For people used to gardening in regions where the winters are freezing cold, the question of when to prune trees and shrubs is thought of more or less in the following terms. Deciduous plants, that is those that drop their leaves in the fall, being hardy to cold, should be pruned during their seasonal dormancy, i.e. the winter, whereas evergreen trees and shrubs, excluding conifers, to the extent that it is possible to grow them at all in cold climates, are not touched until the spring, or at least until all possibility of frosts has passed.
It is also known by many home gardeners and presumably by all professional ones, that pruning deciduous plants in the spring can be highly detrimental to the future health of the plant, due to the loss of sap that would result from pruning in the spring.
However, for those of us gardening in mild winter climates, particularly in areas where frosts are virtually unknown, the issue is somewhat more involved. In fact many a serious mistake has been made because of a misunderstanding of deciduousness and its implications. These mistakes involve pruning a certain type of plant at the wrong time.
The phenomenon of leaf drop occurs in three main circumstances. One of these is when certain plants drop their leaves as a means of reducing water loss during the hot, dry season. The leaf drop that takes place though in cold climates is a genetically programmed response allowing broad-leaved plants to survive the freezing temperatures in the winter. It is a process that starts towards the end of the summer, becomes most obviously visible in the autumn accompanied by often spectacular leaf colors, and terminates at the beginning of winter with the actual fall of the leaves.
Gardeners in cold winter climates are naturally restricted in the number of species available to them. Where temperature fall below -10c, it is highly unlikely that any evergreens, conifers excepted, can be grown at all. In places where winter lows range between say -5c and -9c, it is sometimes possible to grow evergreens like the olive or many species of Acacia.
However in regions where the winter lows hover around the 5c mark, typified by nights that are chilly but nonetheless frost -free, it becomes possible to grow a far wider range of garden plants, including species that originate from sub-tropical and even tropical habitats.
Examples would include Delonix regia, Jacaranda acutifolia, Tipuana tipu and Peltophorum dubium, amongst many.
In such conditions, species belonging to this category drop their leaves, not as a dictate from their genetic code, as is the case with naturally deciduous plants, but as a temporary response to the relative cold of a Mediterranean winter night. And herein lays the trap! Many people on seeing a tree of subtropical origin out of leaf, unwittingly mistake it for a truly deciduous plant, connecting deciduousness with cold-hardiness. The trouble is that the precise opposite is the case. Conditionally deciduous plants are often or not highly sensitive to cold, and are therefore liable to be seriously damaged by winter pruning. *
So what can you do to avoid making such a mistake? In the absence of specific knowledge regarding this or that plant, the simple answer is to find out the natural habitat of a plant before pruning it. This is a piece of information that most people gloss over when they read up on any particular plant in garden literature, but as I hope will be clearer now, it is information that can have significant consequences for the future of some of your trees and shrubs. Therefore, if a tree is out of leaf in the winter but is of tropical or sub-tropical origin, it should not be pruned until the spring or the summer, together with the evergreen plants.
*Note: Plants that are liable to be cold sensitive to any degree, can be seriously damaged by winter pruning, as they have less capacity in the cold season to resist the fungal and bacterial infections, brought on by pruning cuts.