Now is the time to think about all those new garden beds you want to add in 2012. Here is a step-by-step primer on how to construct a raised bed the right way — from the ground up.
The simple, cruel fact of ornamental gardening is that even when we do every other facet right, failure is all too common if our soil is bad. Improving our soil from the very start is an absolute must — especially when starting a new bed.
Fortunately, if you follow the basics, creating a well-made raised bed is fairly easy. A one-time tilling will break up compacted soil and the incorporation of organic matter into your soil at the same time will provide an important component of long-term good soil structure: beneficial microbial life.
Your choice of amendments (based on the type of soil you have) is very important. For topsoil, choose soil similar to your own. Got clay? Buy a clay-based soil. Despite oft-repeated bad advice, do not add sand or gravel to “improve” drainage in clay soils! Some suppliers will make a custom mix of amendments. My default mix is three parts ordinary shredded topsoil, one part mushroom compost and one part pine fines. If the soil is particularly devoid of nutrients or for heavy feeding plants, I’ll sometimes go with more compost. For plants that need lean conditions, I’ll use less. Soil tests are a good idea and can address specific needs. You can also ask your county extension agents specific questions pertaining to soil test results.
The ideal is for your finished garden soil to mimic a good natural soil. From the subsoil to the surface, there should be a gradual increase in organic matter. You do not want a really great soil lying on top of a poor one. This creates an interface, which adversely affects the movement of water through the soil. There must be a seamless transition.
By creating above-grade beds of rich soil, you develop a site that accepts and holds moisture, drains nicely and is nutrient rich — that proverbial “rich, moist, well-drained soil” all the gardening books love so much.
After marking a rough border for your new bed with paint or chalk, spray the existing turf or vegetation with a glyphosate-based, non-selective weed killer. Wait a week or more for the vegetation to die.
Once the vegetation is dead or dying, till the bed as deeply as possible. Be sure to till only when the soil is dry enough to crumble into small, powdery fragments. Never till if the soil is sticky or clumping.
Spread 2 to 3 inches of topsoil mixed with your amendments of choice over the bed. For this first round, you can be a bit stingy with the organic matter.
If possible, avoid walking on the bed as you fully incorporate your soil mix with the existing soil by tilling. A good thorough tilling of old soil with new will prevent the creation of an interface, which can interfere with water movement in the soil.
Repeat steps three and four until you reach the desired depth. As you add materials and your bed becomes deeper, you can gradually increase the ratio of amendments to soil. Ideally, the soil of a general purpose ornamental garden bed consists of about 7 percent organic matter. If you have more than 10 percent organic matter, you can start to have issues.
After giving your bed a nice clean edge, give it a final grading.
You’re ready to plant! Use boards to distribute your weight across a wider area while planting so you don’t compact your newly created loose, friable soil.
Mulch with 2 to 3 inches of hardwood mulch, pine bark or pine straw.
Water the entire bed thoroughly. The soil will settle some, but you will find that your raised bed of rich soil will hold moisture nicely, drain well, retain good soil structure and successfully host a wide variety of plants for many years to come.
(Photos courtesy of Scott Beuerlein.)