Weeds steal nutrients and water from flowers. Most are unsightly and spread rapidly taking over a garden quickly. Weeds can propagate through underground roots and stolons, runners and seeds. Raised beds make weeding easier because you don't have to bend over quite as much. Stopping weeds before they get growing is easier than getting rid of them once they're established.
Established Planted Bed
Remove the plants from the flower bed by digging them up with as much soil around the roots as possible. Place on the ground outside the raised bed. Mist the roots and dirt with water and cover with plastic. Garbage bags will do.
Remove the dirt from the bed.
Cut landscaping fabric to fit inside the raised bed. Place the fabric on the surface of the ground inside the bed.
Replace the soil. Plant the removed plants back in the bed.
New Raised Bed
Cover an area of the garden that hasn't been planted but has good soil with black plastic for two weeks during the growing season. The plastic kills any weed seedlings and dormant seeds because the plastic retains heat. The soil gets too hot for seeds to sprout, in effect sterilizing the soil.
Water the area where the raised bed is to be built. Cover with a sheet of clear plastic. The plastic retains moisture and heat which encourages weeds to sprout. When weeds have sprouted, which should be in a week to 10 days, pull them out.
Build the raised bed. Place a 3-inch layer of newspapers in the bottom of the bed. The newspapers retain moisture and will decompose over time. It also smothers any lagging weed seeds that might sprout.
Fill the bed with a mixture of equal parts of the sterilized garden soil, purchases top soil and compost. Plant.