Just about any other arborist will insist that any pollard is a bad pollard. I am not any other arborist. I have no issue with pollarding certain trees properly.
Pollarding is severe pruning that removes all growth that developed since the last pollarding procedure, leaving only a main trunk and a few main limbs. It is done while trees are dormant in winter, and must be repeated either annually or at least every few years, before the resulting growth develops into major limbs. Pruning must be very thorough and neat, leaving no stubs.
Most new growth develops from where older growth had been pruned away during the previous winter, with only a few adventitious stems possibly developing on the main limbs or trunks. Distended ‘knuckles’ develop where this growth repeatedly gets pruned away and regenerates. All subsequent pollard pruning must be done only on the outside of these knuckles, not below.
It may seem easier to cut entire knuckles off with fewer big cuts rather than cutting all the secondary growth off with so many more cuts. However, as new growth develops, the many small cuts on the distended knuckles will be compartmentalized (healed over) much more efficiently than fewer but larger wounds. Wounds that compartmentalize too slowly stay open to decay.
Once pollarded, a tree will always need to be pollarded, or at lease pruned regularly to compensate for compromised structural integrity. Secondary growth is innately vigorous and heavy, but weakly attached to the main limbs.
Pollarding is done to produce an abundance of lush foliage, to produce an abundance of twiggy growth, or to deprive a tree of bloom. Pollarded mulberry trees provide lush foliage to feed silkworms. Pollarded willow trees provide many uniform limber canes for basketry. Pollarded privets are unable to bloom and bother those who are allergic to their objectionably fine pollen.
Well, enough about pollarding.
I pollarded a blue gum, Eucalyptus globulus, for the second time earlier this year. It was done dangerously late in the season, and at the same time that the roots of the tree were brutally damaged by relocation. (It is a canned tree that rooted into the ground.) The tree has no branches, but only a single ridiculously bare trunk with a silly new knuckle on top. Oh, the shame!
As you can see, the unfortunate tree has not grown much since then. It is now getting to be September, so the tree will not be growing much through autumn. As much as I would prefer to pollard this tree annually, I will likely not pollard it this winter, but instead let it grow for another year before pollarding it again. The blue juvenile foliage is exquisitely aromatic, but scarce.
For this picture, I could have moved the tree away from the fence that it is tied to for support, but the barbed wire somehow seems appropriate.