Any amount of pruning causes some degree of stress to a plant. Ideally, plants of suitable growth and habit should be selected for the site in which they are to be grown but this is not always possible. Also, some people prefer their plants to have a more manicured appearance, perhaps to fit in with a formal garden style.
It is important to remember that different plants have different levels of tolerance to hard pruning. With some plants, you can remove 50% of the foliage and this will result in improved vigour and health. With other types of plants, removing 50% of the foliage will cause death. Occasionally you may encounter a plant which has low tolerance to hard pruning that will survive a hard prune, but this is not typical. It is better to find out what are recognised safe pruning limits for a particular plant before pruning it if you are unsure.
Sometimes it is worth taking a risk with an old plant. If you are have shrub that is old and unsightly but you like it that species in that place, then cut it hard back, perhaps even to the base and give it a year to recover. If it regrows you have the plant you want and if it dies then you have a planting opportunity.
One way of expressing a plant's tolerance to cutting back is a as a percentage of the plant's total size. For instance, Acacias may be cut back by up to 20%. This means that if more than 20% of the top growth is removed, the Acacia is likely to suffer, or even die.
If large branches are growing in a position where you do not want them, it is valid to remove them.
If foliage is spreading over a pathway, it is acceptable to cut it back.
Similarly, if a branch that bears flowers and fruit is growing where it is not wanted then removal of the branch may be warranted.
If a plant is getting too large, then it can be pruned to contain it.
If a plant in a garden is growing out of shape, e.g. a hedge or topiary, then pruning to restore the initial shape is recommended.
It should be noted that when pruning to control size or shape, frequent light pruning is nearly always better for the plant than irregular heavy pruning.
Promoting Healthy, Bushy Growth
The lifespan of many plants is generally lengthened by regular pruning. When plants grow they grow outwards at the tips of shoots, and the stems become thicker. Hormones released by the topmost bud are sent down the shoot and these inhibit growth of buds lower down. This is called apical dominance. Only once the uppermost tip has grown far enough away from the plant do buds lower down start to produce branching stems.
By pruning the tips of shoots apical dominance is stopped and buds lower down stems will burst open sooner. An advantage of doing this is that the tips of shoots are softer and fleshier and the leaves are more susceptible to the ravages of harsh weather such as freezing winds or scorching sunlight. Pruning removes this tender growth. It encourages plants to put more energy into developing woodier stems and denser foliage where it is needed to protect them from the prevailing conditions. The ability of buds further down the stem to start growing reduces as these buds get older. So if you have plant such as Mahonia x media ‘Winter Sun’ that has bare stems in its bottom half, no amount of pruning at the top will induce the buds near the base of the plant to grow.