Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
It’s that time of year again when the lavender clusters of wisteria flowers are gracefully draping over fences, trellises and hedges. The vines are also clambering throughout waste areas where they have been allowed to escape from cultivation.
What is Wisteria?
Wisteria (Wisteria spp.) are flowering bines (not vines) that are members of the legume family which includes peas, beans and peanuts. The legume family is characterized by the plants’ abilities to fix nitrogen in the soil unlike other plants which deplete nitrogen from the soil.
Wisteria are bines because they climb by wrapping themselves around supports. Vines climb using tendrils that cling to supports.
There are four species of wisteria that are popular in the US:
Japanese wisteria (W. floribunda) twines around supports in a clockwise direction. It flowers in the spring before its foliage grows. The flowers are scented. Hardy in zones 4 – 9.
Chinese wisteria (W. sinensis) twines around supports in a counter-clockwise direction. It flowers in the spring before its foliage grows. The flowers are scented. Hardy in zones 5 – 9.
American wisteria (W. frutescens) blooms in the spring after its foliage has grown. The flowers are unscented. Hardy in zones 5 – 9.
Kentucky wisteria (W. macrostachya) blooms in the spring after its foliage has grown. The flowers are unscented. Hardy in zones 4 – 9.
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The two Asian wisterias are considered invasive and it is not recommended that you plant them. They are extremely aggressive growers. They are also quite large, growing to 60 feet in length. The American and Kentucky wisterias are native plants and better behaved. They are also shorter, only growing to about 30 feet.
Is Wisteria Poisonous?
All parts of wisteria contain toxic substances called lectin and wisterin. If consumed, they cause stomach pain, nausea, vomiting, diarrhea, confusion, speech problems and even death. The seeds contain the highest concentration of the toxins so it is a good idea to remove the seed pods from the bines before they mature, especially if you have children or pets that may ingest them.
How to Grow Wisteria
Before you plant your bines, you must provide a very sturdy structure for them to grow on. It’s not a good idea to grow them on your house because they are very heavy and have been known to destroy their supports. It’s better to build a strong arbor or pergola for them well away from other plants. Use weather resistant material such as cedar and provide concrete footings for the posts to keep them from being pulled over.
Wisteria prefers to grow in full sun but it will tolerate some shade. Make sure that your bine is getting a minimum of 6 hours of sunlight each day. It also needs well-drained soil. Be very careful with fertilizing. Too much nitrogen will cause the plants to grow foliage but no flowers. Always use a low nitrogen/high phosphorous fertilizer if you bine is not growing well. Phosphorous encourages flowering. Healthy bines do not require fertilizer. Just apply some compost at the base each spring.
Make sure that your young wisteria is well-watered. Do not allow it to dry out. Once established, the bine only needs about 1 inch of water each week.
How to Prune Wisteria
Wisteria is pruned twice a year. In the winter, prune back half of the growth from the previous year, leaving just a few buds per stem. In the summer, after the bines have bloomed, cut back the remaining stems from the previous year to about 6 inches in length.
If you want a more formal look, after the bines have finished flowering, you can cut back any growth that you think is excessive or that ruins the shaping that you are trying to achieve. You may find yourself pruning every few weeks to keep the bine in check.
What To Do If Your Wisteria Is Not Blooming
- Most wisterias are purchased from plant nurseries as dormant plants in the spring or the fall. These are very young plants so don’t expect them to bloom for 2 or 3 years.
- If you have an established bine that isn’t blooming, withhold all fertilizer including compost. There may be too much nitrogen in the soil.
- Try root pruning. Take a sharp spade and use it to cut the roots half way around and about 1 – 2 feet from the main bine. You will need to go down 8 – 10 inches in the soil to access the roots.
- Weather can also affect blooming. If the previous winter was exceptionally cold, your bine may not bloom.
© 2020 Caren White