Raised beds make an ideal growing environment for perennial and annual flowers, allowing for improved drainage and light non-compacted soils to encourage ample root and plant growth. Building raised flower beds does not have to be expensive--in fact, with some persistence and ingenuity, you can build raised flower beds for free.
Bales and Bags
Hay bales can make an ideal, if temporary, inexpensive raised bed building material, according to the University of Florida IFAS Extension. It is sometimes possible to get hay or straw bales free, particularly if bales get wet or mildewed and are thus rendered unusable for animal feed. However, with some creativity you can bootstrap the hay bale idea into any number of other free materials useful for raised bed construction. Tightly baled newspapers, wrapped with string, can be tied to the desired height and will quickly weather to an unobtrusive yellow-gray, stone-like edging; trailing plants growing over the sides will further obscure the newspaper bales. Alternatively, tightly pack raked leaves into doubled black lawn and leaf bags, place a heavy rock at the top of the bag, tie it tightly, then invert it so that the tie and rock are on the ground. Line the bags up in the desired raised-bed shape and fill the area with dirt and organic material. Plant trailing plants or vines between the bags; the roots will lock them into place.
Wood is the most common raised bed construction material, according to the Florida State University IFAS Extension. But this doesn't have to mean expensive newly-purchased lumber. Look for logs along roadsides, in neighbor's yards after a windstorm, or at a construction sites where crews are often happy to give away downed trees rather than pay to dispose of them. Municipal construction dumps, transfer stations or recycling centers often have tree trunks as well as discarded building lumber. Inquire at lumberyards to see if they are willing to give away twisted or split, unusable boards. Also look for barns, sheds or other wooden buildings being demolished or falling down. Be aware that scavenged boards from construction sites may be pressure-treated, but this is less of a concern for flower beds than for vegetable beds, unless you intend on growing edible flowers. Weathered barnboards or downed trees may not last as long as purchased treated lumber--but you can't beat the price.
Salvaged Construction Materials
Concrete blocks, bricks and pavers also make ideal raised bed building materials, according to the Alabama Cooperative Extension system, and these materials are fairly inexpensive when purchased new from construction material suppliers. They can also be free when salvaged from demolition projects or someone else's re-landscaping venture. Check your municipal construction debris dump; call local landscapers or construction firms who do demolition work and let them know what you are looking for; and keep a close eye on local free-materials exchange boards and websites like FreeCycle.org to salvage free construction materials for your raised flower beds.