Plants which are grown on a wall usually have to be kept within certain limits. To do this the plants need a periodical, usually annual, pruning or training, which is dependent on two factors, the amount of space available and time of flowering. With regard to space this depends both on the area of wall available and on the distance the shrub can be allowed to grow out from the wall. Sometimes plants growing on walls are pruned too much and are kept too close against the wall; how much better to see a well-grown shrub with its branches standing out 1 or 2 feet (30-60 cm) from the wall. It is better to choose a plant which is amenable to training, rather than having to keep continually pruning.
The time to prune depends on the time of year that the plant flowers and the type of growth. Climbers and shrubs can be divided roughly into two groups; those that flower on the current season’s shoots, generally from midsummer onwards to the autumn, and those that flower from March to June on shoots made the previous summer.
In the first group pruning consists of cutting back the shoots that have flowered, if possible to a dormant bud near the base. This should be done in late winter or early spring (February-March) so as to give the plant the longest possible period for growing before flowering later in the year; examples of this group are Clematis x jackmanii types, some types of Buddleja, and Caryopteris.
The second group includes those that flower on the growth made in the previous summer; pruning should be done immediately after flowering, cutting back the strong growths which have just flowered to encourage new shoots. These will produce buds to flower in the following season. Sufficient young shoots should also be left, on which flower buds will be formed for the following year. Examples of this group are Ceanothus, Buddleja davidii, Forsythia, Prunus triloba and Clematis montana.
This hard annual pruning is not necessary for all plants. There are quite a number of plants which are not vigorous growers and make comparatively little growth every year, and they require little or no pruning, certainly no annual pruning, only tying in the new growth as it develops. Young plants that are in the early stages of being trained up a wall need only very light pruning each year to encourage branching and stimulate more growth on which the framework of the plant is built up. It is only when it has reached its maximum height and allotted space that it may be necessary to prune harder.
For plants which are grown primarily for their foliage or autumn colour, such as vines or variegated shrubs, the time of pruning is less important. The most favourable times are winter for deciduous plants and early spring for evergreens, with possibly a second pruning in August to thin out redundant or overcrowded shoots.
While pruning is being done it is wise to cut out any dead or weak shoots and to check and renew where necessary old or weak ties which support the main framework. It is also important to make a regular inspection of all plants during the growing season and tie in new shoots. Often, uncontrolled growths become intertwined with one another, making it difficult to sort them out at the end of the growing season.