By Julie Boudreau
With its incredible clusters of grape juice-scented flowers, wisterias (Wisteria spp.) hypnotize us with their beauty. Everyone wants to grow them! Even northern gardeners!
Wisteria is not the easiest plant to grow north of the 45th parallel! Indeed, the most coveted species are native to Asia (W. floribunda and W sinensis). As for the American species, which are hardier, flowering is less spectacular, which makes them less desirable (W. frutescens and W. macrostachya (now a subspecies: W. frutescens subsp. macrostachya)). So, the first step to obtaining flowers is to choose your variety carefully.
When I think of all the winning conditions needed to enjoy the beautiful blooms of wisteria, I think of a cross between a cosmos, a grape vine and a forsythia!
Too Much Fertilizer… No Flowers, Like the Cosmos!
There are certain plants in the garden that react very strangely to fertilization. This is particularly the case for cosmos and morning glories. If we provide them with too much compost and good nitrogen fertilization, these plants will not flower! They will produce generous masses of beautiful leaves… but no flowers! It’s as if the plant is saying, “I don’t need to reproduce. Life is beautiful!”
Wisterias react in the same way. If we give them too much love, too much compost, too much fertilizer, they thank us… by NOT flowering! But foliage, oh, there will be plenty!
Count the Buds… Like the Grape Vine
When you study the pruning techniques of wisteria, you quickly realize that it has many points in common with pruning grape vines. First, there are two pruning periods, one early in spring and another in summer. Then, you realize that the flower buds develop on very short tertiary branches. I will explain this in more detail soon. So, if you have already pruned a grape vine, prune your wisteria the same way.
Regarding the tertiary branches, this is the third level of branching of the plant. Thus, the main trunk is considered the primary branch. The stems that extend from this trunk are the secondary branches. Then the branches which develop on the secondary branches are called the tertiary stems. In other words, the flowers do not develop directly on the trunk. Some branching is required to produce flowers.
Let’s start with summer pruning, as it sets the stage for spring pruning. Wisteria being a vigorous climbing plant, it will produce numerous very long and twisted stems. These long stems are very often the tertiary branches! Summer pruning involves shortening these long stems. Starting from the point where it meets the secondary stem, therefore from the base of the branch, we count five leaves and we cut! The mission here is to slow down the vigorous growth and encourage these short stems to lignify and nourish the buds for next year. These buds hidden at the base of the five leaves that we kept are the ones that can potentially contain flowers!
Then early in spring, before the leaves come out, first carry out a control pruning on your wisteria. Eliminate everything that does not grow in the right place and remove broken branches. Then comes the fun part! Every five-bud branch produced this summer will be shortened to two or three buds! By doing this, you provide the plant with a surplus of energy that it can invest in the preserved buds. This increases the chances of a more spectacular flowering.
With a good sense of observation, you will notice that the flower buds of wisterias are plumper than the leaf buds. In short, over time, it will be easier and easier to judge whether you keep two buds, three… or five!
It is therefore on these little pieces of stem that the future flowers will develop! And that is… if Mother Nature doesn’t get too capricious.
Flower Buds Sensitive to Cold… Like Forsythia
Because this is another issue, for our unfortunate wisteria grown in northern environments: the intense cold! Everyone has seen a forsythia in bloom in the spring. Very often the flowers are concentrated at the bottom of the plant and you can almost tell if it’s been a particularly snowy winter, because everything that was under the snow has blossomed! And all the flower buds above the snow line are dead.
It’s a bit the same thing for wisterias. If the winter is very cold, chances are the plant will not flower. And it’s a climbing plant! The higher you are, the more wind is added to the equation. This is why, very often, the flowering of wisteria is a festive event which takes place every 7 or 8 years!
And again… this is valid for the hardiest varieties. Our chance of seeing flowering becomes even rarer if you grow Chinese wisteria (W. sinensis) or Japanese wisteria (W. floribunda), which are even more sensitive to cold.
You may be the greatest pruning expert in the world, there is still an element of luck and chance in this beautiful flowering. You can thus arrive at a conclusion which suggests that we should cultivate low, compact or even creeping wisterias. This reasoning makes perfect sense when we better understand the flowering of wisteria. In northern climates, it is often said that you must make the choice between flowering (compact plant) and growing a beautiful large climbing plant with foliage only. Let’s hope that future years will bring increasingly hardy varieties to the market, but above all let’s hope that this will not be to the detriment of this beautiful, spectacular flowering with its divine scent!
