Post G23-009: New garden beds. Working harder, not smarter.

At the start of the pandemic, I recycled some political yard signs and bamboo into a set of raised garden beds (Post G05). After three years of intensive use, a) those are now in disrepair, and b) I know a whole lot more about gardening.

My plan is to replace those beds with something better. With St. Patrick’s day just one week away — the traditional day for planting potatoes — I can’t procrastinate much longer. Time to finish pondering and start shoveling.

This post documents the final design. The next post will show the construction.

Background and design: Block, Bamboo, and Polypropylene

1: Design criteria

1.1 What I don’t want: Tall, narrow beds using lots of virgin materials.

If you look around the internet, you will come across beautiful examples of raised beds constructed this way. You might see a bed built waist-high using cedar 4×4 lumber, with a lovely two-foot-wide planting surface at the top. As in the upper left picture above.

To me, these are primarily backyard decorations, and only incidentally places to grow food. That is usually made clear by the presence of a decorative bench, sundial, or other lawn ornament near the raised bed.

Raised beds like these encapsulate exactly what I don’t want. I don’t want a design that uses lots of everything — lumber, soil, space — for a modest payoff in terms of plantable area. It’s inefficient, except when viewed as a yard ornament. (Or,plausibly, for a wheelchair user or other person unable to reach down to the ground.)

1.2 What I want: short, cheap, and easily replicated.

Source: The Gardening Cook (top), Home Depot (bottom).

I’m looking for something that will retain less than a foot of garden soil, so that it doesn’t wash away. Just a shallow, open-bottomed box. Enough dirt that I can grow a few root crops such as potatoes and sweet potatoes, before those roots hit the impenetrable clay soil that is native to so much of Virginia.

After three years of intensive gardening in thes beds, I have a pretty good idea of the features I’d like to have built into their replacements.

Irrigation.
Trellis.
Deer defense.
Mowing strip.

In addition, I’d like to make these:

As cheaply as possible
Using recycled or grown materials as much as possible.
Easy to expand as needed.
Easy to dispose of at end-of-life.

Irrigation, in particular, is so convenient and so cheap that I would not consider building new beds without it. (See Post G22-026, G22-027, G22-037). In particular, if you use half-inch irrigation line, your backyard garden irrigation system can use either pressurized (municipal) water, or unpressurized (rain barrel) water.

Together, these factors, along with a desire to be as cheap as possible, argue for making one compact set of contiguous (connected) beds. That’s easiest for running irrigation line. That’s easiest for setting up a trellis without wasting a lot of materials on the beds. And that’s best from the standpoint of having my anti-deer defenses cover the entire set of growing beds.

I worked out the general location in the last few posts. The sunniest spots in my yard are right next to my back porch. As a bonus, the local wildlife, including the deer, generally don’t approach the house, but instead tend to socialize at the edges of the yard.

Materials and design. At that point, I could simply buy raised garden beds, but a) that uses a lot of new materials, and b) where’s the joy in unboxing and assembling. As a result, instead of working smarter, assembling pre-made beds, I’m working harder, coming up with the beds from scratch. The design of which is mostly dictated by materials that I already have on hand.

The beds have three components: Block (for the corners), bamboo to provide strength to the sides, and plastic panels (coroplast) to hold in the dirt.

Block: I’m starting with cinderblock corners from Home Depot, pictured above. These blocks are slotted to accept the thickness of a standards 2×4 or 2×6 board, and have a vertical hole sized to accept a piece of vertical rebar. Wherever you want a corner of a raised bed, stack one, two, or three of these to provide the desired bed depth. At present, these are $3.38 each, so every two-block-high (11″) corner costs about $7.

One appealing aspect of these blocks is product end-of-life considerations. The garden beds go together like Tinkertoys. They are friction-fit, with no metal or other fasteners. If I get tired of them, or the next owner of this house doesn’t want them, all they have to do is pull the beds apart. Spread the dirt and plant grass. Send the plastic sides (below) off to the dump (where they were headed years ago.) And recycle the block into (e.g.) a retaining wall, fire pit, or other compact backyard feature.

Bamboo: For reinforcement of the sides I’m using last year’s crop of bamboo poles. To be clear, as long as my neighbor likes having a bamboo grove, I have an inexhaustible supply of bamboo poles coming up in my yard. Even though these will eventually rot, replacing one should be no more difficult than cutting a pole to size and shoving it into place. And if I get tired of that, I can act like a normal American and replace the bamboo with some 2x4s.

Polypropylene: For the sides I’m using coroplast sheets that I just happen to have lying around. Coroplast is “fluted polypropylene panel”. It’s stiff, plenty strong enough, and pure polyethylene is safe for food contact. The sides are “free” in the sense that my son scrounged up this pile of coroplast sheets years ago. I would not consider buying new coroplast if I didn’t already own this.

No 2×6 sides? Nope. It’s not just that a single 8′ 2×6 costs between $7 and $8 currently. (That would be about $48 in lumber for a 4’x8′ bed.) It’s that my local choices boil down to construction-grade softwood, or pressure-treated softwood. I know from experience that un-treated lumber simply does not last, in ground contact, in this environment. Not even if painted or stained. And I don’t particularly want pressure-treated lumber in my garden, even if they mostly don’t use arsenic any longer. The new stuff is treated with “alkaline copper quaternary ammonia” (ACQ). There’s probably no valid, scientific reason for avoiding the new stuff in a garden bed. I just can’t shake a long-standing prejudice, and I’m not all that keen on the cost and the use of virgin materials.

The upshot is that the cash cost for a 4′ x 8′ x 10″ deep bed frame will be about $28, for the block corners. That doesn’t include the trellis or the fill. Each addition to that — taking it from an 8′ bed to a 16′ bed, will cost just $14 more, for two more block corners. The moral of that story is that longer beds are cheaper than shorter beds, when constructed with cinder block corners and “free” sides made of recycled materials.

Finally, there is one more point of design regarding the side of the beds.

Narrow (two-foot-wide) beds are inefficient. A four-foot-wide bed is the most efficient size, from the standpoint usability and materials used. Anything wider than four feet and you can’t easily reach into the middle of the bed. By contrast, a two-foot-wide bed uses almost the same amount of material to construct, but provides only half the planting area.

This results in a penalty for building stand-alone trellis-backed beds. If you can only reach in from one side — if, say, there’s a trellis on the back of the bed — the standard advice is to build a two-foot-wide bed. This makes a long bed with a trellis back inherently inefficient, because the entire bed has to be an inefficient two-foot-wide bed.

I’m resolving this by constructing a “hybrid” raised bed, with a two-foot-wide section running across the ends of multiple four-foot-wide beds. This way, I don’t have to buy no or few additional cash-outlay corner blocks, but I get a reasonably long stretch of accessible, trellis-backed bed.

The final plan looks like this: