Roundup is the best product, but you've got to be careful when working around the flowers in your bed. Roundup is a non-selective herbicide, so it'll kill any plant it comes in contact with. For grass and "tender" green weeds, a ready-to-use Roundup spray will work terrific. Shield your good plants and spray carefully and low-to-the-ground on a day when there's little or no wind. The product must stay on the plants without being washed off by rainfall or sprinklers for at least 24 hours, but 48 hours is even better. If any of your weeds have woody stems, you might need to use what's called a brush killer; a product similar to Roundup but tougher on woody plants.