It would help to know what your are trying to control. 
If it is creeping bellflower then Roundup is the best option. It is VERY hard to kill because it has very deep roots. To use Roundup and not hurt the other plants in your garden cut the bottom out of a paper grocery sack, put the cylinder created by cutting off the bottom around the plant you want to kill and spray the plant inside the bag, gently remove the bag and treat the next weed.  Do this with every weed. Roundup can also be applied using a paint brush and paining most of the leaves on each plant. Roundup breaks down quickly in the environment and if it is applied responsibility does little harm to the ecosystem. This strategy will work on any weed.

Good luck, it is pretty easy to use and what I do.