Killing Grass/Weeds Around the Base of Shrubs

I planted a (very large) row of shrubs 18 months ago, and they are growing well. I have had to spend some time cutting and pulling grass and weeds away from the base of the shrubs. I would like to poison the grass and weeds but am afraid of poisoning the shrubs.

Is there any safe way of doing this?

Is there any selective weedkiller / grasskiller that I could use that will not affect the shrubs?

You could use something to smother the weeds. eg bark mulch or a garden membrane. These would stop light getting to the weeds and they'll die.

See this thread.

Thanks.