealthy citizens of those other cities of the dead.

Mt. Moriah was originally entirely in Philadelphia, but it later expanded out of the city, reaching across Cobbs Creek and over into the Borough of Yeadon.  Now, it’s a few hundred acres of rolling hills, and gravestones, and monuments, with trees here and there dotting the landscape.  And in some places those dots come very close together.  This cemetery, for a number of reasons, has become somewhat overgrown, and in many places the plants have taken over and risen up and over the gravestones, crowding in on them, and filling over the space with green.

On the Philadelphia side however, when I was there last year, the grass had been mown, tangles had been taken out, and it was pretty well open, cut and cared for.  And so as we arrived, walking along Springfield Ave, I saw grass, and open space, and also trees here and there among the headstones.  I was interested to see what those trees were, that were planted there – since this cemetery has a different demographic than that of Laurel Hill or Mt. Auburn, I wondered what trees they would have used – would they have been similar, or perhaps even the same, kinds as those in the cemeteries with wealthier interments?  By seeing what persists in the environment, we can see what was done in the past, and so I was interested to see what the landscaping habits of the middle class had been, at Mt. Moriah.  What trees were there, what trees had been there?

As I got closer, I saw that those trees were mulberries, and black cherries, and they were, estimating by their size, probably about ten or twenty years old, or less.  And I realized that, because people hadn’t been planting mulberries or black cherries at those times when these trees began their lives, and because these are trees that seed in on their own quite readily, for these reasons I realized that these trees that were set among the landscape had most likely been set there without our help, without having been planted by the hand of man and woman.  They were little bits of feral landscape – though the land had been tamed and cultivated around them, cut back and trimmed, managed with thought, the tiny parcels on which those trees sat had been taken over by these plants whose seeds had just come in on their own, with help maybe of the wind, or of animals other than humans, but quite certainly without us.  They had arrived, and they had stayed, without regards to human needs or wants.

And so, I began to wonder – how long has this kind of feral landscaping been going on?   How long has it been a habit for landscapers to just kind of let a tree grow, to not cut it down, to allow it to live until it became a part, indistinguishable unless you look closely, from the sculpted environment around it?  And where else had this kind of element been a part of the landscape “design”?  At a more cultivated cemetery, where most of the plants had been planted, would we also find feral plots?

At the Woodlands Cemetery, there’s a mulberry that can answer these questions, at least a little bit.  The Woodlands, formerly the estate of William Hamilton, a wealthy Philadelphia gentleman and ardent plant collector of the late 18th and early 19th century, became a cemetery in 1840 – and it remains a cemetery to this day.  Rich with beautiful trees – red cedars, American hollies, and so much more, including the rarely seen Caucasian zelkova, the Woodlands provides a lasting record of how a cemetery was landscaped, at least for the tree part, over the last two centuries.

And there is a mulberry tree there – back behind the house, towards the river, you can go there and see it towering up over the path towards the back fence there.  It’s a very large tree, and from a cursory look it would appear to be part of the landscape just like the other trees there, just like the dogwoods and the pines and the English elms nearby.  But, because it is a mulberry, it most likely was not planted there by a person, and it represents a feral part of the landscape, a tiny plot of land that went from tamed to untamed and now sits among blades of grass that are regularly cut, and among trees that arrived there via nurseries and seedhouse – among plants that are cultivated sits a wild tree.

And so by looking at this mulberry, we can see that even the Woodlands, where most of the trees are planted, had some feral landscaping.  But when did this happen?  When did this mulberry start its life?  And more broadly, more generally, how long have wild trees been allowed to participate in cultivated landscapes without being uprooted or cut down?  And more specifically, how long has this wild tree, this mulberry, been occupying this feral patch of graveyard?

We can answer that question by coring the tree – that is, by extracting a thin length of wood from it, going perpendicular to the tree’s growth rings, and then sanding that thin length of wood down and counting the lines, lines that were part of that trees growth rings as it expanded outward as it has grown.  (we can also partially answer a part of those questions by looking for old photographs, such as this one, that shows that the area alongside the railroad tracks was unmown in 1955)

By doing that, we find that the mulberry is well over 80 years old.  We were able to count that many rings, and so get a good lower bound on the trees age.  However, that’s just the rings we could count (some of them were indistinct), and since we only got 14 7/8″ through (i.e., not all the way), it’s most likely quite a bit older – the entire tree is about 53.2″ dbh (diameter at breast height), and so we can roughly approximate it to be over a hundred years old.  This work, by the way, was done in collaboration with and due to the hardworking efforts of Ned Barnard, and Steve Minicola, and Joe Shapiro, and Jessica Baumert, and others who joined us on this day.

I’d like to get another core out of there before saying for sure how old this mulberry is (and just to get  a more precise date on it), but from this tree we can see that “feral landscaping” (that is, where plants that seeded in on their own, as I’m quite sure this mulberry did, are kept as part of a cultivated landscape) has been going on for at least a hundred years, at the Woodlands.  Even in a cemetery, life persists.

In human dominated landscapes, buildings are built and plants are planted, and so it’s easy to think that if a plant is in a park or a cemetery, or anywhere with wide open lawns and open spaces that are cut and cared for, and if that plant were to have grown to the size of a tree, well then it must have been planted there,  While plants quite literally take on a life of their own, and there may be weeds among the cracks and in the lawns, it just seems like common sense to assume that, in a cultivated landscape, if a plant is there and it is not removed for such a long time that it grows to become a tree, then it must have been put there by a person, placed there by someone.

However, as we look more closely and more broadly, we see that this is not always the case, that there are plants that have just kind of come in to these environments, and have been incorporated into cultivated landscapes and become pretty well indistinguishable from the trees that were planted there, and that it can be difficult to tell them apart unless you look closely, and that the wild, the cultivated, and the feral are not so different from one another.

To read about the importance of cemeteries to urban planning, see here:


American elms

There are elms fruiting now.  If, as you walk around the city, you look down at the pavement, or the grass, or the asphalt, you’ll see at some places papery little disks, generally less than an inch across, often quite a bit smaller – some are ciliate (they have fine hairy fringes, that is), while others have entire margins (that is, their edges are unfringed), but pretty much all of them, if the tree they come from has any hope of passing on its genes to progeny, have a seed in there.  Some have their seeds at dead center, while for others they’re positioned a bit towards the apex of the fruit, but all of them, unless their parent tree was unfortunately barren, have a seed.

These fruits are called samaras.  Samaras are fruits with wings – if you look at a maple tree, at some of them about now, the red maples for example, you’ll see another kind of samara, different from those of the elm, with these ones, those of the the maples, making the helicopters or Pinocchio noses you probably played with as a kid.  The maple samaras aren’t in the rounded and somewhat symmetrical form of the elm samara, their wing, the wing of the maple fruit, is extended in one direction, on one side of the seed, and then two are joined together to make the kind of samara that spins as it lofts towards the ground or as it is carried away to be dispersed by the wind, to new places where the seed might grow into a tree (or not).  For the maples, their wings extending outwards make for simple little machines that cause them to spin as they fly away.

For the elms, they are different.  Their samaras don’t make helicopters, their samaras are in a circular or somewhat elliptical shape, some having a notch towards the top, but all having generally a saucer shape, though sometimes uncircled and stretched in one direction to make, I guess, perhaps more of a tureen saucer than one for coffee cups, but saucer shaped nonetheless – and those samaras are all over the place now, falling to the ground beneath the elms’ spreading branches.

The ciliate samaras, the ones that are densely fringed – these rounded shaggy-margined, papery, less-than-an-inch roughly-ovals, the ones you see littering the street that look like that, these belong to the American elm.

The American elm used to be the street tree, the street tree.   Planted in long rows, in lines up and down main streets and smaller streets and all kinds of sized streets, in towns and cities across America, these plantings, they made for a beautiful effect that was somewhat architectural – the branches of the elms from opposite sidewalks meeting in the middle, forming a broad arching ceiling over the street, a covering under which horses, carriages and then ultimately automobiles traveled, and along which people walked in the shade in the summertime and under naked branches in the winter.  This was the tree of America’s streets.

The American elm was also treasured for open plantings.  With its broadly spreading branches zigzagging around and angling outward like the sides of an opening vase, this elm made for a striking shape, standing tall in a yard or park, or in the National Mall in Washington, DC, or in Harvard Yard in Cambridge, MA, or in Rittenhouse Square in Philadelphia, PA, and in places of prominence elsewhere as well.

Put simply, and to quote Andrew Jackson Downing’s Treatise on the Theory and Practice of Landscape Gardening (1841), “Let us now claim for the elm the epithets graceful and elegant.”

The elm was everywhere.  And it had always been common here.  Peter Kalm, the Scandinavian botanist who explored the new world in the late 1740s, writes of this tree as being found throughout the area in and around Philadelphia and New Jersey:

“June the 28th [1749].  The American Elm, (Ulmus Americana Linn.) grows in abundance, in the forests hereabouts.”

And back then as it is now, they flowered about this time of the year, late March:

“March the 21st [1749].  The red maple (Acer rubrum) and the American elm (Ulmus Americana) began to flower at present; and some of the latter were already in full blossom.”

[the above quotes are from John Reinhold Forster’s translation of Kalm’s “Travels into North America”, published in 1771]

And, in the city of Philadelphia, in what is now Independence Square, but was at the time called the State House Square, in the 1780s Samuel Vaughan, the man responsible for designing that square’s landscape, just after the American Revolution, was given a hundred American elms by Captain George Morgan of Princeton.  Vaughan planted them in a double allee along the main north-south path, and the founding fathers of the United States would have walked among them as they worked and lived, in their day to day lives and quotidian workdays that created much of what we live within today.

And so, from the early days of European colonization through to the constructions of the cities and towns of 20th century America, the elm was everywhere.

Then, in 1929, the Dutch elm disease arrived.  First found in Ohio, within a decade it had spread to Indiana, to Maryland, to Virginia – as far as eastern Connecticut and New York City, it had spread hundreds of miles within just ten years, according to Joseph Horace Faull, professor of forest pathology at Harvard (he spent most of his time among the trees at the Arnold Arboretum in Boston), who wrote about and studied this disease, in the 1930s.

Throughout the middle and the latter part of the 20th century, Dutch elm disease spread and spread, wiping out graceful elm plantings in yards and parks and open greens, and caving in the arched vaults formed by elms’ branches across the streets of America. The distribution of the disease rapidly became coterminous with that of susceptible elms – slippery elms (Ulmus rubra), another elm native to North America, can also catch the disease, as can the English elms (Ulmus procera) that had come over to the US from Europe.

A fungus is the causative agent of the Dutch elm disease, well two fungi, actually – Ophiostoma ulmi and Ophiostoma novo-ulmiOphiostoma ulmi, arriving in the US in the early half of the 20th century, caused the first ranging of the disease, and then the more aggressive Ophiostoma novo-ulmi came on to replace that original colonizer in later years.

The fungus spreads by two routes – either via bark beetles, who carry the fungus with them as they go from tree to tree, diving into the bark to live and breed, or via root grafts.  What are root grafts?  They’re pretty much what they sound like – when elm trees grow near to each other, as you might expect, their respective roots can come into contact.  When this happens, the roots can fuse, combining together, even down to the plant’s vascular system.  This allows the fungus to spread because it can travel through this vascular system, the plant’s water transport system, the xylem.  Though the Dutch elm disease commonly spends much of its life growing as filamentous strands (=hyphae), it can also transition to a single celled form, which we call a yeast (to a mycologist, a single celled fungus is a yeast), allowing it to cruise along the xylem like a vascular highway, up and down the tree, unimpeded by bark or air, through shoots or through roots.

And so, you can imagine what this meant for those long majestic rows of elms that were planted cheek by jowl next to each other along the streets of America.  If one instance of a bark beetle arriving brought the fungus with it, the yeast of the Dutch elm disease then rapidly went from tree to tree, by the roots subterreanean vascular system, until the allee fell like a series of arboreal dominoes.

And why exactly did these trees fall?  What is it about this disease that kills the trees?  Well, imagine if you had fungi growing in your vascular system – it would become blocked up, thereby preventing the flow of nutrients to where they need to be.  The same things happen to trees infected with this pathogen – the vascular system is blocked, causing the trees to present the diagnostic early symptom of the disease, called “flagging”, where entire branches in the tree canopy exhibit drooping leaves that yellow and brown, which is then generally followed, ultimately, by the death of the tree.

There are interventions available – removal of diseased parts, insecticides to get rid of the bark beetle, fungicides to attack the fungus, cutting of the roots to inhibit spread underground – but these are intensive and require resources, both financial and in terms of labor spent on these efforts, and so replacement of the elm is generally the more viable option.

And so, through the latter part of the 20th century, American elms have become a reduced part of our urban tree community.  But they’re still there.

There are quite a few American elms in Philadelphia – there’s a large healthy one in Dickinson Square, at Tasker and Moyamensing, and there’s one in the southeast corner of Rittenhouse Square in center city, and another at the northeast corner of 22nd and Chestnut, in front of what used to be a Swedenborgian Church, and right down the street from the College of Physicians.  And if you look in the Kaskey Memorial Garden on Penn campus in west Philadelphia, or on the grounds of Friends Hospital up in Frankford, or in front of what was Lankenau Hospital, on Corinthian Avenue just south of Girard College in north Philadelphia, if you look in these places you’ll find more of the American elms that grow here in Philadelphia.  And we have English elms, too – seven enormous ones at the Woodlands Cemetery right near Penn (two of which date to William Hamilton’s time; in 1921 those two were, respectively, 10′ 1″ and 10′ 3″ around), and at Marconi Square in south Philadelphia, right along Broad Street, on the west side of Broad, there’s one with a trunk over five feet across. (and for an interesting historical record: in John Watson’s Annals of Philadelphia and Pennsylvania in the Olden Time (1899, “enlarged, with many revisions and additions, by Willis P. Hazard”), p. 373, vol. 3, he notes after a brief mention of the balloon riot that occurred in 1819 at “Vauxhall Theater, north-east corner of Walnut and Broad streets”, that “The elm that stands on Walnut street, overhanging the street, was on old tree then.” [Vauxhall was also called Vaux Hall Gardens, and had a very open parklike aspect to it, in 1819; in the 1890s, there was still a fair bit of open space in that lot, as one can see from the 1895 map here, but by 1910 it was all pretty well covered over and the elm would have been gone])

And at Independence Hall, right behind it there, along with the enormous London planes, and nearby to some little chestnuts and not far from a Franklinia, and with dozens, hundreds, or thousands of people walking under it every day, is an enormous American elm, rising high above the square.  There are other American elms there too, five in all – in that smallish square, there are multiple elms.  One, planted in 2004, is a ‘Princeton’ cultivar, a variety that, while known and planted prior to the Dutch elm disease’s incursion into America, ended up being able to co-exist with both species of Ophiostoma and therefore ended up being planted more widely, quite widely actually.  This cultivar grows well, and the example of it that is in back of Independence Hall is strong and healthy, growing  alongside its older relative that towers above the square.  These stand as a reminder of a time when American elms were the American tree, and they are the right plant to have at this historic site – trees deep in history, that are still growing tall, even though organisms that were new to them have inalterably changed their presence in the landscape, the elms stand, and grow.

To read more about elms, including Penn Treaty elms, see here;