Over the past several years at the Noble Foundation Horticulture Center, experiments have been conducted with various bed widths. Crops were successfully grown in 6-foot-, 5-foot-, 4-foot-, 3 1/2-foot-, 3-foot- and 20-inch-wide beds. Every width has advantages and disadvantages.
Each gardener should consider several factors before deciding on a bed width. Some of these include:
Value. As bed width increases, construction cost per square foot decreases. A 5-foot by 20-foot bed constructed of 2-inch by 12-inch lumber is comparable in cost to a 3-foot by 20-foot bed of similar construction (approximately $100, not including labor or soil medium costs). The material cost per square foot is considerably less for the 100-square-foot bed ($1 per square foot) compared to the 60-square-foot bed ($1.66 per square foot).
Utilization of space. Another advantage of wide beds is increased utilization of space. Generally speaking, the wider the beds, the fewer the pathways. Where space is limited, more bed space and fewer paths translate into more yield per square foot of overall garden area.
Climate control devices. Many plastic row covers and mulches are available for use on 40-inch or narrower beds. Also, the narrower the bed, the easier it is to apply these devices. With wide beds, some of these materials must be custom made.
Personal stature. Tall individuals can easily reach the center of a 5-foot bed. Most people find it more convenient to use narrower beds. On wide beds, it is often necessary to place one foot on the bed and the other on the path, in order to reach the center. Stepping on beds is not encouraged because it compacts the soil. However, this practice is easier on the gardener because it takes strain off the back. This may sound trivial, but to anyone with a bad back, placing a foot on the bed makes all the difference in the world. No detrimental effect on yield has been noted from occasionally stepping on beds, especially when a large growing area is utilized. Anyone not wanting to step on beds can place a 1-inch by 12-inch standing board across the bed top. This will reduce the possibility of soil compaction.
At the Noble Foundation, our bed width of choice is 40 inches. This width combines the attributes of both narrow and wide beds.
Theoretically, beds can be constructed to any length. At the Foundation, most beds are 30 feet in length. A bed 40 inches wide and 30 feet long provides approximately 100 square feet of usable space. Most consumer fertilizer and pesticide products give rates of application on a 1,000 square feet basis. Moving the decimal one place to the left gives a rate for a 100-square-foot bed. This is the beauty of growing in this size of a raised garden – it makes computations easy! For example, 10 pounds of 13-13-13 fertilizer per 1,000-square-feet, a common rate for most garden soils, translates to one pound for the 100-square-foot garden.
Bed height is determined by the gardener's personal needs, budget and the nature of the materials with which the beds will be constructed. Most vegetable crops extract the vast majority of water and nutrients from the top 12 inches of soil. Any raised bed constructed on asphalt, concrete or other surface denying root penetration should be constructed to a height of 12 inches.
Good results have been obtained at the Noble Foundation in 6-inch high beds constructed over poor soil. A 6-inch-increase in soil depth above the existing grade will greatly enhance drainage.
It appears that beds constructed 18 inches and above in height require extra fortification, often involving the need for anchored support columns or posts. High beds constructed of such materials as corrugated sheet metal require support posts because they lack rigidity.
By contrast, railroad ties can be stacked several high without the need for support. Small box-type beds are able to stand alone without additional support. The 4-foot by 8-foot box bed made of interlocking landscape timbers is an example of a stand-alone bed. Plan on "beefing up" any raised bed that will double for a bench.
There is no hard and fast rule as to the width of paths between beds. Anyone planning to use wheeled equipment such as a wheel barrow, garden cart or lawn mower between beds should make sure the paths are wide enough to handle the equipment. All of the Noble Foundation's 40-inch by 30-foot beds are on 5-foot centers. Path width was sacrificed for production area. Although the 20-inch-wide paths do not permit access by garden cart, they provide easy access by one person with a harvest bucket.
Ideally, beds should be built on a north-south orientation. This alignment minimizes shading of low-growing plants. If it is impossible to orient the beds north and south, don't worry. Excellent results have been obtained growing crops in east-west oriented beds as well. In this situation, keep the tallest growing plants on the north side of the garden and try to group similarly sized plants together.
Make a scale drawing of the raised bed garden on graph paper. Once a bed width is selected, adjust bed length on the sketch to fit the site in the most efficient manner.