A Gardening Journal
Today in the Garden of a Lifetime: Pinoccheo Hornbeams, Some Pruned, Some Not
- Published: November 07 2012
I planted a columnar hornbeam at each corner of this (woefully messy!) bed, and last Winter I got around to pruning only the front pair. A year later, that pair provides an unexpectedly different look as Fall advances. All four trees received the same exposure to sun and weather and water. And yet the front pair have retained their Fall leaves—which are dead, mind you, so why would they still be hanging around, anyway?—while the back pair has shed them.
Hornbeams and beeches both retain Fall foliage on new growth that forms in response to pruning. (They also retain Fall foliage on their lowest branches even when trees are growing free-range.) This talent is called marcescence, from the Latin "marcere", to whither. Its possible benefits are many, but I'm most persuaded by the deterrence old leaves give to Winter browsers.
Growing free-range, hornbeams and beeches marcesce only on branches that are within reach, up to about eight feet above ground. Young trees might not be that high overall, and, sure enough, they retain their foliage top to bottom. Trees that are pruned into hedges are being kept comparatively low, too, and they also retain their Fall foliage. (This is one reason both hornbeams and beeches make such great hedges: Their Fall foliage ensures privacy all Winter long.)
But the performance of my quartet of columnar hornbeams, Carpinus betulus 'Pinoccheo', is more puzzling still. The back pair has no foliage, top to bottom, which could just be because they have few lower branches that could marcesce. But the front pair have marcesced right to the top, which (trust me) is much taller than eight feet. Fifteen or sixteen, definitely.
What might explain all of these differences in retention of Fall foliage? The mature foliage of hornbeams as well as beeches is unpalatable when green; trees are never browsed in Summer. Marcescence ensures that tasty leafbuds and young twigs continue to be surrounded by plenty of foliage even in Winter. That the foliage is dead might actually be an advantage. Deaf foliage is dry and dusty, providing even better deterrence when forage is scarcer and the pressure to browse almost anything is therefore greater.
Beeches and hornbeams are cold-climate trees, where Winter browsers are large and ground-based, not smaller climbers. Deer and moose, in other words; bears will be hibernating.
In warmer climates, there are many more tree-climbing browsers, such as monkeys and sloths, and they stay active through the Winter. Marcescence in a warmer-climate species wouldn't do much good if only ground-dwelling browsers were deterred. The tree-dwelling browsers could still feed on the higher twigs and buds. And, indeed, trees that are marcescent in warmer climates—such as many species of oaks, which can often thrive in climates warm enough to be termed subtropical—are more likely to be marcescent right to the top.
But how does marcescence relate to pruning? The two of my Pinoccheo hornbeams that retained their foliage right to the top did so in response to pruning, not because they were suddenly vulnerable to browsing by deer who had learned how to drive a bucket truck.
The growth produced in response to pruning is typically much more eager and vigorous—juicy and nutritious and vulnerable to browers, in other words—than the slow-but-steady growth that occurs without pruning. A branch that's broken for any reason, even by accident, has been pruned. It isn't just gardeners with hand-tools that prune branches. Startled deer charge through the woods, tromping and snapping twigs as they run. Bears hoist themselves up into trees, breaking too-thin branches as they find their footing. It's fortunate that the unpalatable leaves born by the unusually enthusiastic growth that results from such inadvertent pruning also protects the underlying twigs and leafbuds from browsers.
In pruning my Pinoccheos far higher than any cold-climate browser could ever reach, I'm taking advantage of the tree's browser-prevention tactic to create an aesthetic goal: Trees that retain Fall foliage right to the top. I'll prune the back pair of Pinoccheos this Winter. I'll also prune the front pair again, to encourage even tighter growth in Summer. And in Spring, I promise, I'll also weed and recultivate the bed these trees are growing in.
Here's how to grow columnar hornbeams.