Habits of a pin oak

Pin oak lined road in Glen Iris, July 2024

Today's post features the pin oak (Quercus palustris), a popular street and park tree throughout the world, including the eastern suburb of Melbourne, Glen Iris. Later in the week, I'll tackle a variant of this species - perhaps - from the country retreat of Mount Macedon. 

(For my international, and perhaps interstate, readers, Glen Iris and Mount Macedon are roughly the same distance from Melbourne as Richmond and Oxford are, respectively, from central London...) 

Despite one of pin oak's alternative common names being ‘swamp Spanish oak’, this species comes from what I would call southern mid-western USA, or perhaps better, mid-eastern. While it does favour swamps in its native habitat, it tolerates drier soils and has proven to be robust in urban settings. Fast growing and tolerant of air pollution, pin oak will often survive in streets and parks well beyond its natural life expectancy of 120 years.

It is one of the more easily recognised of oaks, with deeply incised leaves, leaving pointy lobes with a bristle at their tip – marking it as a member of the Red Oak section of the genus. There are others with superficially similar shaped leaves, but pin oak has broad, almost U-shaped incisions between the lobes (compared to the V–shape in related species, as I illustrated in a post last year). 

And, if you are willing to look more closely, a good character to distinguish this species from other oaks with similarly shaped leaves – mostly in the Red Oak group – is on the undersurface of the leaves. There you’ll find a tuft of fawn hairs, a bit of fluff, where the main veins join the midvein. I mused on the reliability of this 'hairy armpit' character in that earlier post.

The acorns are also readily recognisable and quite different to those of say English (Quercus robur) or Algerian (Quercus canariensis) oak, in being rather diminished, either barely rising above their scaly cup or forming what looks like a ’half acorn’.

Pin oak leaves and acorns, Glen Iris; March 2020

It has a distinctive habit too. The trunk at ground level is flared more than other oaks, possibly due to its roots being closer to the soil surface – better for living in wetlands but apparently not inhibiting its drought tolerance once it gets past the first few years (although I hear of a few failing in the streets of Glen Iris during their first summer or two if not watered adequately). 

Pin oak, Oak Lawn, Royal Botanic Gardens Melbourne; April 2023

The shape of the canopy is unusual for an oak. British garden writer, Alan Mitchell, in Alan Mitchell’s Trees of Britain, suggests the common name is a reference to ‘the rigid, slender and straight minor shoots which grow at right angles from the small branches which spray out in a downward, then outward curved, flared skirt from some eight foot [just over 2 m] up the bole’.

I had no idea what this meant so I headed out into the streets and parks of Glen Iris to find out. When the trees were in full leaf, Mitchell's comments made little sense. But in winter, walking past a young leafless tree, I got it. I think. 

Pin oak branch with 'up flick', Glen Iris; July 2024

The end of each extended branch – and particularly noticeable in the lower branches – has a jaunty little up flick at the tip. It’s as if each year their weight pulls them a little closer to the horizontal (or the ground for that matter) and they compensate but bending upward. That must be what he means, although I'm still not sure what it has to do with pins.

Mitchell has a few other observations on its habit. The lower branches, he says, when not pruned, form a distinctive skirt. That is certainly true and again more obvious in winter when the architecture of the tree becomes visible. 

Young pin oak with 'skirt' and 'long tip', Glen Iris; July 2024

He also describes a zone of more or less horizontal branches, and then small branches curving upwards to create an ‘upswept spire’ and ‘single long tip’. 

The 'long tip' of a pin oak (on the left), Glen Iris; July 2024 

If I squint a little (i.e. use my imagination), I can see this a little in a few local street plantings. More perhaps in some unpruned specimens in the local park. I can also – after spending an afternoon tracking down as many other species of oaks as I could find within walking distance – confirm that other species do not have this form. 

And indeed, neither do the many pin oaks with canopies crafted to wrap around the street's electrical infrastructure...

Pin oak, Glen Iris; July 2024

I'll finish with an odd form that I discovered only recently while walking around the Springvale Botanical Cemetry. I'm assuming it is Quercus palustris 'Pringreen', a fastigiate (columnar) cultivar of pin oak. You'd have to describe the habit of this tree as more needle than pin.

Pin oak, Springvale Botanical Cemetry, July 2024