Wisterias

By Howard Drury

Introduction

A small genus of vigorous twining shrubs. Members of Leguminosae, they are related to runner beans and Laburnum, the typically pea like flowers in shades of white, pink, blue and mauve being borne on pendant racemes between 30cm and 1 metre long. The foliage is attractively pinnate (ash like), each having 10-20 leaflets, is distinctively downy when young in some species. A deciduous genus, most species of which flower during May and June just before or as the new foliage appears. Wisteria are ideal subjects for full sun on south or west facing walls, pergolas or for climbing up into large old trees, but with attention to detail can be restrained to form a standard specimen. Although there are about ten recorded species, only two are commonly planted, there being several named forms of each. Unfortunately, propagation of these named forms is by grafting and somewhat slow and difficult, with a consequent relatively high price on the resultant desirable plants. Many outlets unfortunately supply much cheaper to produce plants from seed, which can take many years to flower, most of which will be inferior to the named forms, which have larger flowers or longer racemes of flowers. It is therefore most important that you buy a specimen in flower, or a named form from a reputable source, be wary of "cheap" offers.
Siting and Planting

Ideally, plant in a site in full sun for best floral effect, reduced flowering will result where growing in shade such as up the main frame of a large, live tree. Wisterias are long lived and capable of achieving height of nearly 20m. Fortunately they do well in any well drained fertile soil, but as these plants are normally used as wall shrubs, it will be wise to thoroughly investigate the site and prepare the planting hole so as to ensure that there is adequate soil and root run rather than a huge deposit of builder’s rubble. If intending to plant on a fence, please take into account the potential size and age of the plant. Eventually it will be the plant supporting the fence, an impossible situation where fence replacement is considered necessary. Being a long term investment, thorough preparation of planting hole is advisable, incorporating plenty of bulky but low nutrient organic matter such as well rotted manure or garden compost. However as is always the risk, such a massive upheaval of the top 0.5 - 0.75m of soil, can result in the formation of a sump in poorly drained soils the young plant drowning in a porridge of improved soil. The great vigour of Wisteria, and the problems sometimes encountered in persuading them to flower, dictates the avoidance of more than minimal quantities of fertilizer, especially nitrogenous types.

Leguminous trees and shrubs generally do not like root disturbance, therefore Wisteria are always supplied as pot grown plants which must be planted at the same depth as in the pot, i.e. surface of pot ball not buried more than 1cm. Planting time, as with most deciduous trees and shrubs, is ideally about or just after leaf fall, or April - early May, weather and soil conditions permitting. By planting in May you have the chance to buy plants in flower proving they are the variety you want and have the ability to flower, they also seem to establish better if planted around flowering time. However where planting is much later in season as a result of selecting a flowering specimen, it will be necessary to pay more attention to after care in the first growing season particularly as regards watering. Wisterias do not transplant readily and once planted should be left to grow for many years.

Support

Wisterias are twining, rather than clinging plants and as such need to be provided with wires or other support to wrap themselves around. The weight of a large specimen covering the whole of a house wall is however considerable, wire thickness and fixings will therefore need to be substantial. The twining habit of the plant will however cause problems for the future where it is grown on a fence with an open lattice, making it impossible to free the fence without severe pruning, to which legumes generally do not respond favourably. It will be necessary to provide some means of support in the first years of establishment to enable the plant to reach up to and become firmly attached to the more permanent fixed framework of tensioned wires. This temporary support need only be a strong bamboo cane, or similar, inserted alongside the plant, reaching up to the wires.

Aftercare

In the first growing season the main concern will be watering, particularly if planted at the base of a wall, once well established after 2 or 3 years (assuming a free root run), the feeder roots will be exploring soil well away from the wall base. Mulching with at least 5cm of well rotted manure, compost or leafmould will help to conserve moisture and also maintain soil structure. Do not allow Wisterias to grow around downpipes or posts, gutters etc. as they can cause considerable damage.

Pruning

Wisterias are extremely vigorous, and is generally the case with shrubs flowering on previous years' growth, unrestrained growth or hard winter pruning encourages excessive growth which in turn can reduce or delay flowering. Ideally, we are looking for a balance between adequate vigorous extension growth to rapidly build up a permanent framework, and the development of flowering wood at the earliest possible time.

This is best achieved by cutting back by 80% all long vigorous shoots not needed for future framework around the third week of August for the Midlands, other regions will vary. This is to allow the exposed spurs to ripen and promote flower bud development, but late enough in the season not to allow for new vegetative shoots to form. The following spring these spurs are pruned again to four buds or less as the buds begin to swell rather than a set time in the spring. Where plants are grown in pots or as standards, these spurs can be cut back to just two buds.

Feeding

In the early years of establishment, whilst the roots are mainly in the area of the original planting hole, it is helpful to apply Sulphate of Potash as this reduces excess growth and helps produce more mature flowering wood. Apply using a watering can in late spring/early summer, after the initial early spring flush of growth as an attempt to encourage ripening of the new growth. This should encourage the development of flower buds for the following season. Once well established, Wisteria being of tree like proportions have such an extensive root system that feeding in the normally accepted close vicinity of the trunk will be of little benefit. Also being members of the pea family they produce their own nitrogen.

Propagation

Most easily achieved by collecting seed, for almost immediate sowing during the autumn, suitably protected from mice and other seed stealers. Place the containers outdoors, keeping moist at all times, and expect germination the following spring. However this is an unsatisfactory method of increasing or renewing your plants as the results are uncertain, and normally a considerable number of years elapse before the first flowers appear. Named or desirable forms must always be propagated vegetatively, using half-ripe cuttings taken during late July-August, and rooted in sandy compost in cold frame. Pot up in the following spring as the new growth commences.

A much more reliable method, ideally suited to the small numbers required by the home gardener, is layering down one of the long pliant branches to the soil which will provide a new specimen ready for severing from the parent at the beginning of the following growing season. Plants produced in this way will often commence flowering within two or three years of being rooted.

Those auteur enthusiasts, always on the lookout for a test of their ability, will find grafting sections of previous year’s growth, onto roots of pot grown seedlings an interesting challenge. Ideally, seedlings of the parent species should be used to ensure a good chance of a successful "take". Seedlings will need to be the same thickness as, or slightly thicker than, the pieces of the scion variety being grafted on to them. An fascinating exercise, but which will take about four years to complete, if you want to do the whole thing, including raising the rootstock seedlings from your own saved seed. For further details of propagation methods,

Species and Cultivars

The two species commonly available are Wisteria floribunda, the "Japanese Wisteria", and the more popular, Wisteria sinensis (chinensis), the "Chinese Wisteria". Both are available in several forms, colours, including unusual double flowered types.

Wisteria floribunda, (W. brachybotrys) may achieve 10m in height, leaves some 30 cm long on clockwise twining stems. Fragrant violet-blue or bluish-purple flowers on 13 - 25 cm (6 - 10") long racemes on previous years growth. Flowers open in succession, starting at the base of the raceme, and are followed by velvety seed pods up to 15 cm long.

Wisteria floribunda 'Alba' (W. multijuga 'Alba'), white flowered form, sometimes with faint lilac flush. Long racemes of flowers up to 60cm (2ft) long, however there appears to be two forms, with variation in length of flower raceme.

Wisteria floribunda 'Macrobotrys' (W. multijuga), The most magnificent form of all Wisteria. The fragrant flowers, borne on racemes up to 1m long are lilac with a tinge of blue-purple are ideally displayed where the plant is climbing up through and over high archways/pergolas or across an arched bridge where the pendant racemes can be reflected in the water. Can take many years to flower after planting.

Wisteria floribunda 'Rosea' (W. multijuga 'Rosea'), pale pink flowers, lightly tipped purple, otherwise similar to W.f.'Alba'.

Wisteria floribunda 'Violacea', violet-blue flowers.

Wisteria floribunda 'Violacea Plena', has double violet-blue flowers, but not so free flowering as the single forms.

Wisteria sinensis, (W. chinensis) the most popular of the Wisteria, an extremely striking climber. Very vigourous, the twining shoots eventually reaching up to 18m, spiralling anti-clockwise as opposed to the clockwise of W.floribunda. The fragrant flowers are shades of deep lilac through to mauve. They can be 2cm long on racemes up to 30cm in length. Flowers open with the unfurling of the young foliage in a most spectacular manner, all flowers on a raceme opening together, instead of the gradually increasing display from W.floribunda.

Wisteria sinensis 'Amethyst'

Wisteria sinensis 'Alba', white flowered form.

Wisteria sinensis 'Black Dragon', double dark purple flowers.

Wisteria sinensis 'Plena', double lilac flowers, with a rosetted form.

Wisteria sinensis 'Prematura', almost guaranteed to start flowering within 3 years of planting. So overcomes one of the main problems of Wisteria in that there is often a protracted period between planting and start of flowering. Also available as a white form.

Wisteria X formosa a hybrid of W. floribunda 'Alba' X W. sinensis. Tends to resemble more closely its W. sinensis parentage, the racemes of pale violet flowers are 25cm long on which the flower all open simultaneously. The stems however twining clockwise as does those of its other parent.

Wisteria x ‘Caroline’ sky blue fragrant flowers

Wisteria 'Issai', possibly a selected form of W. X formosa, the lilac-blue flowers resembling those of W. sinensis. However may possibly be a form of W. floribunda with improved flowers.

Wisterias not flowering

Seedling rather than grafted plant – these are much cheaper to produce but will typically take many years to flower and then the results are like growing an apple from a pip, often very disappointing. Look for signs of the graft just above soil level, look for a named variety as listed above and expect to pay in the region of £20 plus and finally always buy a plant in flower, this way you know it has the capability of flowering and when they are in bloom is also the best time for planting and getting good and rapid establishment.

Late spring frosts will typically cause Wisterias to drop developing flower buds.

Birds will through frustration frequently simply pull or bite off buds if they are unhappy or agitated through bad diet etc, sparrows are known to pull off flower buds.

Do not confuse this with squirrels that are looking for sweet sugars as the sap rises, here they will bite off the buds in search of a rich spring diet.

Dryness at the root, especially when close to a building with eaves is another cause of flower buds simply falling off.

Incorrect pruning or pruning at the wrong time of the year is as serious as not pruning at all, see pruning advice above.

Seedling Wisteria are also not likely to flower for many years, and even then the results are often very disappointing.

Some grafted hybrids do take many years to reach flowering, such as Wisteria macroboytis

Sources of Supply

Although readily available through the better garden centres, very few will carry more than a very limited range of the species, forms and cultivars available. Due to this very limited availability other than through specialist hardy shrub nurseries we refer you to :-

The Plant Finder, published by the Royal Horticultural Society, £15.99 and updated annually. This publication is a mine of information for the enthusiast looking for something unusual, but not necessarily better. It lists many more types than dealt with above. You are however warned that many of these specialist nurseries do not provide a mail order service, preferring personal contact with the customers of their specialist plants.

It is also available online via the RHS plant database, which then shows a link to the Plant Finder if known suppliers have listed it with them.

The information given in this Fact Sheet is provided in good faith. It is however of necessity general information and advice on the topic. Howard Drury will not be under any liability in respect of the provision of such advice and information, and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.

(C) 02/03/2022 This material has been produced by Howard Drury and must not be reproduced in part or full without the written consent of Howard Drury, Kings Heath, Birmingham, B13 0SJ.
JHD/02/03/2022