Raised Bed Gardening
There are many reasons to consider raised bed gardening, including poor existing soil, increased accessibility, and stubborn persistent weed control. Regardless of the reason, raised bed gardening is a centuries-old technique that deserves some consideration for not only vegetable gardening, but also flowers and shrubs.
The simplest raised beds are just elevated areas with walkways between them and no containing structure. This method is good for small areas and is beneficial where poor soil conditions prevent good plant growth. The beds are flat top mounds, usually six to eight inches high and no more than four feet wide, ensuring easy access to the center of the bed. Be sure that the soil brought in to make the mound is high quality and does not contain pathogens or contaminates like lead or pesticides. Start by working up the top 2-3 inches of soil and wait for it to dry. Next, spread a 4-6 inch layer of good organic material or compost. Top that with the soil and work the two materials together. Shape the beds so that there is a 45 degree angle on the sides. Provide a 14-16 inch path between beds.
Let the bed rest for a week before planting. Avoid stepping in the bed at any time. As the season progresses, the soil will settle, but the mound should remain. Each season, add some organic matter to the surface of the bed to replenish what was lost.
More common is a contained raised bed, using wood, stone, brick, or plastic. Avoid any treated wood in vegetable gardens. Many raised beds are available for purchase from the very basic to the very elaborate. The first consideration is the need. If the bed is being designed for easy accessibility for people with limited mobility, the sides should be higher, between 12-24 inches. However, if the bed is simply to mitigate poor soil, the depth can be on the lower end of that range. If it needs to be wheelchair accessible, the height should be 30-36 inches. In any case, the width should be kept to four feet or less so that access across is easy and there is never a reason to step into the garden. Maintain at least a two foot path between beds, ideally mulched to keep weeds at bay.
Once the bed is constructed, the next question is the fill material. Some landscape businesses have begun to sell soil specifically designed for raised bed gardening. This is a mix of top soil, compost, and a drainage material. Otherwise, use a mix of good quality top soil, compost, and drainage material such as perlite or vermiculite in equal quantities. Mix it on a tarp, then fill the bed. Do not fill the bed with only top soil as it is too heavy and does not have the organic matter and drainage material needed for good plant growth.
On an annual basis, incorporate several inches of compost at the end or beginning of the season. A good practice is to plant a cover crop (oats is a good choice) every 3-4 years to give the organic matter a boost. Plant it in the fall and turn it under in the spring. Not only will this boost organic matter, it will help loosen the soil and make it more productive.
Carol Shirk
Certified Master Gardener