Ideally tree works should not be undertaken during the spring time period, when the 'sap is rising' to enable the leaves to flush (come out) and photosynthesis to begin, and during the autumn, when the tree is drawing nutrients back into itself from the leaves as they go brown.
If works are undertaken in the spring then the tree may become more vulnerable to pest and disease attack. If works are carried out in the autumn then the tree will not be able to get all the nutrients that it needs for the next spring and the tree may be put under unnecessary stress, increasing the likelihood of disease.
Outside these periods most trees can be pruned at any time of the year, with a few exceptions:
Cherry, Plum and related trees (Prunus species) should be pruned soon after flowering to reduce the risk of bacterial infection.
Maple, Birch, Beech and Walnut should be pruned in leaf or just after leaf fall and Magnolia in high summer to avoid ‘bleeding’ (exuding sap), which, although not considered damaging, can be unsightly.
If possible, pruning should be avoided when recovery may be impaired, for example during a period of physiological stress following previous tree work or construction-related damage or during seasonal weather extremes such as drought or extended heavy frost.
Another consideration as to whether tree works can be undertaken is to do with the ‘Bird Nesting Season’. Officially, the season runs from February until August. Works can be undertaken during this period, but a responsible person will have to undertake a pre-work survey. It is recommended however, to undertake works outside of this season.