Halifax Tree Project

The Pin oak

Peter Duinker, Halifax Tree Project

2020-07-13

As I pointed out in the article on red oak some weeks ago, the pin oak (Quercus palustris Muench.) has pointy-lobed leaves like the red oak but the leaf blades are much narrower. Let me quote the first paragraph of Farrar’s “Trees in Canada” (p. 252):

“Occurs in southern Ontario at the east and west ends of Lake Erie; locally abundant. Frequently planted as an ornamental because of its symmetrical shape and attractive leaves . . .; tolerant of urban conditions; easily transplanted. Small stiff dead branchlets often project like pins from the trunk and larger branches; hence the name ‘pin’ oak”.

I find it hard to believe that it is frequently planted because of the symmetry and attractive leaves. Most trees are more or less symmetrical when grown in the open, and most trees have attractive leaves. My sense is that the characteristics of tolerance of urban conditions and ease of transplantation would be the key driving forces behind the choice of pin oak as a street tree.

A fact sheet from the University of Kentucky points out some very interesting things about pin oak. The species has numerous small horizontal branches from the main stem, contrary to red oak which can bear huge horizontal branches. The lowest such branches droop downwards, which means that if this species is used in the streets, early pruning to remove the drooping lowest branches will be necessary. Pin oak’s root system is shallow and fibrous, which makes it easy to grow in nurseries and to transplant. One might think that such shallow rooting would lead to windthrow issues with pin oak, but in my experience, pin oaks seem not to succumb to Halifax windstorms like some other species do (e.g., Norway maple).

Pin oaks have rapid height growth when the growing conditions streetside are good. I’ve been following one on Allan St. for the past decade or more. It seemed to be growing so fast and so well. Its leader shot past the power lines above it, and NS Power felt obliged to trim it back from the lines. That action turned a beautiful specimen into an ugly one now delivering still on the biophysical ecosystem services (like carbon sequestration and rainwater attenuation) but not the aesthetic ones. The tree seems to be healthy, but the powerline issue will plague its future from now on.

This quandary of power lines deserves a paragraph or two. In Halifax (and many other North American cities), many of our residential streets have electricity distribution lines on poles on one side of the street, and tree lawns (the grass between sidewalk and curb) on both sides. Trees and power lines are frequently in conflict when the trees grow under power lines. Sometimes I prefer to phrase this the other way – when power lines are installed above the tree lawn. We all know what the conflict means – sometimes it means that the trees have to be eviscerated to keep them back from the lines, and sometimes it means that power lines come down in windstorms because the trees fall on them.

In theory, there are several possible solutions. Some say bury the power lines. This is done in cities around the world. Where the bedrock is deep below the surface, this is relatively cheap and easy. Where the bedrock is shallow, as in much of Halifax, this would be costly. Another strategy is to install supported, shielded cable, or rather a covered conductor system as invented by Bill Hendrix in the USA. Such cabling is much more robust to tree interactions than the standard (read: old!) cabling hung from many of the poles in Halifax. Finally, there is the argument that the only trees that should be grown under power lines are short ones.

If push came to shove, most people would prefer to have uninterrupted electricity during and after a windstorm rather than live with power outages so that they can have abundant tall trees on their streets. Let’s assume that there will be essentially no changes to the electricity distribution system in Halifax. This means that the conflict between trees and power lines will continue. How should we manage, then? We need to choose tree species that can be successfully trained around power lines, prune them with such training in mind, and plant other tree species that have strong single stems on the sides of the streets without power lines.

Pin oak is one such species that, in my view, does not do well under power lines. My favourite example is on Preston St. near the Halifax Central Junior High School. If you go by, notice how significantly the tops of the trunks have been cut off to keep the trees clear of the power lines (see the photo, right). In contrast, notice how wonderfully the pin oaks along the south side of Boland Rd. are growing – there is no power line, but even with a narrow tree lawn, so far this line of trees is doing well (see photo, below).

A final note about pin oak. Besides its leaves being an easy giveaway in identifying the species, look up into the canopy and notice the incredible abundance of twigs in the crown. Arborists hate to climb pin oaks when they need pruning because it is usually impossible to penetrate the canopy without becoming hopelessly entangled in the twig structure. I photographed the pin oak on the Brookfield corner of Almon St. and Robie St. without leaves – the density of twigs is astonishing!