PDF Overview
Raised bed gardening is a practice that goes back to at least 300 BC and the pre-Incan civilization of the Quechua. Their raised beds, known as waru-warn, also incorporated irrigation systems and were quite sophisticated even by modern standards.
At the heart of the practice is an awareness that the less the soil is disturbed, the better for it, the plants, and the gardener. This concept leads to several different implementations, all of which have specific advantages to recommend themselves. One commonality to note, though, is that all rely on the gardener to regularly replenish organic materials lost. This can be through incorporation, mulching / horizontal composting or root salvage. It must, however, be done.
Incorporation, the most commonly used method, defeats the goal of non-disturbance but is quick. The others meet that goal, but work on the natural schedule exclusively. Many gardeners opt to begin by incorporation (most of the methods described below do just this) at the time the beds are formed and then switch over to gentler means in the first growing season. The thinking seems to be “I have to disturb the soil to create the beds. I may as well take advantage of this mayhem to bulk it up with organic materials.”
Disturbing the soil will result in disruption of the natural processes it was already host to. It only makes sense, then, to fuel the re-establishment of these processes with sources of food and energy. Fibrous organic material, alone, provides the nutrition, the soil structure and the moisture retaining abilities needed for this task. This time of disturbance is also the ideal time to make any needed adjustments to pH with rock powders. Most plants favor a narrow pH range and it is easy to ‘kill’ a soil with such amendments, so always test your soil before changing its pH. This is also an opportune time to add greensand, gypsum or other mineral amendments.
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Four Main Methods
Method #1
The first method, and quite possibly both the best known and the most difficult to implement and maintain, is to dig a trench 3-4′ wide, one or two spade depths into the soil. Relocate the soil to the side. Replace the missing volume by filling the trench with organic materials such as compost, manures, fresh vegetable waste and so on. Then cap the trench with the displaced soil. The organic activity beneath the resultant mound will keep the soil above it loose. This promotes deep rooting and drought resistance. Further, any root able to reach down into the organic material will find a bonanza of nutrients to tap into and earthworms will be more than happy to consume the material below and bring it to the surface in the form of very rich and biologically active soil castings.
The draw back to this method is that the initial digging is labor intensive and will eventually need to be done again because, as the organic material in the trench is depleted, the soil level above will drop to fill the trench again. This depletion can be partially offset by incorporating additional organic material into the planting layer of the soil and counting on earthworms to help create new topsoil, but this just delays the inevitable. Subsequent deep diggings will be far easier to perform than the initial foray.
Method #2
A second method is to simply rototill the entire bedding area as deeply as your equipment will allow. This is followed by removing soil from the pathways and placing it in the planting beds which, again, should be roughly 4′ across if there is access from both sides or 2.5 – 3′ across if there is access from only one side. If the entire garden area is first well covered with compost or manures, this method results in soils with a good loft and beds that are (in comparison with the first method) quite easy to establish. The limitation here is that these beds are loose for only a fairly limited depth of 12-18″, have organic materials that will be quickly (2 – 5 years or so) depleted and may result in paths too low to drain well. Paving the paths with gravel or bark chips not only solves the drainage problem, but may also be quite attractive.
Method #3
Thirdly, it is possible to raise the beds to a considerable height by constructing boxes of some sort of reasonably enduring material. These boxes typically range from a low of 6-10″ (nominal lumber dimensions) to waist high of 30 inches or more.
The higher the boxes, the more trouble there is in making and filling them, but the less work there is in maintaining them. Because they nearly eliminate all stooping and reaching up high, they are a boon for those in wheelchairs, of advanced years, or susceptible to back problems.
Crops such as cucumbers or melons can be given a stout trellis to grow on. I use fencing on the back and two sides, allowing me to reach in from the front to weed, water, prune and harvest. Six by six fencing would be ideal, but my local garden supply store only carries 3 x 3 … so that’s what I use. This trellis is also handy for tying tomatoes. I knew that this system worked when I noticed that our neighbors cherry tree had become host to a runaway tendril from one of my boxes and was now sporting a dark green ‘Black Beauty’ refrigerator-sized watermelon! He didn’t think it was funny … but neither would he give the melon back.
A 4×8 sheet of exterior grade plywood will yield two bottomless boxes of 2×2′ dimension. Nail an 8′ long 2×2″ in each corner for rigidity. This will also serve as a place to mount a trellis for climbing crops. If you plan to use such a box for several years in the same location, you might want to begin the filling by cutting a bale of hay in half in its still-compacted state. As the bale slowly rots, it will shrink and cause the soil above it to remain continuously loose. Each spring, replace the subsidence with a shovelful of a soil and compost mixture.
And finally …
There is another method. I have not tried it, but it sounds quite reasonable to me. Lay out your garden into its future paths and beds. Place whatever organic materials you may have available on the future beds. Take soil from the paths and place it on the beds, covering the organic material. That’s it. Future organic material is incorporated into the soil as mulch and by leaving the roots of spent plants to decompose where they grew. Rather than pull the dead plants out at the end of the growing season, cut them off at the soil level.
The nice part about this is that, along with feeding the living organisms in the soil, the decomposing roots will leave literally miles of channels in the soil that water and soil gases can use. I’m going to give this a try as soon as I have my patio completed. It will be time to rebuild my (waist high) beds and I think I’d rather do things this way than dig down to the original trench bottoms again.
In Closing
*** Applying a generous layer of linseed oil, particularly raw linseed oil, to the wooden surfaces in contact with the soil will add years to the life of your garden constructions and thus make the best use of your funds. My current planting boxes are nearing the end of their life spans … but I have gotten 15 years out of (nominal) 1″ thick white pine by liberally sloshing them with raw linseed oil before adding soil.
*** To the extent possible, orient your planting beds along an East – West line. This allows the sun to do a better job of warming the soil in the spring, allowing earlier planting and germination dates. The later application of mulch will insulate the soil against excessive warmth. Most seeds can be planted as soon in the spring as the ground can be worked again. Attention to orientation will thus often pay off in earlier, more abundant yields since the plants get an earlier start and the harvest is also extended. Gardeners in warm climates are to understand that their needs differ from northern gardens. In some areas, it may even be possible to garden year around without resorting to greenhouses.
*** Root crops, especially long ones such as full-sized carrots, are extremely fond of loose soil — and raised beds are generally a great way to give it to them. If you will give plants organic soils, follow companion planting principles, mulch heavily and keep soils moist and loose with compost and well-timed watering, the earth will give up astounding harvests year after year.
