Hi Nay,
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.
http://www.gardenersnet.com/lilac/lilac02.htm
http://www.ipm.iastate.edu/ipm/hortn...993/lilac.html
http://www.bbc.co.uk/gardening/basic...eshrubs1.shtml
Newt
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.