A Gardening Journal
Wire Vine in Winter
- Published: January 24 2017
In milder climates, it's easier to tell if a plant is dead. With only a modest challenge to remain green through a season that is merely chilly, not arctic, if the plant is brown, it's usually a goner. For plants in climates with more intense winters, this basic Green : Alive / Brown : Dead thinking must sometimes be tossed. Plants stay alive on their own terms, leaving the conceptual catch-up to us.
This is a large colony of wire vine—most often seen as a container annual, or a rampant but beautiful groundcover in Zones 8 and 9—that has been thriving for many years here in southern New England. The foliage of the two upper reaches is green.
That of the entire lower portion is brown.
What's going on? The two green portions aren't independent plants growing from higher-than-the-rest pockets of soil. The green growth is resting against solid ledge, and is formed of the highest stems of plants rooted into the soil pocket that runs along the entire base of the colony. Since these upper portions are green, the lower portions must still be alive even if their foliage doesn't look it. Otherwise the upper portion would have turned brown, too—but a brown that really does mean dead. The brown leaves of the lower foliage don't mean that those lower stems themselves are dead: they couldn't be and still be able to sustain the green growth several feet higher up.
For this colony of Muehlenbeckia axillaris in this site, in this Zone 7a climate, then, the reality is Green : High / Brown : Low / Either Color, High or Low : Just Fine. What gives?
Stems cascading down the solid ledge below the soil pocket also display brown foliage, so increasing distance from the plant's roots probably isn't the sole criterion causing foliage to change from green to brown.
Perhaps looking at this puzzlingly bicolored colony while asking yourself questions such as "Is something wrong?" and "Will this plant survive?" is to peer through the wrong end of the telescope. Over many years, this colony has experienced winters both mild and fierce; if it were to succumb, it would have done so long ago. Instead, it has maintained growth covering several square yards—and in a spot that is pitilessly exposed to wind, and too high above ground to be sheltered by any snowfall of less than historic depth. Weirder still, the portions that are maximally exposed—those upper tips—are the green ones.
It's more likely, then, that this colony will pull through. Indeed, the colony might not be suffering at all, let alone displaying brown foliage as a sign of distress. For the moment, think of these leaves not as brown-and-dead but, rather, brown-and-still-in-place: brown-and-retained. To what purpose?
Similar behavior of brown-and-retained leaves of beeches, oaks, witch hazels, and hornbeams offers clues. They can also hold onto fall foliage right through to spring—but, also, only on their lower stems. In these cases, foliage that is higher up doesn't stay green, mind you: it's shed entirely. To me, the connection with wire vine is the retained brown foliage, a quality known (who knows why) as marcescence.
It's thought that marcescence is a helpful attribute for shrubs and trees, in that the dry brown leaves are unpalatable to ground-based herbavores—think deer, bears, sheep, goats, and cows—and, therefore, both hide and protect the juicy stem tips and buds low enough to be within their reach. With those persistent brown leaves in place, browsers would need either to chew through them to reach the buds or strip the marcescent leaves off first. Apparently, browsers aren't able to strip the leaves, so they bypass branches with marcescent foliage entirely.
Could this "lower portion" marcescence be similarly advantageous to Muehlenbeckia? True, there's quite a difference in scale: limbs of trees can still display marcescent foliage even though they are tens of feet above ground. (Bears can climb, remember.) Even so, marcescence is typically displayed only by growth that is relatively close to the ground—and close to the trunk—in comparison to full-size mature growth. This range is known as the cone of marcescence. All the growth of young trees can be within their marcescent cones. Here in New England, the woods are full of American beeches; in winter, when the rest of the forest is almost leafless, American beech saplings stand out because they are totally clothed in their marcescent leaves.
When growng as a groundcover, a colony of wire vine may be just inches high—a foot or so at the most. The entire colony would be in the zone of marcescence, which, then, would be the shape of a mattress, not a cone. Only stems that had found support that allowed them to grow up above the "mattress" would be outside the zone.
Hypothesis: There are browsers so lilliputian that only such ground-hugging foliage is within their reach—and that also need to remain hidden within the mattress of growth while browsing. If they climbed into the upper green portions they would easy pickings for avian predators: the upper growth is too thin to offer protection. There are plenty of such potential critters, including mice, rats, voles, chipmunks, and moles.
Browser-accessible foliage that has turned brown to escape browsing would be dropped in spring as new foliage emerges. Foliage on stems that are too high—meaning too exposed to predators that could "harvest" visible browsers—could stay green all winter. This would be an advantage in fueling quicker resumption of growth in spring.
True, this hypothesis also assumes that the stems of Muehlenbeckia aren't that palatable. They remain easily accessible no matter what color their foliage is. Even so, the hypothesis is plausible. I must identify additional colonies of wirevine to determine if the bicolor performance of this one is consistent. (My own colony has petered out; this spring, I'll see about re-establishing it.) Being a landscape designer, I'm also creating other gardens, too. Perhaps some of them can also host wire vine, so that my exploration of this species' winter performance can be even fuller.
Here's how to grow Muehlenbeckia axillaris.
Here's another look at marcescence, this time as displayed by roadside oak trees that get pruned to keep their growth below power lines.
Here's a look at the stunning marcescence displayed by young American beeches growing in the dappled shade of native woodlands.