One of the best ways to reduce tillage and increase our enjoyment of gardening is to construct permanent beds. When we stop tilling, it is important to avoid compaction as much as possible. On a garden-sized scale, this means separating the growing area from walkways. In many cases, the best way to do this is by constructing raised beds and elevating the surface of the soil.
I have tried gardening with permanent beds the same level as the walkways. There are several potential problems with this. First, if you use mulch to control vegetation growth in the walkways, you’ll end up with raised walkways and subsequently sunken beds. This is not a good situation, as it becomes harder to reach the growing beds for planting, weeding, and harvesting.
I am told that in desert environments, raised walkways with sunken beds are an advantage and were constructed on purpose by Native Americans. In the desert, significantly sunken beds help protect tender plants from the wind, and the edges are in shade for some of the day. This reduces evaporation. In more temperate, well-watered climates, sunken beds are to be avoided.
There are several reasons why growing beds in most climates should be raised above the walkways. I have tried using grass strips for walkways and mowing between garden beds. Without a solid edging material between the grass and the growing bed, I found that maintenance was too high with grass inevitably creeping into the growing beds.
If we attempt to maintain walking paths at the same level as growing beds in many temperate environments and most soil types, we will end up with too much moisture in the growing beds for some of the season. Compacted walkways do not absorb as much water and naturally increase the concentration in the beds.
It is common for commercial growers to create raised beds by simply using machinery to move soil into a raised bed, leaving a sunken walkway between. This mostly involves tillage and the re-creation of beds every year. It is possible to use low-till methods to maintain these beds permanently by using a coulter to re-form the walkways and bed edges every year without disturbing the centers.
This is feasible for commercial growers with acres of production. However, for most home gardeners, I highly recommend constructing raised beds with permanent edging materials. Many different materials can be used for permanent raised beds, but some work better than others. Putting a little thought into the construction can reduce maintenance requirements in the long run.
What materials work well for edging raised beds? It really can be almost anything solid and rot-resistant. Wood, metal, stone, concrete, plastic, and rubber are all useful bed edging materials.
Wood is probably the most commonly used. Wood used in contact with soil needs to be naturally rot-resistant. Species such as locust, ironwood, redwood, and several types of junipers and cedar provide naturally rot-resistant logs and boards. Pressure-treated wood can be used for ornamental plantings, but avoid growing food in beds edged with pressure-treated lumber. Pressure treatment involves using poisonous ingredients for the purpose of killing any bacteria or fungi that attempt to consume the wood.
Metal can be used for some designs. Use either galvanized steel, stainless steel, or aluminum. A couple of gardening catalogs used to advertise a raised bed kit that was basically heavy-duty aluminum flashing snapped together into circles of different sizes that you could stack on top of each other. Filling each level with soil created a pyramid effect. This was functional but a bit flimsy, and the center of the pyramid was out of reach from the edges.
A more practical use of metal was the very first raised beds I ever built. I used a hammer and a three-foot-long board with a groove down the center to drive eighteen-inch-long by three-foot-wide sections of metal roofing ten to twelve inches into the ground in a straight row. This was an extremely fast, inexpensive way to build a raised bed of any length. The limitations of this method include the necessity of having a stone-free soil; it would be impossible to drive this thin metal into stony soil. Also, it is inexpensive only if you have access to used or otherwise scrap sections of metal roofing. The top should be covered with J channel or screwed to a strip of wood to protect from the sharp edge. Also, though this is a fast, easy way of building a bed in stone-free soil, the aesthetics may leave something to be desired. The look may be more industrial than beautiful.
Stone and concrete are perhaps the best materials for creating permanent borders and raised beds. They are physically the hardest to use because of the weight. It is difficult to create edges that stay in place with small stones, and large ones are heavy to move. Natural stones are usually not uniform, and it can be difficult to make a nice stack that will stay in place without being excessively wide, which works against the purpose of being able to reach into the center of the bed from the walkway.
If you want to build a nice permanent raised bed system, consider using twelve-inch-square manufactured pavers vertically for the edges of your beds. Lay one to two pavers for the walkway between the beds. You could also use different sized or smaller bricks for the walkway, but it is important to have a solid masonry walkway between the beds to support the vertical pavers.
Without additional support, about two-thirds of any thin bed edging material needs to be buried to support the pressure of the raised bed. If you utilize a solid unyielding walkway, you can get six usable vertical inches out of the twelve-inch paver; without it, probably not more than four.
This is the ultimate of raised-bed construction. However, along with substantial cost, it requires a lot of labor and some skill to do a neat job. The same idea could be accomplished with poured-in-place concrete, but that requires even more labor and skill.
The latest versions of garden-catalog raised-bed kits most often utilize recycled plastic lumber or a wood plastic composite. These look good, go together quickly, and are light and easy to handle. The downside is cost, and they’re mostly limited to relatively small beds.
Another small-scale alternative is used rubber tires for bed edging. Round beds can be rather space inefficient from the aspect of mowing around the outside, but tires do last a long, long time, and I haven’t found any evidence that they leach harmful chemicals. The best tire raised beds I have seen are constructed from truck or tractor tires with the upper side wall mostly cut out, allowing planting next to the edge of the bed. Tires are easy to paint and can be hidden with earth-tones or decorated to the individual’s taste.
Next month we will look at more details and considerations in building permanent raised beds, including size, fill materials, and some construction tips to reduce maintenance.
