Any salt will kill plants be they grass weeds or your choice flowers. Not the best of ideas as it stays in the soil and pollutes it. Dig, hoe and dig and hoe some more.

Dig out what you can then put down newspaper, whole sheets, two or three thick and cover with compost. If the grass is spreading from your lawn then use a spade to a depth of at least six inches and slot your newspaper in to the slot, push the soil back against the paper before laying the excess paper flat over the top of your bed. Rake some bonemeal in to your bed before doing the paper laying as it can rob the soil of nitrogen as it breaks down. The paper can be torn to fit around plants. You can never make your garden entirely weed free as they seem to have a built in attraction to the slightest amount of soil available to grow. This in turn allows a build up of soil around the plant and if you do not keep on top of it you will be back to square one.