Pruning Trees
The focus of this pruning information is for amenity trees not orchards or fruit production and is aimed at Brisbane region.
It's important to understand some plant biology first as to what pruning does to the tree and then you'll understand why we try to minimize it. That's right, less is best, smaller cuts are better and it's fairly commonly accepted that pruning cuts should be kept under 100mm or 4" diameter. Hence why we try to formative prune a juvenile tree rather than make large cuts trying to make a big tree fit our landscape ideas.
Pruning is wounding and removing resources
Trees have a balance of foliage to roots, I call this the foliage:root ratio. How it all works is the leaves of the tree make food via photosynthsis. Water and nutrient is drawn up through the root system, up through the vascular system of the tree to the leaves. When the tree photosynthesizes sugars are made to feed the tree. These sugars then travel throughout the tree including the roots.
When we remove foliage we remove the food factory of the tree. The tree simply makes less food for itself. Also, surplusses of sugars are stored in tree parts including branches, so that too is now discarded and unavailable to the tree. Then there's a cut, an open area where insects and pathogens can enter the tree. The larger the cut the longer it will take for the tree to grow over and seal the area (if it's cut right!) so we try to keep cuts small to make this sealing process quicker.
Reactions to pruning
In response to the above the tree will try to compensate by adding foliage either where you cut or elsewhere. Study's have shown that trees tend to replace the foliage mass removed via pruning within 3years. Now don't confuse foliage mass with size, we are talking volume of leaves, you may now have a shorter branch but it will be denser with foliage.
Another reaction is new shoots growing, mainly near the cut. This can in most instances be undesirable as they tend to be weakly attached and grow rapidly and spindly. We'll cover this in the section below on topping and lopping. But the heart of the matter is the tree is responding by trying to replace the foliage lost.
Trees unlike us or animals etc dont repair damaged cells. What they do is compartmentalise the damage, like try to wall it off from the rest of the tree. There's 4 barriers, vascular system is shut down, the harder heartwood slows or stops inward progression, rays slow or stop radial progression and new cambium growing over the wound.
Callus wood should start to grow around a correct pruning cut and eventually seal the cut. This callus wood later becomes woundwood and is some 40% stronger than the original wood and has a slightly altered make up as it's composition is designed to fight any pathogens. Internally the tree attempts to block the advancing pathogens by setting up barriers and shutting down vascular activity. In instances where there's branch collars the tree actually has increased defences by having protection zones, harder altered wood. Chemical changes occur within the tree, PH alters and anti bacterial/fungal properties are made. However this is also species related as some trees are better compartmentalizers than others.
So as you can see a whole bunch of stuff is going on, and the more cutting you do the more stress a tree goes under. On mature trees it's accepted that no more than 5% to 10% of the foliage should be removed, on young vigourous trees no more than 25%. As per any rule of thumb there's exceptions and the one that comes to mind is pollarding certain species, we'll cover that below in the pollarding section.
Where to cut a branch
This would be the simplest thing to know however you rarely see it done correctly. People have an unusual way of complicating the cuts, but I can show you how easy it is because there is basically only 3 cuts.
You first need to understand the types of branch attachments. There's 3 types and rather than reinvent the wheel I'll link to a thread at Tree World about it, it has diagrams and all.
When to prune
In Brisbane we tend not to have a genuine dormancy period, many of the trees are evergreens and winter is certainly warm enough for trees to grow. Growth can be varied depending on soil moisture levels as winter is the drier season. So for Brisbane I can comfortably say you can prune whenever you like.
Pruning deciduous trees at the tail end of a dormancy period is preferable. When the trees come into bud and start to grow the sealing process will commence on pruning wounds.
Trees getting sunburn
A word of warning. Some trees may get sunburned when limbs that have been shaded for life are suddenly exposed to sun. This tends to be thin barked trees and certain rainforest trees. Keep this in mind, if you are going to open up the interior canopy of say a mango tree in the middle of summer you risk sunburn on the limbs which is real bad news. So to play safe I prune some species in winter. You can also apply a whitewash waterbased paint to the limbs to prevent sunburn, you might see this on avocado and mango farms after heavy pruning.
Pollarding - a rare and precision pruning method
First understand what this practice is as there are numerous forms of bad pruning called pollarding, which is simply not the case, they are topping or lopping and covered later.
Also it's important you understand the species that it applies too, not all trees can take it. Generally
the species are deciduous (drop leaves for winter) and because pollarding removes all growth it's good to have
a species that atleast has become accustomed to being defoliated. These deciduous trees store energy for the spring burst
of leaves and shoots so they are better able to recouperate from a 100% prune. Some of the suitable species
are willow, lime, ash, field maple, holly, hornbeam, oak, plane and beech. See something familiar? You dont grow those up here! We're
in a subtropical environment not temperate.
Pollarding is also commenced on young trees, it's not what you do to a mature tree. It's a common practice to commence pollarding at stem diameters of 75mm to 100mm.
Pollarding is the removal of all stems back to their point of origin, the pollard head. You do not cut off any collars or pollard heads, you cut just outside them. New stems grow from the pollard head. The trees have to have this repeated every year or two. The original reason for this pruning was to keep a regular supply of wood and fodder annually from the same trees and to also contain the size and form of trees. Correctly done to the right species doesn't seem to decrease the lifespan providing it commenced at an early age.
The pollard heads are very hard (woundwood) and also loaded with energy as a storage point just near the wound for
defence and to supply new growth. Decay seldom advances much past the pollard head.
You'll see some attempts at this but I can honestly say the last time I saw a proper pollarded tree in real life was some 20 years ago in Adelaide, it was a London Plane tree. Some people try to practice this on their Crepe Myrtles but they dont have the technique right nor do they usually start it on a young specimen. My advice is keep clear of it and leave it to a professional