There are three species of Wisteria grown commonly in New Zealand.
W. sinensis flower on bare stems, W. brachybotrys tends to flower with the first leaves and W. floribunda forms which tend to flower with the first flush of foliage but they have longer racemes which make them better candidates for pergolas and other growing frames. All forms are suitable to train as standards.
Wisterias prefer a soil rich in organic matter to hold adequate moisture through the summer months, since moisture availability is important for flower initiation. A little lime is also beneficial, as it is to all legumes. Once the plants are established, too much water and fertiliser can encourage excessive growth at the expense of flower production.The red pigmentation in most varieties appears to be enhaced by warm temperatures at flowering time. This is particularly so with 'Royal Purple' and 'Amethyst'. Being climbers they need full sun for at least half the day.
When obtaining plants it is helpful to know if a plant is grafted or cutting grown, since grafted plants will sometimes sprout from the base, and these stronger shoots can overwhelm the original plant. To avoid this, and if you are unsure as to whether the plant is grafted or not, keep it trimmed to a single stem for the first 30cm of trunk.
The pruning of wisterias is an important aspect of their culture since it can markedly affect the amount of flowers produced. The main structural shaping should be done in early summer, straight after flowering, and any further training should be limited to cutting unwanted shoots back to 3 leaves, not removing them completely.
The basic principle to keep in mind is that a wisteria is like a set of twiggy shrubs along a common vine. As these twiggy portions develop then the flower production will increase, especially when spurs develop, which consist of a short stem with 4-6 buds on it. It can be tempting to "tidy up" the vine in winter, but this should be limited to trimming longer shoots of last summers growth back to 3 or 4 buds. Also remove any dead wood.
During this cleanup operation it is good to keep an eye out for borer holes, since borers love wisteria. A shot of spraying oil directly into as many holes as you find will give control.
Varieties worth cultivating:
Amethyst: A sinensis selection with dark reddish violet florets on racemes up to 20 cm long, most perfumed variety available.New growth is tinged bronze.
Blue Saphire: Similar to Amethyst but flowers soft violet blue, and perfume not as strong.
There is a widely grown form of W. sinensis called 'Consequa' which is the original form of sinensis introduced to England. Blue Saphire is considered superior. Blue Saphire may be the same clone as 'Cooke's Special', an American selection..
brachybotrys Shiro Kapitan: (sold as White Silk) This variety flowers late in warm areas but is reputed to flower earlier in cool climates.The racemes are rather short compared to other wisterias, being only 15-20cm long, but the florets are up to 3cm across the standard. The flowers appear with the foliage in the spring and are sweetly scented. It is easy to grow and flowers freely, and in the nursery situation it is difficult to find wood without flower buds to propagate from.
Peter Valder in his book mentions this form sometimes produces double flowers toward the end of the flowering season. He was unable to find any double white forms recorded in cultivation in Japan.
Caroline: possibly a hybrid introduced in California about1953.Early flowering on bare stems. It is one of the earliest flowering forms with an overall appearance of pale mauve.
floribunda Macrobotrys: 11-17 leaflets, 80-128 florets on racemes 47-100 cm long, better length developing as the plants get established. The pale standard and darker keel give an overall appearance of mauve from a distance. The lenght of the racemes results from the elongation of the rachis, or central stem of the raceme, resulting in a rather sparsely furnished raceme, but en masse, the effect on an established plant is most impressive.
Macrobotrys is an imaginary group which upon close inspection merges into floribunda, and is not really a botanical variety as traditional nomenclature would suggest.
floribunda Honbeni: (sold as Pink Ice) Otherwise known as floribunda 'Rosea'. This is definitely the pinkest variety available with racemes up to 45cm long, with a faint to medium strength perfume. Upon close inspection the colour is actually more lavender, but the combination with white and the yellow of the eye gives an overall impression of pink. This variety can be slow to settle down to flower but it is well worth the wait. Careful pruning can encourage better flower set.
floribunda Royal Purple: If you can obtain the true form of this cultivar you won't be disappointed, since it is free flowering and one of the darkest cultivars available with racemes up to 40cm long. It is a deep violet colour which is enhanced in warmer conditions, though this will depend on careful site selection since it flowers early in the season. Its foliage is distinctly different to other varieties, with narrow leaflets which closely resemble a long lobed Robinia leaf. It is not usually a strong climber, and is probably best treated as a shrub.
floribunda Shiro Noda: (sold as Snow Showers) This is the latest flowering cultivar. It is often refered to as floribunda Alba, but Shiro Noda is the most common name for this cultivar in Japan, and the designation "Alba" may lead to confusion with a W.sinensis form. Shiro Noda produces well furnished racemes up to 50cm long, and is best grown as a pergola plant but responds well as a tall standard.
floribunda Violacea Plena: This is the only double flowered variety in cultivation. It takes a few years to settle down to flower due to its vigour, but once established it never fails to put on a brilliant display. It is one of the earliest varieties to flower. The violet-blue flowers also have quite a strong perfume.The autumn foliage is a spectacular, rich yellow colour.
Lavender Lace: A probable hybrid, origin unknown apart from Brooklands Park, New Plymouth. It has large florets that overlap to produce a very full raceme up to 50cm long, with a sweet perfume. It takes a few years to settle down to flower, but is well worth adding to the collection if you have the room.
This information is a combination of first hand experience and information obtained from "Wisterias, A Comprehensive Guide" by Peter Valder.
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