Pollarding is a type of radical pruning that involves cutting off all top branches of a tree. The branches are carefully cut back to a stub of a few inches. The process normally spurs the tree into a regrowth of more branches, which give pollarded trees a very distinctive look.
Pollarding is a very useful technique for a variety of reasons.It is commonly used in urban environments to prevent root invasion and control the overall size of the tree. Once a tree gets to a desired size then heavy pruning, leaving just a stump, will stunt further growth. It is important, however, that pollarding is done to a tree at the right time and then repeated every few years to maintain both the look and size.
The other traditional role of pollarding is to encourage fresh growth, use as livestock feed and source of firewood.