I have several globe-shaped yews that I trimmed around late June, after the first flush of growth. There was another flush of growth this fall that needs a little trimming to maintain the tight, globe shape. I have heard some say that nothing is supposed to be trimmed after August 15 and then others say a final, late fall/early winter pruning is OK. Please advise.
As a general rule, heavy pruning should not be done after August 15. The reason for this is that trimming stimulates new growth which may not have sufficient time to harden off before the onset of frost and cold weather in the fall. Late fall or early winter pruning of evergreens is also not recommended since the open wounds created at the tip of each shoot will be likely to dry and cause further dieback which will require further trimming come spring.
In this case, one of two methods may apply. The first would be to selectively remove any feather growth (any which extends beyond the main body of the plant) with a pair of hand pruners. This will usually stimulate little, if any, growth. In the event that much more needs to be trimmed, the second approach would be best to use. Wait until early spring (around forsythia bloom time), then trim away as much as you would like. This means looking at a larger, more natural-looking plant through the winter, but since evergreens provide great winter color, the more the merrier!