I recently had a question from a lady in London about her Wisteria, which had failed to flower for the past 20 years. This is quite a common question, along with "How and when do I prune it?" so I thought I'd repeat and expand my answer to her, here, for everyone's benefit.
Firstly, there are three reasons why Wisteria refuse to flower: lack of sun, incorrect pruning, and lack of nutrition.
Well, four, if you include "Wisteria grown from seed take 20 years to flower, so don't waste time collecting the seeds, buy one in a pot, and preferably already in flower so you know what colour it will be."
Nutrition: Starting with the easiest one first, nutrition: if the plant has been there for many years, and in particular if it is planted right hard up against a wall, it is quite possibly getting a bit low on nutrients. I would suggest giving it a small fist-full of bone meal once or twice through the summer: scatter it around the main trunk, scratch it into the soil and then water it in.
You can also give it a feed of something like Tomato Feed, diluting it down exactly as instructed on the container, in the late summer/early autumn. This will (theoretically, at least) help it to make flowering buds for the following spring.
Should you give it tomato feed every two weeks after flowering during the late spring? Personally, I don't think so, but a lot of people recommend it, so why not give it a go - again, dilute it properly, there is nothing to be gained by giving it a "double dose" (*wags finger warningly*).
Sun: Wisteria will not flower if they do not get enough sun. This means they really need to be on a south-facing wall, and a south-facing wall means that if you stand with your back to the wall, you are facing towards the south.
If you have planted one on a north-facing wall, it will never flower. West-facing, well, you might be lucky if it gets a lot of late afternoon sun. East-facing: you will only ever get sparse flowers. If you realise that your non-flowering Wisteria is planted in the wrong place then you have two options: either dig it up and start again, or - if you are lucky - train it around the corner to the south-facing wall.
I have one client with exactly this problem, they planted the Wisteria hoping for flowers all across the house, but it was on a very shaded west-facing wall. As it was the corner of the house, I put up some wires on the south-facing wall, and trained the strongest long whippy shoots round the corner and onto the new wires. They are thickening up nicely, and I'm hoping for flowers maybe even next year. The roots of the plant don't need to be in the sun, but the flowering wood does.
It's worth mentioning that even if your plant is on the south-facing wall, it still won't flower if it is seriously over-shadowed by trees, other buildings etc. If trees are the problem, you might be able to do some crown-lifting and/or thinning of their canopies to allow more light through.
Pruning: Ah yes, the question of pruning. Right. It's quite simple. Each year, from early summer onwards, your Wisteria will produce dozens of long, thin, bright green, whippy shoots. These should be pruned back, leaving just 2 or 3 leaves. I do this pruning probably three or four times during the summer, as no matter how much you cut off, they send out more! I would suggest you go out every three or four weeks with the secateurs, and chop them off.
Winter pruning - once it has finished growing and has dropped the leaves, prune it again, cutting back all those shortened new shoots - now brown and lifeless - to just one or two buds. Pull out all the spindly dead stems you can find, and shake out any clumps of dead leaves caught up in the branches. You should aim to be left with a framework of stout stems.
If you have ever seen a proper old vine, this is what your wisteria should look like. Thickish stems, with gnarly bunches of knobbles. These are the flowering buds. So, go over your plant and cut out every tiny spindly little stem.
If you get out there now and do the summer pruning, you shouldn't have all that much to do in winter. But if you just can't find the time to do it now, well, you will have more to do in winter!
I will take photos later on in the year when I do this, so I can add a step-by-step guide.
Three points about pruning:
1) If your wisteria isn't quite big enough yet, don't let all the whippy stems grow in the hope that "it will quickly cover up the pergola/trellis/building." Choose just a few of the whippy stems, and tie them neatly to the support. These few will become your "framework of stout stems". If you can, arrange them in a neat fan shape, or on parallel wires. You may think that this is a bit of slow start, but it will repay you time and time again in the future.
2) The actual cut: to remind you, when pruning pretty much anything, aim to make a clean cut at an angle, slanted away from the bud above which you are cutting. Reasons:
a) a ragged cut will allow infection in, so keep your tools sharp and cut cleanly. If the branch is too thick for your secateurs, get the loppers!
b) if you leave a longish bit of stem above a bud, it will die back, as the energy of the plant will get to the bud and stop there. Once a stem starts to die back, it tends to continue dying back, which can be disastrous for the plant.
c) sloping the cut allows rain and dew to slide off the cut and away, thus reducing the risk of rotting. If the cut slants towards the bud, then the bud will catch all the water, and will rot.
If you are unsure, put "pruning cut" into Google and click on "images" and you will find dozens of illustrations. If you really can't get the hang of it, then you might benefit from some private tuition, click on the page button for details. Or if you are not in South Oxfordshire, ask a neighbour or a friend to show you how.
3) If it's a big wisteria..... "How do I reach up there!" you cry. The simple answer is to buy a long-handled pruning pole. I have a lovely lightweight aluminium one which allows me to reach first floor window height: several of my senior clients have wonderful old wooden versions. They are basically just a pole with a knife or a blade on the end, and a mechanism for operating the blade, either a handle to pull, or a rope. Of course, you don't get quite the finesse with these tools - for instance, you can't do an accurate sloping cut when you are working 8' above your head, but it means you can get the job done yourself, and you don't have to go up ladders to do it.
And proves, incidentally, that although it is Best Practise to do a sloping cut just above a bud, it is better to hack wildly than to not prune at all!
Which Wisteria Do I Have?
There are two types, Wisteria sinensis or Chinese Wisteria, and Wisteria floribunda or Japanese Wisteria.
Sinensis or Chinese: the flowers appear before the leaves, the stems twine anticlockwise if you imagine yourself at the base of the stem, looking upwards.
Floribunda or Japanese: leaves and flowers appear at the same time, the length of the raceme (the flower mass) is much longer than in Chinese wisteria, and the stems twine clockwise.
There, now you know! Both of them, by the way, are fairly poisonous in all parts, and particularly in the seeds, which are temptingly velvety when fresh. As always, and you will have heard me say this before, TRAIN YOUR CHILDREN NOT TO EAT ANYTHING UNLESS IT IS ON A PLATE.
Honestly, I'm not kidding, I know that modern children are allowed to graze constantly (no wonder most of them are as fat as little piggies, ha ha!) but there are a huge number of poisonous things in the garden. I often get over-anxious parents asking for lists of plant that are not poisonous, and I have to tell them that it is almost impossible to make a garden entirely from "safe" plants.
Did you know that daffodils are poisonous? And Columbine? Crocus? Buttercup? Box? Bindweed? Beech? Chickweed? Laurel? Delphinium? Ground Ivy? Hellebores? Holly? Ivy? Iris? Laburnum? Lily of the Valley? Larkspur? Oak? How many letters of the alphabet do you want me to go through?
It is far better to teach your children not to eat anything that is not on a plate.
I know that there is a movement to get small children interested in gardening, and to get the adults to grow herbs and veg at home, and articles on this subject always show happy little children with a tiny carrot, or a strawberry half-way to their mouths. Sometimes without an adult even within sight. This makes me cringe, as so many plants are not edible. Please, please, take the little children down to the veg garden or the allotment, but teach them to bring the edibles back to you, the adult, who can then check them, wash if necessary, and put them on a plate to be eaten.