How high should a raised bed be? I find 6-8 inches (15-20 cm) to be a very practical height for growing most crops. With less than six inches, you may start losing some of the benefits of drainage. Also, lower beds require that you bend over more to plant, weed, and harvest. Waist-high beds are sometimes built for handicapped individuals or for luxurious herb or flower gardens, but require substantially more investment in materials and skill to assemble.
How wide should a raised bed be? In the commercial world of machine-formed raised beds, the crop often dictates the row spacing. Some growers will adjust bed size based on the crop being grown. With permanent beds constructed of solid edging materials, we have to set the size and work with that regardless of the crop.
There are two primary considerations in determining bed width. One consideration is efficient use of edging materials. The wider the bed, the more square feet of growing space per linear foot of edging material. This is true regardless of the shape, but becomes especially pronounced with long, narrow configurations. Anything less than 3 feet (1 m) for beds that are equally accessible on both sides is probably inefficient from a construction cost perspective.
The other consideration is being able to reach the center of the bed comfortably. One of the main reasons we build permanent raised beds is to avoid compaction. We really don’t want to step out into the bed at any time. This puts the maximum width at about 5 feet (1.5 m) for large adults. I find 4 feet (1.2 m) to be the maximum practical width when children will be assisting, and even that might be a little wide. Two feet (.6 m) is not too far to reach when kneeling and working a few inches off the ground, but it becomes a stretch for comfort when doing something like picking green beans 2 feet over and 2½ feet up.
Both of these considerations together cause me to recommend permanent raised beds between 3 and 4 feet in width. I have worked most extensively with 4-foot beds but plan to install mostly 36- to 38-inch-wide (1 m) beds in the future, because of the ease of use and greatest flexibility between crops.
How about bed length? In some cases, the building materials dictate length. For wood planks, 8-10 feet (3 m) is maximum between cross ties. The simplest way to do this is to limit each bed to this length and use the ends to hold in the sides. Longer wooden beds can be built from planks if they are tied together at intervals along the length of the bed with rot-resistant wood or galvanized wire between. Such ties can get in the way of weeding and transplanting, so I’d only recommend this technique for perennial plantings.
Where multiple beds are being constructed adjacent to each other, one could drive stakes into the ground and support them with cross pieces under the level of the walkways from bed to bed. I’ve not had great success relying on stakes by themselves for support.
Thicker wood, such as logs or sawn beams, allow you to build longer beds between supports. If you have rot-resistant logs available at little or no cost, this can be a viable method. The negative of wider materials such as these, is the fact that your bed width is effectively narrowed by the width of your edging material.
Edging materials that are self-supporting, such as masonry or some types of steel, as I described last month, do not create limitations for the length of beds. In those cases, there are other practical considerations. If a bed exists by itself surrounded by lawn, it could be longer. But where multiple beds exist side-by-side, you probably don’t want to go more than 40 to 50 feet (12-15 m) long. It becomes inefficient to move from one bed to another if the distance is
too long.
When filling a raised bed initially, what goes inside is very important. This is the place to create garden soil, not necessarily just work with what you have at the location of your garden. Topsoil should be the majority of what fills our beds. However, that should be amended with compost and, depending on the soil type you have, things like peat moss or vermiculite could be very beneficial.
Biochar is a great bed amendment and is increasingly available in many areas of the country. If you have a source of wood readily available and live where open fires are allowed, you can make your own biochar. Just make sure you do the research on technique before you start.
In any case, you need a source of good topsoil. One can certainly dig down the walkways and build up the centers of the beds with material found at the immediate location. But, depending on the lay of the site, doing this could create drainage issues. You really don’t want to drop your walkways below the level of the surrounding terrain.
In most cases, it is better to fill beds with material brought in from off the site. Do you have an area on your property with extra topsoil that could be stripped away? If you have any place that you would like to build a driveway, parking lot, or future building site, these would be obvious places to harvest topsoil.
Regardless of where topsoil comes from, I strongly recommend solarizing it before using in raised beds. This technique can give you a big boost in weed control. Lots of viable weed seed in your imported bed materials is a recipe for frustration. Perennial roots are even worse. A few pieces of bindweed root imported into a raised bed is a disaster! Yes, I know from experience. Shiver and sigh!
Take it from someone with painfully hard-won experience. Solarization of bed-fill topsoil is well worth the bother. The technique is simple but takes time and the right weather conditions. I will go into detail about solarization next month.
My final consideration for construction of raised beds is walkway width and type. A walkway does not need to be very wide for walking. But, in most cases, we need room to comfortably kneel; this requires at least 16-18 inches (.5 m). If we have a wider bed that we need to face squarely to comfortably reach the middle, the walkway needs to be at least 2 feet wide.
Paved walkways are nice for the use of wheeled carts. They are especially worthwhile in more formal garden settings, where gardens are incorporated into living spaces, adjacent to patios, or in a high-traffic area.
Wood chips and other forms of organic mulch are more commonly used between beds in less formal gardens dedicated to the growing of edibles. On the plus side, organic mulches provide a softer kneeling surface. However, the softer surface is not as conducive to using wheeled machinery. To maintain an organic mulch of sufficient thickness without decreasing the desirable height benefit of the raised bed requires a little taller construction to begin with.
Another possibility for wider walkways is to maintain them as lawn and mow in between. In this case, I often bury wood planks or flat stones along the edge of the raised bed to provide an impervious edge for the mower to run over, eliminating the need for string trimming. If stones are used, it’s a good idea to start with a layer of plastic to prevent weed growth between the joints. The downside to this technique is that rocks or boards make the working surface hard. The extra string trimming might be worth the comfort of working on grass.
