There are about as many ideas for a raised garden as there are gardeners who use them. It is rare to find two gardeners who use all the same techniques and materials. This is a blessing, since it means that it is difficult to go wrong building a raised garden, as long as you follow some simple guidelines.
Raised or not, plants in a garden have some basic needs: sun, proper growing medium, water, and nutrients. When you make sure your plants get the optimum amounts of these, your garden is likely to be a success. (I say likely, because in gardening, there are times when circumstances beyond our control make it difficult. Think drought, hurricane, unexpected animal visits.) As long as the above needs are met, you can have a field day designing and setting up your raised beds.
Remember to leave a wide enough gap between raised gardens. Three feet is often recommended as a good distance that allows you, your wheelbarrow or cart, the bags of mulch you are dragging, and the overhanging leaves of productive plants enough room to avoid injury and conflict.
Keep the garden beds narrow enough to reach in to weed, water, fertilize, plant and harvest.
For the minimalist, consider simply hilling (piling up) a low mound of soil slightly higher than the surrounding area. The mound can be linear, square, or any other shape that fits with your overall garden design.
Soils have what is called a natural angle of repose. It means that each type of soil (and there are many) will pile up with a certain angle to the edge of the pile. Soils with more sand and less organic matter will have a lower angle, and won't make a nice tall pile, and therefore, not a neat raised bed. Soils with more clay and silt and organic matter will be able to hold a steeper angle, and it will be easier to keep neatly piled up.
These raised beds require no siding material, but since they are just a bit higher than the ground around them they will drain a bit better. Edges need to be maintained by periodic hilling with a hoe. Aside from any soil amendments to improve nutrients and pH, the existing soil can be used. Low spaces between raised beds can act as mini-reservoirs for rainwater that drains off the beds. Mulch on the beds reduces weeds and holds moisture also.
The next step up is the bag garden. It may not sound very high brow, but it is inexpensive, simple to build, and can be quite productive for small spaces. It is a technique being promoted in developing countries where soil quality, labor and materials are an issue.
A large, sturdy bag, such as one used for grains, flour or animal feed is placed on the ground, with the open end up. A large food can, open at both ends and placed upright in the center of the bag, is filled with small to medium stones. Growing medium is then placed in the bag, around the can, up to the top of the can. The can is carefully slid upward, till the top of the stone column is just inside the bottom of the can. The process is repeated until the bag is about half full. At that point, stakes are placed around the bag for stability, and string or wire is used to secure the bag to the stakes. The stone and growing medium process continues upward again until the bag is full. The top part of the bag is secured. Small openings are cut into the sides of the bag. These can be planted with trailing plants or, really, any kind of plant with a fairly sturdy root system. The growing medium in the open top of the bag is planted, also. The center column of stones evenly drains the bag.
Keyhole gardens are another kind of columnar raised garden. Shaped like a keyhole, they are made, usually, from stones piled onto one another. In the center of the keyhole is a column for compost. Feedstocks (items that will break down into compost) are added regularly to the compost column, and plants are placed in the growing medium around the compost column. In areas where rabbits or similar garden pests are abundant, this type of garden can keep your vegetables out of their reach. It is also useful in areas prone to flooding or poorly drained soils.
Some gardeners are stuck with an impermeable or nearly impermeable surface, such as asphalt, concrete, or poor, compacted soil where they want their new raised bed. They will need to make a box with a bottom that holds growing medium but allows for drainage. Of course, you have countless options.
The material for the raised garden needs to:
hold its shape
not leach anything toxic into the soil and plants
be at least six inches high
allow water to drain out
Thousands of gardeners use found objects such as old bricks, cinder block, dresser drawers, or tires. Some find non-toxic sealants or liners if they are not certain that the support material won't leach.
Bricks, cinder blocks, or flat stones can simply be stacked neatly on one another, the individual pieces of each level placed over the point where the ends of two pieces on the level below meet. Some have put growth medium into the hollow parts of cinder block to grow herbs. Depending on climate, this may work, or may require much additional watering. Consider whether this will work for you.
For the creative do-it-yourselfer, hypertufa is a fascinating material (concrete, perlite, fiber and water) that is relatively lightweight and can be formed into all sorts of fanciful shapes, using forms made from cardboard boxes, old buckets, or any number of objects and colored with natural pigments. Remember the minimum six inch depth, and the need for drainage at the bottom.
A fancy variant of the raised bed is the herb spiral, a spiral of soil edged with stone or similar material, that rises to a peak in the center, something like a snail shell. Herbs that are low-growing and wide-spreading are planted at the bottom, and those that are tall and narrow are planted at the top, with a mix of plants in between that get taller and more narrow as you travel up the herb spiral. This avoids too much shading and crowding, but it still needs to be managed by regular harvesting. It requires that you do some homework, and understand the mature shape of each herb. This can be done with fruits, vegetables and flowers, also.
Several old logs that have been sitting in a corner of the yard were reorganized this spring into sides for a raised bed. I lined up two rows, parallel, for a length of about twelve feet, with logs at each end. I love the look of the darkened wood, the mosses growing on it. The decaying wood will last for several more years, providing beneficial fungal colonies for the soil and habitat for worms and other soil-building organisms. I simply filled the eighteen inch space between them with a mixture of soil, compost, coconut coir, and coarse vermiculite.
If you have room, and know how to use a saw, a drill, and a screwdriver, wooden box beds are a snap to construct. The ends of two-by-eights (I used untreated spruce) are attached with three screws. For extra stability, plywood triangles are attached at the corners of the bottomless beds. Plywood with one inch holes provide the stability for the beds with bottoms. I use the Square Foot method, in which I have dividers at one foot intervals across the top of the box, and I plant something different in each square foot. It is a pretty way to garden, provides me with quite a bit of variety, and is helpful for managing nutrients and pests.
You can hand water your raised gardens, or use a hose for overhead watering. The latter is the least efficient method of watering, and can cause problems for the plants if they are watered too vigorously or if a plant is infected with a disease that can be spread by spraying water.
For raised gardens, drip irrigation lines or soaker hoses can be used to efficiently provide water during dry spells. There is also an ancient technique, called the olla, for providing a steady supply of water during dry times. A non-vitrified clay vessel is buried up to its neck in the soil. Water is added to the vessel regularly, and it slowly releases the water into the surrounding soil.
By adding simple supports to the raised garden bed, it can be converted into a low tunnel (covered bed), protected from the elements, thus extending the growing season.
Now you have a list of ideas for ways to build your own raised garden, one that meets your needs, tickles your fancy, and can provide you with years of enjoyment and satisfaction!