Pruning your trees
Silvicultural pruning is carried out to remove the lower branches of plantation trees. Its primary purpose is to produce clearwood. Branches are removed from a section of a tree's stem to allow a knot free timber to form around the defect core.
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Is pruning expensive?
Pruning is the most expensive silvicultural operation. Before you start pruning, look at market access and how this will affect your investment – work out the costs and benefits before you start any work.
Types of pruning
There are 3 pruning methods – form pruning, sail pruning and clear pruning.
Form pruning develops a strong, dominant central leader. This is important for species which tend to grow multi-forked stems. Depending on the species and growth, you'll do this in the trees' early years, from age 2 to 6 years.
This pruning is done with secateurs or light pruners.
Sail pruning (crown lightening)
Sail pruning is done on exposed sites to:
- reduce the sail effect of the crown
- maintain stability in younger trees.
It involves removing 2 or 3 branches.
This pruning is done with pruning shears, a jacksaw (for bigger branches) and an epicormic knife (for removing epicormic shoots).
Clear pruning produces clear timber, free of knots. You do this by pruning all the branches off the lower stem when the tree diameter is 10 to 18 centimetres. This pruning is done with long handled secateurs (pruners) or saws.
The stem diameter of the largest pruned whorl is called the diameter over stubs (DOS). Following pruning the tree will:
- continue to grow in diameter
- repair the tissue around the prunding wounds (occlude).
The zone of inferior wood is called the knotty core. The knotty core includes the DOS and occlusion zones. Further tree growth outside the knotty core produces "clearwood".
When to prune trees
Timing clear pruning operations is vital if maximum clearwood is to be gained. A target DOS normally sets the programme for pruning operations.
If the interval between pruning lifts is too great, both stem size and branch size will be large. This will result in a large DOS and, therefore, a large defect core. Larger branches also means greater pruning cost. On the other hand, it is also expensive to prune trees too often since it costs money every time the stand is visited.
How to prune trees
All branches being removed should be cut as close to the stem as possible without either:
- cutting the branch too far out from the stem (leaving coat hangers)
- damaging the branch collar or stem by pruning too close.
Correctly pruned branch stubs heal over with minimal grain distortion and knot formation.
Things to be aware of when pruning
- Branches should be pruned cleanly, without leaving broken ends and slivers of wood or bark.
- Pruning should be at right angles and as close as possible to the tree stem.
- Damage to the tree stem should be avoided.
- Remove any cones, green shoots and green needles growing on the tree stem inside the pruned area. They may develop into branches if left.
A pruning operation is often called a pruning lift. This is because the bottom of the green crown is being lifted to a new position on the tree. A lift is the length of the stem from the top of the section pruned previously to the base of the green crown.
Two or three lifts are usually sufficient for Radiata pine. The first lift will usually be at around age 4 to 6 years (depending on how quickly the trees are growing) and the others at 1 to 2 year intervals. Four lifts may be needed on some species such as Cypress.
There are 2 main types of pruning lifts:
- A fixed pruning lift prescribes that all trees in a stand are pruned to a specified height.
- A variable lift prunes each tree on its merits to leave enough green crown for the tree to continue good growth. An advantage of variable lift pruning is that tree growth and vigour can be optimised. A disadvantage is that, unless the last prune is done to a prescribed height, the final crop can contain a mixture of pruned log lengths. This requires careful management.
Tools to use when pruning
The tools you select for pruning will depend on:
- branch size
- pruning height
- personal preference.
The standard pruning tool in New Zealand is pruning shears. These are long-handled secateurs which can handle branches up to 35mm in diameter.
A jacksaw is generally used on larger branches but is much slower than pruning shears. Light weight chainsaws are sometimes used for pruning. For safety reasons these should only be used by experienced and trained professional operators.
Cones, green shoots and green needles can be cut off with a knife. If they're small, they can easily be brushed off by hand.
Using ladders safely when pruning
- Aluminium ladders are often used to reach branches above 2.5 metres.
- A safety helmet is essential when working off a ladder.
- A safety strap which passes around the tree and clips onto the operator’s belt is recommended.
- On flat or easy land, a ladder which leans against the tree is adequate.
- On steeper slopes, a ladder which clips onto a tree is safer and easier to work with.
- Pruning shears are preferred to jacksaws when pruning off a ladder to save time.
More information on pruning
Check out the below websites for more information on pruning.