Raised flower beds are an ideal solution for properties with clay soils and limited space. They can also act as a showcase for bedding plants and annuals, even if you must install them on concrete. To provide proper drainage for your raised flower beds, replace the base of the bed with pea gravel to catch the water and rainfall. A series of drain holes drilled into the sides of the bed at the same level as the pea gravel will allow the water to flow outward.
Drill small holes in the sides of your raised bed. Place holes about 15 cm (6 inches) apart in a zigzag pattern between 5 and 7.5 cm (2 to 3 inches) above the ground or concrete slab.
Line the bottom of your raised bed with hardware cloth. Top the hardware cloth with a layer of newspaper about an inch thick.
Add a 5 to 7.5 cm (2 inch to 3 inch) layer of pea gravel atop the newspaper. Level the pea gravel with a sturdy, steel-tined garden rake.
Fill the raised beds with soil. Water well to test drainage at least 48 hours before planting your flowers. Saturated soil should release the water into the pea gravel and out the bed's side drainage holes.
Enrich the soil with a shovelful of compost for each square foot of soil to help improve soil structure. For deeper-rooted flowers, use open-base containers that can sit atop the raised beds.
Try scoring the concrete first with a masonry drill before you install your raised bed to roughen the concrete and create channels for water drainage.