The cascading flowers of wisteria create an intensely romantic vision in spring. Its rapid growth and woody structure often cause confusion, leading many to wonder if they are cultivating a true tree or a powerful climbing plant. Wisteria is a popular choice for covering pergolas and arbors, but understanding its biology is key to managing its vigorous nature.
The Definitive Classification
Wisteria is classified as a woody perennial vine, also known as a liana. This classification is based on its natural growth habit, which involves twining stems that lack the self-supporting trunk structure of a true tree. The genus Wisteria, belonging to the pea family Fabaceae, includes several species native to East Asia and North America. Its stems are designed to wrap tightly around a support structure, allowing the plant to rapidly ascend toward sunlight. Without external assistance, the plant would naturally sprawl across the ground or climb the nearest vertical object.
Training Wisteria for Structure
Despite its nature as a vine, wisteria can be skillfully trained to appear as a tree, a form often referred to as a “standard.” This transformation requires significant human intervention and continuous care, forcing the vine to develop a single, upright trunk.
The process begins by selecting the most vigorous stem as the leader and securing it to a strong support post, such as a metal pipe or heavy wooden stake. Continuous pruning removes all lateral shoots and side growth along the leader, redirecting the plant’s energy into thickening the central stem.
The leader is tied to the support at regular intervals, typically every 8 to 10 inches, to ensure straight, vertical growth. Once the main stem reaches the desired height for the canopy, the tip is pruned, which encourages the development of lateral branches at the top. This rigorous shaping over several years allows the woody vine to mature into a freestanding structure that mimics a small tree.
Managing Aggressive Growth
The vine’s inherent vigor means that managing its aggressive growth is the primary reality for any wisteria owner. Chinese and Japanese species, in particular, are known for their fast, rampant growth, capable of adding many feet of length in a single season. This rapid expansion can cause structural damage by wrapping around and constricting gutters, downspouts, and wooden porch supports.
To maintain control and promote flowering, wisteria requires heavy, two-part pruning every year:
- The first pruning occurs in mid-to-late summer, after the flowers fade, cutting new, whip-like shoots back to about six inches in length.
- A second, more severe pruning takes place in late winter while the plant is dormant, trimming those same side shoots back to only three to five buds.
Homeowners must also remove root suckers, which emerge from the base or roots, as these pull energy away from the main structure and contribute to the plant’s spread.
Toxicity and Safety Considerations
Wisteria presents safety considerations because all parts of the plant are toxic if ingested. This toxicity is due to the presence of lectin and a glycoside known as wisterin. While the entire plant contains these compounds, the seeds and the velvety, bean-like pods that appear after flowering contain the highest concentrations.
Ingestion can lead to a burning sensation in the mouth, followed by severe gastrointestinal symptoms, including nausea, vomiting, abdominal pain, and diarrhea. Children and pets, who may be tempted to chew on the attractive pods, are at the highest risk. Even half a seed can potentially require hospitalization in young children. Immediate medical attention should be sought if ingestion is suspected.