Pruning is used to prevent or correct problems with a plant.  Plants should be selected based upon many factors, one of which is mature size.  Why select a shrub that will grow to be 6' tall and continually prune it to keep it at 3' tall?  This is poor planning.

As most trees and shrubs develop next season's flower bud during the summer, pruning at this time will remove the flower buds.  One client of mine has a large lilac against their house which has not bloomed.  Their previous gardener had been pruning at the incorrect time and had removed all the flower buds!  

For spring blooming trees and shrubs, pruning should be done immediately after flowering. This allows next season's blooms to form.  Some examples include lilac, Nanking cherry, forsythia, and roses that bloom once per season.

Some trees are considered "bleeders" and will leak sap when pruned when not actively growing. They must be pruned during summer and early fall.  Some examples include Birch and Maple trees.

Conifers usually don't require pruning except to produce denser growth or shorter stature.  These are best pruned before the end of July.  Pruning after this time can produce a dwarfing effect and also remove next season's buds.

If your plant has dead or diseased areas, these should be removed at any time. You do not want to allow any infections to spread throughout the plant.  For diseased tissue, ensure removal of an additional six inches of healthy wood.  Clean your pruning tools between cuts with an anti-bacterial product such as Lysol, and do NOT apply any wound dressings.  

I know many of you desire a low-maintenance weed free garden and who doesn't?  You may be tempted to add landscape fabric in an effort to eliminate or reduce weeds.  Don't do it!  

Over time, organic matter will find it's way on top of the landscape fabric.  This will then provide a media for weed growth on top of and through the fabric. As most landscape fabrics are covered with inorganic or organic mulches, weeds will also grow into these mulches.  As many weeds are perennial in nature, the entire root must be removed to eliminate regrowth of the weed.  This becomes difficult when the roots are growing into the fabric and mulch.  If you thought weeding was tedious before, you will be pulling your hair out once you have a tangled mess of weeds, landscape fabric, and mulch.

In addition, your garden plant roots may also become tangled into the fabric.  Any attempt to remove the fabric can damage the roots.  If you like to split your perennials and occasionally move plants around, this task becomes much more difficult.  

Landscape fabric will degrade with time, especially when exposed to sunlight.  It is not a permanent solution. The best solution for your garden is an organic mulch with no landscape fabric.  See my blog article on Benefits of Mulches.