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How to Build a Raised Planting Bed

March 16, 2000

Building your raised planting bed garden

The best time to begin building a raised bed is during the fall or winter, while your primary gardening projects are on hold. By the time spring arrives, the soil will be settled into place and you'll be ready for planting. Raised planting beds should be set up in north/south orientation to allow the maximum direct sunlight to both sides. Avoid using creosote or pentachlorophenol treated lumber (such as railroad ties) for the frames, to prevent these chemicals from leaching into the soil and injuring your plants. Use pressure-treated lumber, redwood poles, logs, concrete block or brick (Warning!... the cement in concrete blocks will raise soil pH over time, so soil pH testing will become necessary in the future).

Special situations

If you intend to use this planting bed for bog plants or other moisture loving plants, or... if you are using treated lumber for the construction, the entire bed should be lined with black plastic sheeting before adding the soil.

Creating A Raised Bed

Start by marking out the perimeter of the bed using stakes and string (or a hose) as a guide.
Dig a shallow (2 inch) trench inside this guideline so that as you lay out the first level of your chosen building material, it is recessed slightly into the ground. If the ground is uneven, dig the high end deeper into the ground, so that the end result is more or less level. This will make further construction much easier.

A simple level...

If you don't own a level, you can make a simple one which will work just fine for rough landscaping purposes... Lay a straight 2x4 across the surface you want to check. Fill a glass with water almost to the rim (a glass with a line around it would be perfect). When the glass is set on the 2x4, the distance from the rim to the liquid will be the same on both sides of the glass if the 2x4 is level. Raise or lower the most appropriate end.

Depending on your situation, design, and needs, you may opt to add all or part of the topsoil to the raised bed at this time. It is sometimes easier to have most of the soil in place to act as a back support as you build the retaining wall.
In other cases it is more prudent, and easier to add the soil as you build the wall.
(Don't forget to mix the bottom layers of the added topsoil in with the native soil... Remember that any additional compost, manure, leaf mold, or other organic material which you add to this planting mix will provide the soil with essential nutrients for plant growth, and at the same time, greatly improve drainage. Now is by far the easiest time to do this!)

Add successive layers of your building material

Tall planting beds...

If your raised planting bed will be over 12-18 in. high, it may be necessary to take additional measures to ensure the stability of the retaining walls. Landscape timbers should have holes drilled through them, so that a piece of steel re-bar (used in concrete work) can be driven through all timbers, and into the ground.
If the retaining wall is to be made of loose stones, they should be piled so that the base is the widest point and subsequent rows taper inward. The largest stones should be used in the bottom courses. This will help to prevent the weight of the soil from "blowing out" the side walls.

Each subsequent course should be set slightly farther inward.
If you are using logs or timbers, align them so that the seams are staggered, and nail them into place with 6 inch or longer galvanized spikes. (Pre-drilling the nail holes will prevent the wood from splitting.) At the corners, the exposed ends should be alternated (log cabin style) for strength and stability.
Concrete blocks and bricks should also be set so that the seams don't line up. For strength, steel bars should be driven into the soil through the existing holes in the blocks. The holes can then be filled with cement, soil, or gravel.
Building the retaining wall with stones requires the same abilities as to do jigsaw puzzles. The base of the wall should be considerably wider than the top (pyramid style). Interlock the stones together as closely as possible, using small rocks and gravel to fill the voids.

Fill the bed with quality topsoil, allow it to settle for a week or so, and add more soil as needed. Because the soil will continue to settle, you may have to add more soil the following year.
Dig a small trench around the entire perimeter of the planting bed, and fill it with gravel to divert excess water away from the pathway around the bed, during rainy weather.

Happy Gardening! Your raised bed is now ready for planting.

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