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A Small Kitchen Garden in a Raised Bed

Several of my recent posts explained how to plan and build a small kitchen garden. Most recently, I showed the simple steps to create a planting bed in an existing lawn. In that discussion, A Small Kitchen Garden In Your Lawn, we got as far as removing sod, but we didn’t actually prepare the underlying soil for planting. Before we do that, let’s talk about alternatives to cutting garden beds into your lawn. Let’s talk about raised bed gardens.

Materials for my raised bed garden are minimal: landscaping timbers and 12 inch twisted spikes. There’s a lot of fuss about pressure-treated lumber leaching poison into the soil… and a lot of fuss that such leaching is negligible or non-existent. My raised bed retaining walls are of pressure-treated lumber, as are my tomato stakes. I’ll report in this blog when my health deteriorates as a result. If you fear leaching, use untreated lumber—something that rots slowly such as black locust. If you buy construction grade lumber, find a stable sealant and coat the lumber heavily before installing it. Perhaps four or five coats of spar varnish would extend the life your raised bed retaining walls for several years.

Raised Bed Gardens

Landscaping timbers are about 2.75 inches thick and 3.75 inches across. With rounded sides and flat tops and bottoms, these timbers stack easily. Their weight holds up well against pressure from your garden soil; they don’t bend easily. If you have your timbers cut to length at the lumber store (most will do this without extra charge), you need only a hammer and a drill with a long bit to assemble your retaining wall. If you buy timbers at full-length, you’ll also need a saw to cut them. (I’d use an electric saw, but the hand saw asked to be included in this photo.) For each rectangular bed you need four timbers of matching length for the length of the bed, and four of matching length for the width of the bed. For example, to create a planting bed six feet long and four feet across, you’ll need four 5’ 8.25” sections, and four 3’ 8.25” sections.

A raised bed is a planting bed that is higher than the existing ground. There are several circumstances in which you simply can’t grow fruits and vegetables in the ground in your yard. In those circumstances, if you want a small kitchen garden, you have only two choices: plant in containers, or build raised planting beds.

But a raised bed provides benefits beyond relief from poor site conditions. A raised bed…

  • …can reduce the amount of bending to work a garden bed
  • …creates a barrier between your planting bed and potential invasion of unwanted plants that propagate via roots, rhizomes, and stolons.
  • …can protect the soil from becoming compacted and in so doing reduce the amount of work needed to keep the soil productive

Strategies for Building Raised Beds

There are no rules for building raised planting beds. Because we’re talking small kitchen garden, you may be trying to cram a planting bed into a very limited, and perhaps oddly-shaped space. In many cases, the shape of that space will dictate how you build your raised bed garden. If you’re lucky enough to have at least some open space in which to work, you’re a little less constrained.

A simple test

Where you plan to put your planting bed, dig a hole about a foot deep and a foot in diameter. Fill it with water and check on it periodically. If it takes more than eight hours for the water to soak in, you should create a raised bed. Even once you till the soil and amend it with sand and humus, your garden bed will be surrounded by the compacted, non-porous soil that won’t absorb water in eight hours. In a wet season, your planting bed could become a mud pit: water will soak into the garden quickly, but it could take days to soak into the surrounding soil.

At its simplest, a raised bed could be no more than a heap of soil sitting on the turf, on an old concrete slab, or on just about any surface. However, when the heap has no barriers to hold it in place, it will flatten out from one season to the next and require regular rebuilding to keep it together.

Because of this, most gardeners build walls to contain the soil of a raised planting bed. Those walls might be wooden boards, concrete blocks, brick walls, or cleverly-placed rocks. The walls must be strong enough to withstand pressure from hundreds of pounds of soil. Also, the walls should be durable; if made of unprotected wood, they may rot away in a matter of three or four growing seasons.

The Raised Bed in My Small Kitchen Garden

My house came with what looked like a planting bed for a kitchen garden (along with many beds with ornamental plantings). This was a raised bed 13.5 feet square. I don’t know how the previous owner created the bed, but it seems likely to have been a three-step process:

You’ll be stacking the timbers two deep around the perimeter of your new planting bed. Start by laying out a rectangle according to the scale model in this photo. (Yes, I simulated the layout of your landscaping timbers using Popsicle sticks.) Drill holes through the timbers to accommodate the spikes. The drill bit should be the same diameter as the spikes, or a smidge larger; for my spikes, I needed a three-eighths-inch bit. Put holes about two feet apart, but at this point don’t put holes in the last four inches of the outer ends of the timbers.

First, I believe the bed’s creator cut out a square of sod following the procedures I illustrated in my previous blog post.

Second, the bed’s creator built a retaining wall around the excavated square using gardening timbers.

Finally, the bed’s creator added topsoil and tilled—mixing the added topsoil with the “Lewisburg Soil Clay” he or she had found under the sod.

In fact, it’s possible that the previous homeowner had intended to have an in-ground planting bed, had cut the sod, and had then realized a problem… and there were two obvious ones:

  1. Most of my lawn sits on clay. It’s amazing that grass survives in it.
  2. The garden bed is at the bottom of a hill that drains my neighbor’s yard. Any depression at the bottom of that hill fills with water during rain. Perhaps the former homeowner cut out sod for a planting bed, watched it become a wading pool in a damp growing season, and then converted it into a raised planting bed.

How Deep Your Small Kitchen Garden Beds?

The illustrations in this post show how I would build a raised garden bed to match the one that was in my yard when I bought the house. You can follow these steps to build beds of just about any dimension—though it’s easiest to build rectangular planting beds this way. But there’s an important consideration we haven’t yet discussed: how deep will your crops grow?

Make sure the timbers are in the right places, and drive spikes through the holes into the ground as far as they’ll go. (If you’re looking for a perfect rectangle, measure both diagonals and adjust the frame until the measurements are equal.) Then lay the second tier of timbers on the first, overlapping pieces at each corner as these photos illustrate. Drill holes. This time, align a timber as you want it and hold it in place as you drill through it and the timber beneath it. Drill a hole at a corner first, drive a spike through it, move about two feet along the same timber, drill a hole and set a spike, and so on. If you don’t maintain alignment of the timbers when you drill holes, setting the spikes can be quite difficult.

Most vegetables that produce above ground tend to have shallow roots; they’ll be comfortable in four inches of soil. Even some foods that develop in the soil, such as radishes and beets, do so near the surface. Root crops such as potatoes and sweet potatoes can make it in six inches of soil, but you may need to mound soil on top of exposed tubers that push to the surface—this can happen even in very deep soil.

Depending on the variety you choose, long tap root vegetables (carrots and parsnips, for example) provide the deepest challenge: some want to grow ten or even twelve inches deep (though there are many varieties that tend to be shorter). So, for greatest versatility, I suggest that your raised planting beds provide at least 12 inches of soil depth.

The inside corners of your retaining wall will look like this. When you’ve spiked all the timbers to each other and the ground, you’re ready to prepare the soil. We’ll talk about that in an upcoming post.

Here’s the trick: if your raised planting bed will be on existing soil, you might be able to include some of that soil in your measurement. So, on an existing lawn, you could build a bed four inches deep, and till down some ten-to-twelve inches, using six or more inches of the existing soil for planting.

That’s the situation in my yard: my raised bed provides only about three inches elevation above the surrounding soil, so my plants’ roots can still reach excess moisture that collects from the hill above my garden. If this becomes a chronic problem, I can add another two inches of soil before I’d need to build the walls of my raised bed higher.

In an upcoming post, we’ll explore other ways to create walls for a raised planting bed. As well, we’ll talk about soil amendments that might be necessary to prepare your garden beds for spring planting.

Here are some other posts about building raised planting beds:

  • Our Raised beds on Gardencentre TV « The Recycle Works Blog – We thought you might like to see this video of our raised beds being reviewed on Gardencentertv website. It shows both how easy the raised beds are to assemble and how versatile they are. They say a picture can say a thousand words …

  • Innovative Raised Beds – So instead of writing a longer post, I thought I’d share a photo of my traditional raised beds built from recycled cedar fencing (top) and some alternative raised bed ideas I discovered at The Garden …

  • Making a Raised-Bed Garden – Back in 2002 when we landscaped our yard and started our garden we decided to create a raised bed garden. We decided on raised bed gardening for a number of reason. The main reason being the soil in our yard seemed to be composed of …

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