Flower beds are meant to be for flowers and not the grass from your lawn or weeds. This extra vegetation competes for the nutrients in the soil and can weaken your flowers' health. It is best to monitor the garden and treat for weeds and grass that are growing up in the flowers as soon as you see it occur. There are several ways that you can kill this problem vegetation without harming your plants.
Pull weeds up by hand as you see them crop up. This is the most effective way to kill weeds without harming flowers.
Spread mulch several inches thick around the flowers. The thick material blocks the light and keeps the weeds and grass from being able to grow. Move it around the flowers with you hands to work it into all the crevices.
Spray the weed or grass, and nothing else, with full strength vinegar. Fill a spray bottle with vinegar and set it to a "stream" setting so that you can do pinpoint applications.
Lay down a plastic cover under the flower bed if you haven't planted flowers yet. The plastic cover will block air and light and kill the grass and weeds.