Overcoming Wisteria Woes

In last week’s column, we took a look at the magnificent and exotic woody vine known as wisteria. It comes in many varieties, but a word of caution about the species Wisteria sinensis — also known as Chinese wisteria.

There has been a move to classify this species — though not the entire group — as invasive. It’s not that it’s running rampant and taking over the Northeast, but the vine can get easily out of hand if left untended or abandoned. It can topple trees and damage buildings as it tends to cling, invade and weigh down anything that supports it, including trees. So beware when buying this plant — and check its lineage, carefully.

Wisterias do climb by nature, but can be controlled best on wires, trellises, arbors and pergolas. They will also climb up a gutter, which, in time, the vines will overpower and destroy. They can be grown on solid, vertical surfaces if proper supports are provided, such as rows of wire attached 4 to 6 inches from the wall. Use sturdy, durable materials, like galvanized wire, steel pipes, tubing, or wood. Copper and aluminum wire or tubing are preferred over other metals, since these don’t rust, and use pressure-treated or rot-resistant wood for arbors and pergolas. However, do not plant wisteria where the stems can invade and clog building gutters, sneak under shingles, or climb into window sashes.

Wisteria can also be grown as a single trunk standard, or in a tree form. To accomplish this, the plant must be staked in an upright position and, when it has reached 4 to 5 feet tall, its top cut off. Side shoots are allowed to develop on the upper part, but continually remove them from the lower stem, and prune each winter — from 6 inches to 1 foot in length — until the top is as large as desired.

Future pruning consists of cutting back summer shoots to the sixth or seventh leaf as soon as they expand, and trimming secondary shoots just beyond the first or second leaf. In winter, these secondary shoots are cut back to within an inch of their base.

Living trees are often used as supports, but this must be done carefully. Trees less than 10 inches in diameter, and even larger trees, can be quickly killed by girdling of a twining wisteria stem. Be sure to inspect them every few years and, if wisteria is girdling a tree, cut it to the soil line and allow it to grow back — and physically remove the old stem from the tree to avoid future damage.

With all of that said, if you still want to get started, check our local garden centers in a few weeks for potted wisteria in large containers. These will already be several years old, properly pruned and tied to training stakes. All you have to do is plant and install your training system. Don’t try to retrain the vines; rather, get the vines to follow your intended form.

Once the soil is prepared and the support system is in place, the vine can be planted. Place the root ball in the hole, so it is no deeper than it originally grew in the nursery or pot. If your wisteria is grafted, set it so the graft union is slightly below the soil surface, about 1 inch. Fill in the hole with the prepared soil mix and firm it around the root ball with your fingers. Water well after planting, soaking the entire area. New plants will require 1 inch of water per week, through either irrigation or rainfall.

Young plants can be lightly fertilized annually and organically with a low-nitrogen, high-phosphorus fertilizer. Since vegetative growth is being encouraged at this stage, do not expect young vines to bloom, though large potted wisteria may in just one to two years. A lack of flowering is rarely the result of a lack of nutrients, so once the plant is established and has filled the allotted space, do not fertilize unless there are known soil fertility issues. Water only if the foliage wilts, as it might during drought conditions.

To train plants on a wire trellis or an arbor, select a vigorous, upright stem to serve as the main leader and attach this to the support. Remove other side shoots. As the main leader grows, it will develop side branches that will then produce more shoots and flower buds. Continue to train the main leader upward and the new side branches as needed — maintaining about 18 inches between them — to form a framework to fit the allotted space. Pinch off the tip of the main leader when it reaches the desired height.

Allow only one strong leader to develop from the end of each main framework branch without pruning it, and stretch and attach this shoot along the support. During the summer, cutting off the ends of all new side shoots will help form new shoots as a result of those cuts — which should be pruned as soon as one or two leaves develop.

In the winter, cut unpruned leader shoots back to one half or two thirds of their original length, and trim side shoots 1 or 2 inches from their base. This method of pruning allows permanent framework branches to extend each year by half the annual growth and side shoots to become short flowering spurs.

For maintenance, once the vine has been trained into the allotted space, follow summer pruning practices as outlined above. In winter, prune leader shoots back to only four or five buds, and remove any suckers that appear at the base of the plant.

If you have neglected vines, in winter, prune away all but a framework of branches. Shorten these drastically, removing any that are crowded and poorly spaced. Then, follow pruning and training as described above for a new vine. Hopefully, the plant will bloom within two or three years.

Root pruning is sometimes done in late fall to stimulate young plants to bloom, or to restore blooming on older plants. Use a spade to cut vertically into the soil about 18 inches deep, about 4 feet from the main trunk in older plants, all around the vine. It serves to check top growth and favor flower production, and must be combined with summer pruning to be effective.

The biggest frustration gardeners face when growing wisteria is that plants have a longer-than-average juvenile period and sometimes fail to bloom as expected. Start with grafted plants, or those produced from cuttings, rather than those grown from seed. A plant will also fail to bloom if it does not receive full sunlight, if it is pruned improperly, or if there is excessive vegetative growth that may have been stimulated by an abundance of nitrogen fertilizer. If it is pruned heavily in winter and spring, this can also encourage vigorous vegetative growth.

In severe winters (you do remember severe winters?), flower buds may be injured or killed. The following practices may help induce non-blooming vines to flower: A heavy application of superphosphate (0-20-0) in early spring, about 3 pounds per 100 square feet of bed, and severe pruning of new growth in late spring or early summer, then root pruning in late fall. But, again, these tricks won’t help with vines that are just too young to flower.

Buy top quality, named and grafted parent plants. These may have the vigorous W. sinensis as the root stock, but never as the shoot stock, and should flower in three to five years. Otherwise, you will just have to wait and wait and wait and …

Keep growing.

Oregon State University has a good series of videos on pruning that can be helpful if you do your own work, or want to understand what your tree people are doing. If you have a home orchard, there’s also a series specific to pruning it, as well. For more information, visit the homepage at bit.ly/3tvqynt, and navigate from there.

Some vendors and nurseries are reporting seed and plant material shortages, as well as shipping delays. So don’t procrastinate on placing your orders, even on plants and bulbs that may be shipped later in the year.

I’m a stickler for gardening tools that last. I think I own one hand trowel that I’ve had for 30 years. When it comes to digging tools, I’m not impressed by fiberglass handles (they splinter) and tines and blades on garden forks and spades that bend when you need strength. Spear and Jackson still makes two excellent spades and two forks, which both come in two sizes. They’re not feather light and are built with strong forged steel, with a great double-riveted attachment between the tool head and the handle that won’t come loose, and D-grip wooden ash handles. With minimal care, these will last decades. You can buy them from Kinsman at bit.ly/3hnovuh, and prices range from $59 to $76. Note: Spear and Jackson has several tool lines, and those noted above are specific models.

As a reminder, lawn fertilizer should not go down in April, even though your gardener and landscaper will insist on it. Early to mid-May is much better for the turf. And, once again, spring is not the time to seed your lawn. You’ll have greater success in late summer and into early fall.