Wisteria is a popular flowering plant known for its dramatic, cascading blossoms that transform gardens in the spring. The fragrant flowers often appear in shades of purple, pink, or white. This aesthetic appeal has led to its widespread cultivation. Because wisteria develops a woody structure when mature, many people question its true botanical classification.
Wisteria’s True Classification: Vine vs. Tree
Wisteria is botanically classified as a woody vine, or liana, not a true tree. This means it lacks the natural, self-supporting trunk structure that defines a tree. The plant’s vigorous growth habit involves twining its stems tightly around any available support structure, such as a pergola, trellis, or even a neighboring tree. The strength of its twining, woody stems can become immense over time. In the wild, wisteria uses this climbing behavior to scramble high into the tree canopy to reach sunlight. This aggressive, twining nature necessitates training and support to achieve the more formal, upright shape often seen in cultivated landscapes.
Training Wisteria into a Tree Form
Achieving the “tree-form” or “standard” wisteria requires specific, long-term horticultural training. The process begins by selecting the most vigorous young vine to serve as the single central leader or trunk. This leader must be secured to a sturdy support, such as a metal conduit or a large wooden stake, and tied loosely at intervals as it grows vertically.
It is important to remove all side shoots and lateral branches that emerge from the main stem below the desired height of the canopy. Once the main stem reaches the intended height for the head, typically three to six feet above the ground, the tip is cut just above a growth bud. This cut encourages the development of side shoots that will form the crown of the “tree,” creating the initial framework for the canopy.
Managing Aggressive Growth and Ensuring Blooms
Wisteria is known for its aggressive growth, meaning that successful flowering and shape maintenance require two distinct annual pruning cycles. The first, or summer pruning, occurs after the plant finishes blooming, typically in July or August. This initial cut involves reducing the new, whippy green side shoots back to a length of about six inches, leaving approximately six leaves or buds on each. This curbs the plant’s vegetative growth and channels its energy into developing flowering spurs.
The second cut, the winter pruning, is performed while the plant is dormant and leafless. During this time, the shoots that were shortened in the summer are cut back drastically to just two or three buds. This two-part pruning system directs the plant’s resources toward the compact buds that will produce the next season’s flowers. If a wisteria is established but fails to bloom, avoiding fertilization, especially with nitrogen-rich products, is recommended, as this encourages leaf growth over flower production.
Safety Considerations
Despite its beauty, wisteria contains toxic compounds. It contains poisonous substances, specifically lectins and the saponin wisterin, which can cause adverse reactions if ingested. The highest concentration of these toxins is found in the velvety, bean-like seed pods and the seeds they contain.
Ingestion by humans or pets can lead to symptoms that include an oral burning sensation, abdominal pain, nausea, and vomiting. Because the toxins are present in all parts, and the seeds pose the highest risk, gardeners with children or pets should exercise caution. In cases of suspected ingestion, immediate medical attention should be sought.