Understanding Pruning
To really understand how to prune you need to know a little bit about how plants grow. The above ground parts grow larger in two main ways. Firstly they grow outwards from the tips of shoots, and secondly they grow by stems becoming 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.
When you prune to control size and shape you are cutting off the most actively growing part of the plant, and breaking apical dominance.
It is important to consider 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.
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 can 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. 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.
Of course, there are many reasons to prune other than controlling size and shape. Pruning is also done to encourage fruiting or flowering, to remove dead or diseased tissue, to promote healthy growth and to rejuvenate old plants.
Pruning is an essential gardening skill whether for general horticulture, garden maintenance, or tree care. Indeed, it is one of the single most important practical skills any horticulturist or garden lover should learn.
