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! 

The deck in my back yard needs power washing, but it is surrounded by plants, mostly perennials and clematis. Can you suggest some cleaning solutions and/or methods to clean the treated lumber without harming the plants?

This is a difficult question to answer since new cleaning materials and methods come about every year. A good place to seek advice on which solutions are safe to use would be your County MSU Extension Agent (the phone number can be found in the phone book under the government section, county name, MSU Extension). Keep in mind that power washers can be damaging to plants just by the force of impact against the foliage so provide protection with canvas tarps or cardboard. If cleaning solutions are used, be sure to wash off foliage with your garden hose as early as possible and soak the surrounding soil as well to dilute any solutions that may enter the soil.

When planting clematis, I saw where someone cut the young plant back to just a couple inches in order to stimulate a thicker growth habit.  Is this recommended for our area?  Does it apply to all types of clematis?

When planting dormant clematis (either new or transplanted) it is beneficial to hard prune back to 6 to 12 inches to promote a fuller vine. If the plant is already growing, keep the plant intact for the first season so the leaves can nourish the root system and aid in establishment. You may then hard prune the following spring to promote the thicker growth.