Research has shown, and certainly my own experience agrees, that blackwood growth rate and stem form are positively correlated. The faster the blackwood grows, the better the stem form and the less pruning is required to create a single straight-stemmed tree. Good growth rates are achieved by good site selection, good establishment techniques and good ongoing management.
But some pruning is inevitable. For a variety of reasons blackwood loses its growing tip and then goes on to develop multiple leaders that need to be managed. Here’s a recent example of form pruning to help keep these blackwoods on the straight and narrow. It’s not hard work, and can be enjoyable.
The following excerpt on blackwood pruning comes from the NZFFA website:
There has been an increasing acceptance that pruning is essential in managing blackwood. The method has no resemblance to pruning radiata pine; it is not difficult, but requires a commitment to visit the trees annually during the establishment of the 6 metre stem. There are two stages, which overlap:
· form pruning
· clearwood pruning
Blackwoods have poor apical dominance. Periods of stem growth are interrupted by abortion of the shoot tip. When growth resumes, several shoots compete for leadership. A blackwood stem is therefore formed by segments of straight growth which are interrupted by zones of disturbance which contain double or multiple leaders. The aim of form pruning is to identify and remove these competing leaders while they are still small (< 3cm diameter), and this requires an annual pruning visit during the formation of the 6 metre butt log. Long-handled pruners are useful.
The aim is to confine the defect core. Clearwood pruning is done annually, starting at about year 3, and is complete by about year 8. It is carried out in stages, in which the largest branches on the stem are removed first, using a 3cm calliper. The trunk is then pruned to the diameter of your defect core (10 to 12cm). No more than a third of the foliage is removed at one visit. It is recommended that about 3 metres of the crown is left after each lift. Ladders that grip the trunk should not be used.
Perhaps not the best description of clearwood pruning. I would define the aim as controlling side branching, and then to progressively removing all branches from below to a height of 6 metres, while retaining about 3 metres of green live crown after each pruning.