Ensuring an enduring pergola at Duke Gardens

Jun. 14, 2014 @ 01:18 PM

The Terrace Gardens are one of the most popular destinations in Duke Gardens.  Each April, people flock there to enjoy the dazzling display of bulbs, cherry trees and the Pergola’s iconic Chinese wisteria.

In May, Sarah P. Duke Gardens celebrated the 75th anniversary of its dedication in 1939. Now that the festivities have ended and graduation pictures are taken, the gardens will commence a project that focuses on restoring the pergola’s metal frame structure as well as rejuvenating the original wisteria vine that envelops it.

This important restoration was made possible largely by the Duke Campus Club, which chose the project in celebration of its own centennial.  

In her original plan for the terraces, landscape architect Ellen Shipman specified a flowering vine to adorn the pergola. She chose Chinese wisteria (Wisteria sinensis). It was planted most likely in the fall of 1938 or winter of 1939, when the majority of the terrace plantings were installed.

Chinese wisteria was first introduced to the U.S. in 1816 to support the nascent horticulture trade. With its fast growth and long trailing racemes of lilac-colored flowers, it became a very popular vine for shade and fragrance, used primarily adjacent to porches, on gazebos and rambling down fences in the Victorian landscape.

Unfortunately, its beauty belied its aggressiveness, and as with many introduced plants, its hostile tendencies were not realized for many years after its introduction. Over two centuries it has escaped to areas throughout the South, and it frequently climbs 50 to 75 feet high, engulfing trees, houses and anything else in its path. Because of this aggressive nature, this wisteria is on invasive plant lists from Virginia to Texas.

How does it spread so fast? Wisteria’s natural growth habit sends 20- to 50-foot shoots from the base of the trunk. These shoots grow just above the ground and contain buds that will sprout and grow into new vines when they reach a preferred location.

Wisteria has another mode of propagation that’s much more interesting -- dehiscence. Dehiscence is the process of drying and splitting of seed pods in preparation for dispersal. In wisteria’s case, dehiscence is a more violent activity called explosive seed dispersal!  Wisteria seed pods are 4 to 6 inches long, with a velvety coating. The bean shaped seeds are about the size of a dime. When the pods dry out in autumn, the fun begins with a sharp crack as the pods explode, flinging the seeds many feet away. Other plants perform similar ballistics, including our native touch-me-nots (Impatiens capensis) and Africa’s dynamite tree (Hura crepticans), which can hurtle its seeds up to 300 feet away.

wisteria is such an aggressive plant, we have kept its growth in check by annual summer pruning, removing the spent flowers and thus eliminating most chances of pods forming and seeds flying about.

Another amazing feature of many plants is that their growth habits can be extremely malleable and may respond positively to manipulation. Poinsettias can be tricked to bloom in July if given the proper lighting requirements. Bulbs can be forced to bloom in December if given the proper cold treatment. Lemons, limes and oranges can grow on one tree by grafting. And giant sequoias can be trained as diminutive bonsais if pruned properly. Wisteria also tolerates severe pruning and it is frequently used as espalier on garden walls in England.

We’ll use this manipulation to our advantage later this summer, when the pergola restoration project kicks off.  Just prior to the renovation, on Monday the vines will be pruned back to ground level. As the new sprouting vines emerge later this summer, we’ll train them up around the pergola columns. 

Another benefit to cutting back the wisteria will be to improve the flowering display. As plants age, their vigor diminishes -- yes, even wisteria vines can show symptoms of old age! The flowering of our wisteria has become very inconsistent over the last 10 years, and we believe it may be a result of some girdling taking place from the vines having grown through the columns, or it may be indicative of an old specimen. By cutting the wisteria back to the ground and applying a specific fertilizer, we should see new growth and an improved flower display in a few years.

I am looking forward to the 75th anniversary of the pergola and Duke Gardens, and celebrating everything that they mean to the Duke community. But I’m equally excited about the renewal process for an old vine considered a horticultural patriarch, which has created decades of splendor and captured the hearts of thousands of visitors.

Bobby Mottern is director of  horticulture at the Sarah P. Duke Gardens. This article originally appeared in the gardens’ donor magazine.