PHOTO PROVIDED American wisteria (Wisteria frutescens ‘Amethyst Falls’) growing on an arbor in Clinton County.

Wisteria are among the most beautiful vines when they flower. They can also be very frustrating when they don’t. “Why won’t my wisteria bloom?” is a question I’ve heard many times. Learning a little more about the different species of wisteria that can be grown in Central Pennsylvania and the care they require can help gardeners choose the right wisteria and avoid the disappointment of wisteria that fail to bloom.

Asian wisteria, including Chinese (Wisteria sinensis) and Japanese (Wisteria floribunda) are widely available at nurseries and garden centers and are often sought after for their long showy racemes (clusters) of flowers in shades of purple, blue, pink, and white, some of which are fragrant. One look at an Asian wisteria in bloom is often all it takes for a gardener to decide to plant one. What many don’t realize, however, is that a considerable amount of time and effort are needed to achieve and maintain that beautiful appearance.

Asian wisteria are aggressive growers that do best in full sun and require extensive pruning and training. In ideal conditions, the vines can grow to as much as 70 feet. Proper pruning done regularly is directly related to flowering and without it, flowers will be sparse or nonexistent. Asian wisteria vines often produce multiple fast-growing shoots that can reach several yards in length in a single season. To encourage flowering, much of each year’s new growth must be pruned away, leaving only a few new shoots which can then be trained to climb in the desired manner. Wisteria set buds for next season’s flowers on “old” wood (growth from previous years), so care must be taken not to remove too many of the vines that contain those buds when pruning. Guides and step-by-step instructions for pruning wisteria can be very helpful, especially for those new to the process.

An Asian wisteria needs something sturdy, such as a well-built arbor, pergola or fence on which to climb. The importance of sturdy support cannot be overstated. The woody vines become very heavy as they grow and can damage or crush inadequate support structures. As the wisteria grows, it should be pruned and trained to climb the structure. Using an existing mature tree as a support for Asian wisteria has many drawbacks. The vines can wrap around and strangle the tree, break its branches, prevent sunlight from reaching tree leaves, and cause serious harm or possibly death to the tree.

Even with proper pruning and a solid support structure, it’s important to note that the young plants of Asian wisteria must mature before they are able to produce flowers. Some may take seven years or longer to develop sufficiently to bloom. Asian wisteria are considered invasive in many parts of the United States, including Pennsylvania.

American wisteria, Wisteria frutescens, have become more widely available at nurseries and garden centers in recent years and are increasingly popular choices. Although the racemes of American wisteria are somewhat smaller than those of Asian wisteria, they are beautiful nonetheless. The blooms and leaves appear at about the same time in spring, and the green leaves provide a vivid contrast with the color of the flowers. While still requiring pruning, American wisteria are less aggressive growers, making them easier to train and maintain. Vines can reach about 30 feet in length. A sturdy support structure is needed, even though the vines do not become as heavy as those of Asian wisteria. American wisterias begin to bloom at a young age, usually two to three years old. They can be prolific, sometimes mildly fragrant bloomers and are available in shades of purple and white.

Many years ago, before I became a Master Gardener, I purchased two Japanese wisterias. My husband built a large sturdy arbor in a full-sun location and we planted one on each side. I can honestly say I was not prepared for what came next. Their growth wasn’t only vigorous, it was rampant. We tried for several years to keep up with the pruning, but it was too much for us. Not only did the vines cover the arbor, they spread from the top of it to a nearby tree and climbed it as well. The more they grew, the less they bloomed. We finally made the difficult decision to remove them. Fortunately, my husband has a backhoe. Otherwise, we might still be out there digging.

We replaced the Japanese wisteria with an American wisteria cultivar called ‘Amethyst Falls’. It was a good decision and one I have never regretted. Pruning and maintenance are a fraction of what was required for the Asian wisteria and the results have been very rewarding. Our arbor is now covered in lovely lavender-purple flowers offset by bright green leaves every spring and I’m not too exhausted to enjoy them.

For additional information or other home gardening questions, please contact the Clinton County Master Gardener Hotline at clintonmg@psu.edu or call (570) 858-0198.

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Debra Burrows, PhD, is a retired Penn State Extension educator. She can be reached at dcb3@psu.edu.