In an ideal world, there would be no need to prune your Acer. Generally speaking they are fine without being subject to the secateurs. But as with many shrubs that do not actually need pruning, we sometimes have a need to prune.
Other than formative pruning for those Acers which go on to make large shrubs or trees, the Acers and Japanese Maples in particular are best left alone. The main exception to this non pruning is the removal of dead and dying branches which have succumbed to the cold in the previous winter.
If Acers or Maples are planted in the right position, and do not suffer overcrowding from other shrubs, then the least done by way of pruning the better. The problem with positioning Acers is the fact that most are bought as relatively small shrubs, with a general habit of slow growth, so planting in the right position often means gaps to be filled around the newly acquired plant.
Even those bought with the intention of being specimens somehow seem to need the protection of a few small shrubs or simply a covering up of the bare earthy around. Being reluctant as we are to remove these sheltering shrubs once they have grown and turned into smothering rather than sheltering, the Acer suffers, and the secateurs are seen as the only answer.
Most of the mails I get about pruning Acers are generally about the Dissectum varieties, which have decided upon their own course of action - or direction of growth when planted in a rock garden or container. Acer palmatum Dissectum - and its many different cultivars, are normally dome-shaped shrubs with a very uniform habit of growth. However, when planted as a specimen in rock garden or container, they often veer off to one side and invariably not the side you wish.
Pruning is sometimes seen as the only course of action. This is often the wrong thing to do, for if you cut a sizeable section of the Acer away, you still have a root system below ground, that wants to support the size of plant is has become part of. It will keep finding and sending its food reserves to try and re-grow the plant as soon as possible.
New growth if it happens invariably starts again on the removed side, and in a short time the shrub is again trying to assume the shape that it wanted in the first place.
It is far better to allow the Maple to grow the way it wants if at all possible, and to alter the surrounding and aspect to suit!
If any pruning is done, then it should be carried out during late, and in any event before the early spring sap rising. Again, the exception is if there is any damaged wood after a severe winter.
Regardless of all the preceding information and advice, someone, somewhere, will want or need to prune their Acer or Japanese maple. We accept that the world is not always ideal so this is how to do it.
Pruning Mature Acers
Acers sometimes fall into neglect, normally by way of being planted in the wrong position. Life happens and the Acer has to be saved. Acer trees, normally have quite a dense canopy of foliage and can be a nuisance casting shade over large or unwanted areas of the garden. They do not normally respond too well to having a large central leader taken out. This often leads to dieback over a few years.
However, lateral branches other than the ones that emanate directly from the main trunk, can be removed or reduced in order to curb the growth habit, so such pruning can allow light to penetrate through the tree canopy to the garden or house below.
Formative Pruning of Tree Acers.
Most newly bought Acers which are ultimately to form trees - Acer cappadocicum; Acer platanoides; Acer pseudplatanus etc. will have had their central leader removed, to allow the start of a branching hait by the two opposite buds - which will then form branches. If this is not the case, then simply cut back the central leader to two strong buds, to allow for this to happen. Further formative training of this type can take place by pruning the central shoot of the laterals. Bear in mind, that this will then be aiming towards a dense foliage canopy if carried too far. Far better to allow the lateral side shoots - branches - to spread out and allow for a relatively open centre of the tree as it grows.
When to Prune Maple Trees
Always carry out such pruning in late Autumn or Winter. NOT in the early spring or summer. By all means decide in the summer which branches are going to be troublesome - make a note - and do the work later.
The reason for this dormant season pruning, is that Acers bleed their sap if pruned in the spring or summer. (So much so, that some Acers are actually grown for the purpose of sap collection!). It is said to make good wine!
If the steps are followed below for the Japanese Acer group, then it is possible to open up the centre of the shrub - which might be good if it is a large specimen. The resultant form can be attained over several years of light formative pruning. Much of the beauty of the Maples is their branch structure - other than the beautiful foliage for which they are normally grown.
Often after severe winters, there is a case for cutting out damaged shoots and branches on Japanese Maples. Sometimes this winter damage does not show through until leaf break time in late Spring. Simply cut the dead wood back to a pair of live sprouting buds - about 2cm - 1in above.
This group of shrubs rarely need anything other than the lightest pruning (a) to bring back the shrub into a more desired shape, and (b) to take out 'rogue' crossing and week shoots.
In this diagram, you will see some 'crossing' shoots and a few small twigs - which if allowed to grow - will spoil the shape of the shrub.
In the summer growing season, it is a good idea to loosely tie a little tape or string where you think best to prune for the desired shape in the coming Autumn.
This diagram shows the proposed pruning cuts at the red marks - to sort out the few potential problems mentioned above. The aim should simply be to open up the centre of the shrub. This is more preferable for Acers, so that the attractive branch form can be seen,
Always make the pruning cut back to within a few inches of the main branch. There will be a little dieback of the stub, but this is normally self-healing and does not require any of the tar based pruning treatments.
The pruning has been done! 2 further small branches 'instantly better' shape, but I would wait to see what sprouts on the left and side to even things up a little!
The least done to this group the better! If really necessary, can be reduced in size at this time of year, but far better to allow it to grow to its proper maturity. Most of the foliage growth will be around the perimeter of the branches - so there will be plenty of room to under-plant attractive low growing shrubs or perennials.