Tips on Raised Garden Bed Construction

Of course, first, you need to decide on a location. See our article on planning a raised garden bed to guide you in choosing a good location.

Lay out the Perimeter

If you’ve purchased a kit ( Yardiac.com has over 100 raised garden beds to choose from), there will be directions for preparing the area for the bed.

If the bed has a rectangular shape or some other shape with straight lines, use string or twine to outline the outer perimeter of the bed. Attach it to stakes put at the corners of the rectangle. For an irregular shape you can use rope to outline the edges. Pay attention to the surrounding elements of the landscape (for example, leave enough room to get the lawn mower between the bed and any adjacent fences or walls.)

Eliminate the Existing Vegetation

You will have to start preparing the area a few days or weeks ahead of time, depending on how you want to clear out the existing vegetation. Start with the larger plants, shrubs, and the like. Remove those larger plants with loppers, hand saws or or even a chain-saw if necessary. You will have to dig or pull out the roots. To kill the remaining grasses and weeds you can apply a systemic herbicide (like Roundup) to kill perennial weeds and prevent them from returning. If you would rather take an organic approach you can kill vegetation without herbicide by covering the bed area with a large clear plastic sheet anchored over the area for up to 60 days. The intense heat created under the plastic will eventually kill the plants. Once the site is bare of vegetation, till the area thoroughly, and you may want to remove the remaining roots to clear out the soil.

Erect the walls

This will vary based on the choice of materials. For metal edging which usually comes in varying lengths, you will connect them with stakes installed in overlapping notches. Install them along the perimeter of the bed and hammer the stakes into the ground to hold them in place. Don’t drive them all the way in until you have place all the edging in place to make sure you have everything aligned properly.

To build a bed with brick or concrete block walls, you may want to put down a footer for the wall. You may want to pour a concrete footer, especially for brick that is over a foot high. For a short cinder block wall you may consider using concrete pads (12 or 18 inch square) put into a sand base (similar to a patio construction). For the concrete foundation you may consider using forms to shape the edge, but if you are careful you can avoid that. After the footer has cured for a few days, the brick wall can be installed. For brick you will need to mortar the bricks in place, as well as cinder blocks if you plan to install them more than a couple of block heights.

Stone walls are the most attractive, but tend to be the most work. Depending in the size, you may need a hydraulic lift to put them in place, or you may be able to use a wooden ramp to roll them into place. A dry stone wall (one with no mortar) still needs to have some dimensional stability, so the base needs to be made wider than the top of the wall, so choose your stones accordingly. Make the wall wider at the base than at the top, with the stones canted inward for stability. If the wall is to be more than 24 inches tall it should be mortared.

Using landscape timbers can be the easiest way to install the bed, although it’s not usually considered the most esthetic. Once again you need to prepare a footing, or dig a level trench to lay in the first level of timbers. After laying in the foundation and making sure it is level the you can add the next layers. Anchor the corners by driving a rod through the ends of the timbers into the soil to create an interlocked pattern. Check every level of timbers to be sure they remain level.

Install Your Irrigation System

After the walls are installed and before the soil is added is usually the right time to plumb in the irrigation system. If it is going to be its own zone, be sure that the control valve is in an area that can be easily accessed after everything is installed.

Add Soil Mix

If you have aggressive lawn grasses, like our southern favorites bermudagrass and St. Augustine, you need to install some type of weed barrier inside the wall. Then you are ready to add your soil. Sandy soil will not hold water well, while clay soil generally doesn’t drain well. A mixture of the two, a loamy mix will be good for most plants. Adding decomposed organic matter to any soil will improve it. Peat moss, compost, or mulching materials are good, along with cottonseed mulch, which will add nitrogen. If you can’t find a good mix of soil, in addition to organic matter you may want to add perlite to improve the drainage.

As you put the soil in the bed, be sure to shape the grade to fit your irrigation needs. Typically , sloping away from the center will allow the bed to drain well. In arid climates you may choose to do the opposite and forma a slope towards the middle of the bed to do a better job of retaining moisture.

Finally, especially for ornamental beds, you may choose to put a mulching material on the bed to suppress weeds around your desire plants. Otherwise an organic mulch of some sort is usually a good choice.