Growing Food In Raised Beds
There are a variety of reasons for building raised beds for vegetable gardening. Reasons include compacted/clay soil, soil that has been tested positive for lead (often from old paint), a sunny spot that happens to be in a driveway. For some, raised beds can provide a dual function of growing food and delineating outdoor space. Raised beds can be any height and built from a variety of materials, ideally ones that are being reused for this purpose. Raised beds sometimes have a bottom (with drainage holes) if they are going to be on asphalt or to keep out gophers (wire mesh bottom is cheapest and most effective). There are a few things to keep in mind when you decide to put a raised bed in your garden.
The layout of beds in small gardens is often predetermined – there is an ideal sunny spot for a bed. If you have more space, consider a lengthwise east to west layout (ideal for sun), wide enough paths for wheelbarrow access (about 3 to 4 feet) on between beds and over all garden design.
Using wood with paint on it can introduce lead to the soil, and treated wood contains arsenic so should not be used near any food crop. Redwood and cedar both work well, and are resistant to rot. Also a good option is composite decking. Often people have leftovers of various shapes and sizes from building a deck.
Beds can be up to 2 feet tall for wheelchair access or as low as 6 inches. If there is only going to be access on one side, 3 feet wide is the maximum. For a bed with access on both sides, you can go to 4 feet.
Drip irrigation is the best way to use the least amount of water for the most plants. Plan your irrigation before building your beds.
A good option to determine how a bed will work in your garden is to use an old hose to lay the bed out on the ground. Walk around it, imagine planting in it. You can also purchase sun meters to determine how many hours of sunlight an area of your garden gets. You can always plant in that area of the garden and see how a favorite garden crop grows before building a raised bed.
Further reading on raised beds can be found here:
http://ucanr.org/freepubs/docs/8059.pdf (scroll down to raised beds)