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Why prune?

by Timber Press on January 15, 2019

in Design, Gardening

For passionflowers (Granadilla), prune to control the plant and to encourage flowers. Avoid hard pruning, which may reduce flowering for the next year or two.

At its simplest, pruning is a means of manipulating a plant’s growth, shape, and productivity by cutting and training it to achieve what you want to happen. To prune plants well is not so much about knowing how and where to cut but about knowing what you are trying to achieve.

The main reasons for pruning are to train a plant to grow in a particular way, to balance its growth, to control the production of flowers and fruit, to maintain its health, and to restrict its growth. A final type of pruning, remedial or renovation pruning, may also be necessary from time to time.

Careful pruning in the early years—often referred to as formative pruning—will allow you to create a plant that is well-proportioned, attractive, and that carries flowers or fruits where they are visible and easily reached for picking. A tree or shrub with well-spaced stems and branches with good angles will reduce the risk of breakage and stem splitting. Plants pruned correctly while they are young are easier to care for in later years. Time spent on training and pruning young plants should be regarded as an investment in their future and as a time-saving, long-term benefit for the gardener.

Balancing Growth
A healthy plant should show signs of vigorous, active growth, especially when it is young and establishing itself. Most plants will start to flower earlier in their lives if they are allowed to grow naturally. Young woody plants will often produce only a few flowers until they are established. As plants mature and begin to flower and fruit on a regular basis, the production of shoots will slow down, with fewer and shorter new shoots being produced each year. As plants age, there is less annual growth. While leaves are produced on older and on younger wood, it is often the younger wood that produces the flowers.

From the gardener’s point of view, it is important that a plant’s shoot growth and flower production are going on at the same time. Pruningshould strike a balance, allowing woody plants to continue producing young woody stems while providing a regular display of flowers and fruits. Often, the timing of pruning can maintain this balance. Pruning plants in late winter and early spring, for example, often encourages the plant to produce lots of new shoots, whereas pruning in midsummer can induce a plant to produce more flower or fruit buds for the following year. Removing old flowerheads (deadheading) to prevent plants from producing seeds will help to extend the flowering season if their energy is not devoted to producing seeds.

Controlling Flower and Fruit Quality
As plants develop a cycle of flowering and fruiting regularly, they often slip into overproduction. You have only to look at a rose or crabapple that has been left unpruned for a number of years to see that the more flowers and fruit a plant produces, the smaller they become. Often, too, the flowers and fruits on the inner sections of the branches are not only small but of poor quality. Pruning away some sections of stems and branches allows you to remove some of the poorer stems altogether. Pruning weak stems also diverts energy into the production of larger, though fewer, flowers and fruit. A good example of this is the butterfly bush (Buddleja davidii). On an unpruned bush there may be profuse quantities of flower spikes, each about 4in (10cm) long. A plant that is pruned regularly and at the correct time of year, however, will bear a smaller number of flower spikes, but each may be 12in (30cm) or more long.

Prune plants grown for attractive fruits in late winter—once the birds have eaten the fruits.

Creating a Pattern of Growth
Some plants don’t have particularly nice-looking flowers—in fact, some plants produce flowers that are barely noticeable—but other characteristics do make them attractive garden plants. A number of deciduous shrubs, including dogwoods (Cornus spp.) and willows (Salix spp.), have colored bark that is especially bright in winter, and other plants, such as some hazels (Corylus spp.) and elders (Sambucus spp.), have large, colorful leaves in spring and summer. These colored stems and large leaves are produced only from the current season’s growth, and the more vigorous this growth is, the better the effect will be. In both instances, the vigorous growth can be achieved only by severe pruning, often cutting down whole plants to within 4–6in (10–15cm) of ground level each year.

Maintaining Plant Health
Combating pests and diseases is a vital part of gardening. Often the best method of control is prevention, either before a problem becomes established or even before it begins. Good pruning can preempt some serious problems, and good formative pruning to encourage strong stems and wide angles where branches join the main trunk will reduce the chance of branches splitting or breaking and providing a site where pests and diseases can take hold.

Many of the diseases that attack woody plants damage the wood and hence the whole structure of the plant. Disease often enters through dead tissue, such as a wound or injury, and is spread throughout the live, healthy parts of the plant. This is why the first part of any pruning process should be removing dead, dying, diseased, or damaged wood (the four Ds) before the real pruning begins.

If there is any suspicion of disease, look for telltale signs, such as a brown staining in the wood on or just under the bark. Always cut back to healthy sections of branch or stem where there is no staining. Pruning to create a good, open structure will allow a free flow of air around the branches. This reduces the chance of diseases, including mildew, and helps to reduce hospitable areas for pests such as aphids that find shelter and become established in weakened and sheltered sites on plants.

Simply changing the time of year that you prune your plants can combat certain diseases. Oak wilt can kill strong, healthy oaks within a few years if it gains a foothold. The beetles that carry the oak wilt disease are active from late April through June in most parts of the country, so it is best to prune oaks in winter, when the beetles are not active. Cutting down tall roses to half their height in an exposed garden will prevent them from rocking in the wind and suffering root damage through the winter.

Restricting Growth
Perhaps the ultimate example of restrictive pruning is the practice of bonsai, but in a garden the most common use of clipping and pruning is to make rows of plants form a dense shelter or screen—a hedge. Many plants will keep getting larger if they are left to develop naturally. In gardens and along pathways this can be a problem if space is restricted. In a natural setting it is often survival of the fittest and biggest, so large plants often crowd out smaller ones. Most gardeners face this at some stage and need to prune routinely to keep plants within their allotted area as well as encourage balanced growth and production of flowers and fruit.

Remedial Pruning
This type of pruning—often referred to as renovation pruning—is usually used to gain control of a plant that is not growing in a desirable way or one that has been neglected and becomes misshapen or unsightly. Remedial pruning varies in effectiveness. Some plants respond well and often recover, growing for many more years after getting a new lease on life. Unfortunately, other plants, such as brooms and many conifers, will not respond to this treatment and often die after severe pruning instead of regenerating themselves.

Even if plants respond positively, problems sometimes arise if they have been budded or grafted onto a rootstock (a healthy plant used as a root system for a grafted plant base), because the rootstock may grow as vigorously as the cultivar that has been grafted onto it. Also, if you are doing remedial pruning on a grafted plant, it is important to discover where the rootstock and scion (a young shoot from the desired plant) are joined together. If the plant is cut below this union, the cultivar will be removed and only sucker growth from the rootstock will emerge. Remedial pruning can improve the plants in your garden, but don’t expect miracles! Years of neglect cannot be rectified in one season.


Steven Bradley spent over 20 years teaching horticulture at colleges around England and is now a freelance garden writer and broadcaster. He studied horticulture at Writtle, Cannington, and Pershore Colleges, achieving the RHS Master of Horticulture diploma. For more information, visit him at sungardening.co.uk.




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