Gardeners in cold winter climates are usually well aware that the autumn is hardly the ideal season for pruning both evergreen and deciduous trees and shrubs. Is sometimes however, less obvious to gardeners in Mediterranean and other mild winter climates, where frosts if they occur at all, are light and occasional.
Pruning evergreens in cold weather greatly increases the possibility of bacterial and fungal infection, resulting in the development of rot around the pruning cuts. While on the other hand, it is common knowledge, hopefully, that deciduous plants should be pruned in their dormant season, which is of course during the winter.
The trouble in Mediterranean type climates is that the transition from warm summer weather to cold wintry temperatures is often sudden and abrupt. One day at the end of October, it can be 27c in the shade. The shrubs appear too open and leggy, looking as though they need a good prune to encourage dense and compact growth. Pruning the trees and bushes at such a time, can be highly detrimental to their health, because the new juvenile growth will be damaged, inviting infection, if early frosts occur in November. These may be rare, but they do happen.
Concerning deciduous species, the danger of rot that is caused by fungi or bacteria entering through the pruning wounds, is no less acute. It is for this reason, that pruning woody plants should be delayed to the end of winter, so that the young growth, which has been partially simulated by the pruning, will not appear until all danger of frosts has passed for the year. The advice you may read about pruning at the onset of winter, applies to cold winter climates, not to mild ones such as in California, The Middle East, and South West Australia.
A further danger lurking for Mediterranean climate gardeners concerns some trees and shrubs of tropical or sub-tropical origin, that drop their leaves in the winter, but are not naturally deciduous species. These are often more sensitive to cold temperatures than many evergreen plants, like Olive and Viburnum, and are never seen in cold climates. They are however, especially vulnerable to the damage caused by autumn and winter pruning. It is essential therefore, to know what your tree or bush is and where it originates, before assaulting it with the saw and secateurs!
A simple rule, worth remembering, is that evergreen, or cold sensitive trees and shrubs should only be pruned from the spring through the summer. The end of the summer is probably the last chance for a major pruning operation. If the weather is still warm, but winter only a couple of weeks away, then it is best to accept that you have missed the boat and delay the pruning until the following spring.