The best time to prune spring flowering shrubs is within two weeks of the finish of bloom.* After that they set their buds for the following year.* The best way to prune a lilac is to cut the oldest canes flush to the ground.* Never do more then one third of the shrub each year.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.