While this post is part of a series that explains how to build your own raised planting bed, you could save much aggravation by assembling a planting bed from a kit. As you might expect, a kit costs more than twice what you pay for materials to build it yourself. Still, if you have absolutely no woodworking skills (or tools), seriously consider getting a kit. Follow this link for more information.
There are few limits to how you design a small kitchen garden. Once you’ve decided on location, you face the considerable challenge of designing and creating planting beds. So far, this blog has shown how to cut a planting bed into an existing lawn, and how to build retaining walls using landscaping timbers for a raised bed that sits five inches above an existing lawn—or another area with soil that can become part of the planting bed.
What if the soil in your future garden area simply isn’t usable? Or, what if there is no soil, but you happen to have a concrete pad left over from a demolished barn, house, garage or carport? Or, what if you just don’t want to bend down so far to reach the plants and weeds in your planting beds? These are all reasons to build deep raised bed gardens. This post explains how to use lumber other than landscaping timbers to build deep raised beds.
Small Kitchen Garden Raised Beds
In an earlier post on building raised beds, I suggested that at the very least, your planting beds should have soil to a depth of one foot. Were I to build on a surface through which roots couldn’t grow, I’d make my raised beds 14 inches deep. This way, I know that if I need to dig with a shovel, I won’t likely strike bottom when I push the shovel down with my foot.
Construction-grade lumber is typically milled from white pine trees. The wood is light-colored and light-weight, and it rots easily. If you build retaining walls for a raised bed garden from white pine, you’ll likely need to replace them in four-to-six years. Find a leech-proof, non-toxic sealant, and apply several coats before installing your raised bed.
You need to create a retaining wall to hold that much soil in a pile. Typically, people use lumber to build retaining walls, but there are many other fine alternatives. The downside of using lumber is that it has a tendency to rot; a wooden retaining wall in contact with soil can lose integrity in three or four years. Worse: in many climates, putting wood in contact with soil is like putting up a welcome sign for termites. If your planting bed will be close to your house—or snuggled up against it—take extreme caution not to provide a bridge for termites to get from the soil in your yard to the wood frame of your house.
Lumber for your Raised Beds
Termites and other agents that destroy wood have preferences. The most affordable lumber, for example, is pine, a very soft wood. Pine absorbs and holds water easily which leads to cracking and deterioration especially where the wet wood freezes, thaws, and dries out over the course of a year. Boring insects can make holes in pine easily, and microorganisms that cause rot find pine an easy target.
Hardwoods resist rot far better than pine does, but they are also considerably more expensive. To boot, hardwoods aren’t as easy to find in the same variety of dimensions as pine… and the very most rot-resistant hardwoods may simply not be available at all.
There are several varieties of cedar. You often see red cedar lining linnen chests and saunas, and white cedar used to build outdoor furniture and fences.
Cedar is a popular choice for building raised bed retaining walls. As it’s also popular for building decks and outdoor furniture, it may be available in a local store… but it is expensive.
Because it’s inexpensive and abundant, many people use pine to build raised bed retaining walls. But if you use pine, you’ll need to replace it every three-to-five years unless you treat it with something to protect it from rot. This is where the worlds of evil corporations, government regulation, an overzealous legal system, and fearful consumer gardeners intersect.
Pressure-Treated Lumber for Raised Beds?
From pre-history, people have turned to chemicals to protect wood from rotting. Creosote has proven extremely successful—it’s on telephone poles and railroad ties all over the United States. For many years, you could buy creosote-soaked railroad ties to build retaining walls in your own yard… but this has fallen out of practice: creosote protects wood effectively because it has stuff in it that’s really bad for biology. We have a lot of biology going on in us, so we shouldn’t be getting into creosote.
Pressure-treated lumber typically has a green tinge to it. If you use it in your garden, wear a dust mask when cutting it. Thoroughly clean up any sawdust you create and put it out with the trash. Don’t burn or compost unused pressure-treated lumber.
The lumber industry has found alternatives to creosote, and in most places you can find pressure-treated lumber that is extremely resistant to rot. In the United States, until recently pressure-treated lumber had arsenic in it, and experts have argued the danger of coming into contact with such lumber. About 20 years ago, I read a discussion in which an expert explained that if you collected the soil around a piece of pressure-treated lumber and ate it, you might consume a dangerous amount of arsenic after twenty or thirty years of daily consumption; even within an inch of pressure treated lumber in the ground, there was no detectable arsenic.
Despite these types of findings, all do-it-yourself gurus warned people not to use pressure-treated lumber where humans might come in contact with it. You certainly wouldn’t use it in a produce garden. And, because of lawsuits and finger-pointing, the industry agreed to stop putting arsenic in pressure-treated lumber destined for home use. In the US, a ban went into effect five-or-six years ago.
Does this mean you can use today’s pressure-treated lumber to build raised planting beds in a small kitchen garden? There’s still a lot of paranoia; but, living in the United States, I’d do it. To address any discomfort you might feel about it, there are two precautions you can take:
- After assembling the retaining wall, line the inside of the wall with heavy plastic before you fill the frame with soil.
- Stain or paint the surfaces of the retaining wall that face out or up; if you’re going to come in contact with the wood, paint it to lock in whatever chemicals are in it.
Here’s a terrific article about pressure-treated lumber, in case you want a thorough understanding about what was wrong with pressure treated lumber when it was made with arsenic: Does Pressure-Treated Wood Belong in Your Garden?
Plastic for a Small Kitchen Garden?
With the near necessity that every new house includes a deck, the building industry has introduced several materials to replace and out-last wood. Composite materials are waste wood—sawdust, for example—mixed with synthetics such as plastic and then formed into boards. These boards have the same dimensions as certain standard cuts of lumber and they have similar woodworking characteristics. That is: you can cut them with a saw, drill holes, and hold them together with screws. They’ll also take paint, though they don’t require any. Best of all: they’ll last for 40 or more years with no maintenance.
I’ve also read about “plastic lumber” which I can only guess is not a composite, but is plastic shaped and textured to resemble boards. If you don’t object to surrounding your garden with plastic, then composite decking is the most maintenance-free material for building a classic “wood-frame” raised planting bed.
I visited a home improvement store so I could estimate the costs of materials for raised planting beds using different materials. Here’s a summary:
|Retaining Walls Cost of Materials for 4′ x 10′ Bed
These estimates assume a 4 foot by 10 foot bed with 14 inch sides (technically, they’ll be 13 inches when assembled)… except in the case of the composite materials. The dollar amount in that column will build a 12-inch-deep raised bed.
One other caveat: I haven’t included costs for paint or sealer or plastic sheeting. If you build with untreated pine boards, figure to spend another $15 to $25 for sealant to protect the wood against moisture. I’d apply several coats of marine spar varnish to all the wood before assembling the retaining wall. That should extend the life of the walls for a few seasons.
Enough about Lumber
At this point, you have a pretty good idea of the materials you might use to make a traditional raised bed for a small kitchen garden. In an upcoming post, we’ll list materials to buy and the tools you’ll need to do the work. We’ll also explain how to assemble a frame and set it in place.