The Ideal Size for Raised Beds

The 4’x8’ garden bed is iconic in the raised bed gardening world - but with recent increases in the price of lumber, other configurations might yield better value for money. Let's do the math!

Nothing lasts forever, and this is especially true of wooden boxes that are left outdoors all year filled with wet soil. Given the price of lumber these days, it makes sense to consider alternative designs to the classic 4’x8’ bed.

Height

Slightly raised beds that are 6” (15cm) high tend to be ideal. They require less lumber than higher beds, and less soil is needed to fill them. They also do not need to be watered as often higher beds. A 6” high bed will have soil that is about 4” above grade (the soil is never right at the top of the bed). This 4” of soil is all that’s needed to deal with most spaces that have drainage issues. For added value, since the soil level is only 4” above grade, mature plants will actually be able to reach into the ground to find water, and by virtue of capillary action, ground water will be drawn up in the that 4” of soil when it rains. The extra 2” of board above the soil level leave just enough room for a mulch, which will further improve upon moisture retention in the soil. The only reason for going higher than 6” is back/knee/mobility/etc. problems. If you need them high for those reasons, then there’s no way around it; but if you can kneel, squat and touch your toes, there’s no point in going higher than 6”. Besides - all that bending and squatting is good for you right? Well.. that’s what I tell myself anyway!

Length and width

For any bed, we can think of “optimality” in terms of how much area is made available relative to the amount of lumber required to make the bed. In this sense, it can be argued that optimality can be measured in terms of area per linear foot, since lumber is typically sold by the linear foot. Using the 2”x6” for the sake of this thought experiment, consider the following options for building beds that are 8’, 10’ and 12’ long in terms of length and width:

For each table, we have the options for dimensions in the first column; the area in square feet (Sf) in the second column (length times width); the linear feet in the third column (total length of all the lumber needed to make the box (therefore L*2 + W*2)); and, area per linear foot in the fourth column. Looking across the tables, it can be seen that the perfectly square beds always have the greatest A/Lf, and the larger ones have more A/Lf than the smaller ones. To put that another way - you get more gardening space per board-foot when beds are larger and more square.

For example, consider the difference between having two 4’x8’ beds, and one 8’x'8’ bed. The two 4’x8’ beds will require 48 Lf of lumber; whereas the single 8’x8’ will only need 32 Lf of lumber - yet the total area of the two 4’x8’ beds (32 Sf + 32 Sf = 64 Sf) will be the same as that of the single 8’x8’ bed. In other words, one 8’x8’ bed gives you the same area as two 4’x8’ beds, yet needs 16’ less lumber. Right now, an eight foot 2”x6” costs about $8.00, so by going with a single 8’x8’ bed instead of two 4’x8’ beds, you save $16, and still get 64 Sf of garden. To put that another way, the 8’x8’ bed costs $32, whereas the two 4’x8’ beds will cost $48, so for 64 square feet, the 8’x8’ bed costs 33.3% less than two 4’x8’ beds

Workability Matters

The 4’x8’ garden bed is ideal in many ways. It is the perfect size because it’s not too long to walk around, and from either side the middle can be reached comfortably. This latter feature is important because there is no need to step on the soil, which minimizes soil compaction. Unfortunately, contemporary economics do not care about raised bed gardeners, so it is difficult to justify the expense of these “ideal” beds when larger square gardens offer more garden per board. Still, this leaves us with the problem of how to work an 8’x8’ bed without stepping all over the soil. In my experience, this can be achieved easily by placing an old board laid flat at the side of the bed, or large flat rock - or anything like that - on either side near the middle (see graphic below). In this way you can step/kneel on the soil without really stepping/kneeling on the soil.

The Keyhole design is not a solution

One approach to making the large garden more workable is to employ a keyhole design. Using the 8’x8’ bed as an example, a keyhole design would have a slot built into the side that allows the gardener enough reach to access all areas of the garden without needing to step on the soil. While this design does achieve that end, it uses more lumber than does the simple 8’x8’ bed. To be precise - with a 2’x4’ keyhole (as shown below on the left), the 8’x8’ keyhole garden requires 40 Lf of lumber, whereas the simple 8’x8’ (shown below on the right) bed requires 32 Lf.

Yes, the simple 8’x8’ requires kneeling boards for access, but almost anything can be used for these - and you’d be surprised how often people throw away 3’ long planks, or old pallets. It’s also the case that old beds eventually fall apart, but this wood can still be used for kneeling boards! It’s also the case that you can make a “luxury” kneeling board that is maybe 12” x 36” and padded - and just move it from bed to bed when needed. Even multiple layers of stiff cardboard taped or tied together make nice soft kneeling boards (highly recommended).

Different beds for different crops

Another way to get the best value from lumber in terms of bed dimensions is to have some beds that are 4’x8’ and some that are 8’x8’ depending on the crop. Crops that need intensive weeding, thinning, harvesting, and other maintenance (e.g. kale/lettuce/spinach/beans) might be better suited to the easy-access 4’x8’ beds; whereas as crops that just grow with little maintenance and only need to be harvested once (e.g. squash, potatoes) are fine in an 8’x8’ bed. By using this approach you can save money, but still have a few 4’x8’ when you need them.

What about circles?

So squares have more area relative to their perimeter than do rectangles. This is no mystery to anyone that knows basic geometry. For that matter, circles have an even better area to perimeter ratio… but good luck making circles with 2”x6” lumber! Perhaps someday there will be a building material that is ridged and durable enough to make garden beds, while simultaneously being at a low enough cost for the ordinary gardener to justify the expense - but I don’t think we’re there yet. There are options out there like galvanized steel and some types of plastic - but the cost is very high on a per-foot basis. Of course, if the price of lumber continues to rise, the high price of these other materials may begin to make sense.

Final thoughts

The title of this article refers to the ideal size for raised beds - but it is the subtitle that gets to the heart of the issue. Lumber costs money, and it also does not last forever - so value for money matters. For those considering raised bed garden designs this fall or next spring, it might be worthwhile to sit down with pencil and paper and consider incorporating a few large square beds into the mix. The numbers don’t lie.

Postscript: I know that some people are going to argue (perhaps vociferously) that I should be using the term “lineal” instead of “linear”. I spent about an hour trying to figure out conclusively what the right word was in this context, perusing about dozen English language resources… with no conclusive answer to my satisfaction. It should also be noted that I spent a summer working in a lumber yard where guys would use both terms interchangeably all the time, and I spent a number of years in the construction industry where the same thing occurred. For me, since linear means “straight line” - and that is what I mean when I say “linear feet” (straight line feet) - I think “linear is the right word. Feel free to convince me otherwise in the comments section - but please only do so if you can refer to to an authoritative source (that I can read) to back up your argument.

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Here’s a video where I discuss a range of different garden border materials and their various pros and cons: