Types of tree pruning
Arborists and tree surgeons tend to classify pruning according to where abouts in the crown branches are removed from. What ever type of pruning you are doing, you should remember that trees need their leaves in order to produce food - never remove more than 30% of the live foliage from a tree at once. If the tree is less than perfectly healthy, you should remove less than this.
Here are some of most commons types of pruning.
Crown lifting involves removing the lower branches in the crown. This is often done to provide clearance, over paths or roads for example, or sometimes to allow more light to pass beneath the crown. Crown lifting often has limited impact on the amenity of a tree because it does not change the more visible higher parts of the crown. On the other hand, lower branches tend to be bigger so the wounds are larger, which can have an adverse effect on the tree's health.
You should avoid leaving a clear stem that is more than one third of the tree's total height. Lower branches have an important role to play in dampening the sway of a tree in high winds.
Crown thinning involves the selective removal of branches throughout the crown, such that the overall shape of the crown is not significantly changed. Crown thinning increases light penetration and air circulation throughout the crown.
It is important to work throughout the crown, including its outer edges, focusing on removing small diameter branches. Removing too many branches from the centre of the crown can result in a tree with poor structure with few points that can be pruned back to in the future. It can also result in long, thin branches that have little foliage in their lower parts to dampen swaying, putting extra stress on the tree in high winds.
Crown reduction is the reduction in overall size of the crown by shortening branches, cutting back to a suitable growth point. Crown reduction is usually used where a tree has outgrown the space it stands in. Crown reduction often results in large wounds at the branch ends, which may start to decay. It should be seen as a last resort.
When carrying out a crown reduction branches should be cut back to suitable side branches and a flowing branch line maintained, reflecting the natural shape of the tree. Poorly executed crown reduction is known as topping and is very bad practice - see the 'Useful links' box for more on why.
Crown reduction should not be carried out on trees with a pyramidical shape, such as many conifers and birches.
The term Pollarding is often misused. Pollarding is really a maintenance regime begun when a tree is young and repeated at frequent intervals throughout the tree's life. All the branches are removed back to a framework of secondary branches off the main stem. Regrowth since the last pollard is then removed back to the 'knuckles' at regular intervals. Branches removed are all of relatively small diameter. This is a management technique traditionally used on willows to provide a constant supply of small diameter poles, much like coppicing hazel but above ground, out of reach of grazing livestock.
The term is often misused to refer to the removing of all branches from a mature tree (as in our picture). This is a very poor practice leaving large wounds and a tree with no foliage with which to produce food. It causes considerable stress to the tree.