Grass/weeds amongst my lilacs...

[tpierrepont]

I would really appreciate some advice on how to improve the health of my lilac trees. We have a row of lilacs along the back of our property that seem to be overtaken with grass and other weeds growing among them. They do not flower like they used to and really don't want to lose the trees altogether. What can I spray on these weeds/grass without harming the trees??
Tammy