The simplest raised bed is just mounded soil, high in the center, tapering to the edge. Most have a border made of almost any material; lumber, logs, rocks, bricks, cinder blocks, etc., to retain this soil. The goal is to hold the growing media above ground level and truly define a place not to walk.
Raised-bed gardening offers several advantages:
• Soil raised above ground level warms up faster in the spring.
• High-quality soil, and amendments, can be used more effectively and efficiently.
• They are smaller and easier to maintain than a conventional garden.
• They are more accessible for adults and kids.
• Avoids compaction of the growing media
• Improves drainage
• Helps level slopes
• Are adaptable to almost any shape or configuration
• Are easier to water, but require more water
• Can be used for flowers, shrubs, trees or vegetables
Most can work within a 14- to 18-inch reach, and with tool in hand 24 inches is near the max. A raised bed should be no wider than 24 inches and, if accessible from both sides, 48 inches. Remember, we do not want to be in the growing bed. Six to 10 inches of depth is good. If it’s under 6 inches, we defeat the purpose, keeping the roots in good soil. If it’s over 10 inches, the side supports need to be much more substantial. There is no need to line the inside of the bed unless there are large holes in your retaining wall, and a lining on the bottom is certainly not recommended.
Unprotected lumber used outside will deteriorate. Common pine will not last long while cedar, although the most expensive, will last the longest. There were some concerns with the old CCA treated lumber. This was phased out in 2003 and is no longer available. The new outdoor treated lumber (labeled ACZA or ACQ) is acceptable to use. There are new, man-made, boards — basically recycled plastic — that are starting to show promise.
As we age, or become physically limited, raised beds certainly offer an advantage in bringing plants to an easier level. The structure needs some serious attention but wheelchair height is not out of reach. Kids, too, benefit from a defined space. It suddenly becomes more acceptable as their space and not just a hole in the landscape.
The weeds will not magically disappear. They just become more destruction friendly. New media will limit the weed seed population and allay some undesirable soil pathogens. As in all growing habitats, keep in mind the benefits of proper watering, fertilization and the benefits of mulch.