A Gardening Journal
Must Have: White Oak, as a Hedge
- Published: January 15 2014
Countless trees right along the roads retain their Fall foliage well into Winter—even though the taller trees immediately behind them do not. What's going on?
Part of the answer is the species involved—here in southern New England, mostly two species of oaks. Notice the leaves below, one with rounded lobes, and the other with pointy lobes.
In the picture below, you can see a close-up of the round lobes. They are characteristic of the leaves of white oak, Quercus alba, a predominant native species in eastern North America.
The pointy lobes are characteristic of the leaves of another oak species native to eastern North America, the red oak, Quercus rubra.
Goodness knows the "red" and "white" of each species isn't from their Winter foliage. The bark of white oaks is lighter gray than that of most other oak species, but I've never seen any individuals whose bark was truly white. Red oaks are reddish brown (at least by comparison with white oaks) in their bark, lumber, and "young" Fall foliage of October and November, which hasn't yet transitioned to its tawny tan Winter color.
Why do roadside trees of each species retain their Fall foliage into Winter? This characteristic is known as marcescence, and is common among oaks, beeches, witch hazels, parrotias, and hornbeams. I've referred to it often in connection with beeches, hornbeams, witch hazels, and parrotias that I've established in my gardens. Although entire trees in these genera can sometimes marcesce, the habit is more common among trees that are young, and on branches of mature trees that are lowest. It turns out that the key is being closer to the ground and to the trunk, at any age, not simply being short overall, or just low to the ground, or merely young. All the growth of younger trees is close enough to the trunk as well as low enough to the ground and, so, marcescence is likely to be maximized. Leaves at the ends of long and low branches of older trees are usually not likely to marcesce regardless of how close to the ground they are. Instead, it's the foliage of shorter and lower branches that arise directly from the trunk that are mostly likely to marcesce. In short, there's a zone of marcescence—a cone or a cylinder—and as long as leaves arise from branches within it, they're likely to marcesce.
To the degree that pruning removes any portion of branches that are outside this zone, most of the foliage that remains is likely to be retained. Beeches and hornbeams that are grown as hedges have all of their "outside the zone" growth removed regularly; no wonder the retention of leaves on the remaining branches is so noticeable. It also helps that any plant grown as a hedge is inherently under closer observation than one growing free-range: Its architectural geometry is striking, its placement is intentional, and the attention it receives (via the pruning itself, as well as by being such a functional component of the landscape) is both regular and intense. We notice the marcescence, in part, because we're noticing the hedge itself.
But these roadside oaks aren't being pruned with the intent that they will grow into hedges. They've been brush-hogged only to keep them from growing tall enough to interfere with overhead lines, or wide enough to interfere with road traffic. What gives such oaks potential for gardens is that, as their roadside performance shows, they might be used in a landscape to provide nearly year-round privacy. Plus, if they were grown near plants whose retained foliage was evergreen, the contrast of their tan foliage with those plants' green foliage would be handsome, indeed.
These roadside oaks also haven't been pruned with any particular thought to their long-term health and, yet, they seem to be thriving. Oaks aren't normally thought of as trees that accept regular pruning and, yet, these roadside oaks by the countless thousands seem to accept it well. I'm drawn to the idea of growing white oaks as pruned specimens—as a hedge or not—because these native trees are otherwise difficult to incorporate into a garden. Free-range specimens can be as gigantic as any beech and, with as remarkably cantilevered limbs, as impressive. Few gardens have room for either. Further, white oak is difficult to establish at any but the smallest size, and mature specimens are prone to decline if the ground beneath them receives much foot traffic or disturbance of the native undergrowth. Quercus alba is usually a slow grower, too, yet another reason why it would be a fool's goal to try to establish one with the hope of enjoying it as an enormous specimen within your lifetime.
Establishing white oaks by growing them from direct-sown acorns is usually the best way to establish them. Pruning them into dense shapes (a hedge, say, or a column or cone) creates full growth to the ground, which preserves that ground from any disturbance. Even so, it might be best to grow a pruned white oak within a larger swath of native groundcover, which would preclude any foot traffic in the surrounding area but the gentle and only occasional steps of the gardener.
I'll profile the white oak in late Summer, when its green foliage and acorns are prominent.