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Not sitting on the fence

The main work in our garden this week has been the overdue repair of part of a boundary fence. Most of the posts holding it up had rotted at ground level and one end of it had a much worse lean than the Tower of Pisa.

It was a bit complicated because it was also supporting two incongruous plants. The fence is the eastern boundary of my neighbour’s house, and only about a metre away from their house. In that dry and shady place someone, several owners ago, had optimistically planted a climbing rose. It put up a valiant fight but usually only provided a few flowers each year.

On our side of the fence is a shrubbery, containing several different daphnes, a camellia or two, a michelia, a lily of the valley shrub, a rhododendron and a couple of kalmias. That makes it sound as though it is rich, moist soil with a low pH, when it is in fact very dry and requires a lot of summer watering. I probably should grow South African and Australian shrubs there, but the others are so appealing!

In the shade of these shrubs, and for some years unnoticed by me, a wisteria established itself from seed and in the manner of wisterias, quickly twined along the fence, through the rose and into the neighbours’ trees. It thrived better than the rose, but a shady, dry place is not really suitable for a wisteria either.

In the event, I severely pruned both the climbers and will now have to await their reactions. I’m not sure about the rose, but I imagine the wisteria will put on an excess of growth as it regains its lost ground.

Although this is not an absolute rule, the roots of a plant are generally in proportion to the above-ground growth, and many plants will strive to keep the equilibrium. Anyone who has pruned a plum tree very hard will know that it will respond by making an excess of new growth.

Wisterias, which are very energetic growers generally, will certainly do the same. Cut a wisteria severely, and it will reply by exponential growth. Don’t forget that wisteria can grow up to 30 meters or so.

In theory you could try restricting the root growth by root pruning that as well, but that is difficult with strongly growing plants. With some plants you could drive a spade into the soil around the edge of the drip line, and that would help keep the roots under control, but a wisteria’s underground growth is too vigorous for that.

If you restrict the root growth of a wisteria it is possible to create a shrub-like plant rather than a rampant climber. With care, wisteria can be grown in any large container, but the most spectacular way the grow a constrained wisteria is to turn one into a bonsai.

You will probably need to start your own bonsai wisteria from seed, as the plants usually sold in the nursery trade are too big. You will need patience – it will take between 10 to 15 years before you will get the wonderful arching racemes of pea-shaped flowers, and you will need to establish a tall plant for the arching stems to have effect. This will not be an indoor bonsai as it will have to be kept outside so the plant will get enough sun to set the following year’s flowers.

A friend grew one of these for many years on a veranda, and it was a spectacular sight when in flower.

It is more usual to see wisterias outside, clambering along fence lines or stretching over pergolas where they have a bit of space to express themselves. At their best they are spectacular. In Taranaki I have visited a number of gardens where they stretch along the side of a house, their flowering period being stunning. Similarly, there is a pergola-covered bridge at Kuirau Park in Rotorua that is hung with purple wisteria each spring, making an unforgettable sight.

These plantings are generally of Wisteria sinensis, the Chinese species, and usually in the mauve through purple range, although there are white forms, and some that could loosely be described as pink. The Japanese species, W. floribunda, has longer racemes of flower, but is also slightly more spaced out so the effect is perhaps not quite so strong.

To get the best out of a wisteria, make sure its roots are in cool soil, and the upper growth is in the sun – remember, it won’t flower unless it gets some sun. Keep it well pruned too. Establish a framework with just one or two leaders, then pinch them out when they are as long as you want. After that trim the ends of the stems each year, occasionally replenishing from new shoots at the base of the plant.

And make sure it does not tip the fence over.

Roger Parker
Roger Parker
Roger Parker is the Times-Age news director. In the Venn-diagram of his two great loves, news and sport, sports news is the sweet spot.

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