Friday, July 18, 2008

Designing and Building Raised Beds

As spring approaches, we wait in excited anticipation for delivery of our first seed catalog order, or impatiently mark off the days to our first sowing. New gardeners wonder how to best lay out their gardens to squeeze in all the vegetables from their recipes, impulse buys at the seed rack, or favorite flowers. Veteran gardeners want to manage planting schedules or crop rotation better, provide easier access, and even squeeze in a few more plants than last year. Raised beds are a great way to help organize your garden plot and improve yield.

Benefits of Raised Beds

Gardening in raised beds offers several advantages over growing in flat earth, or even mounded rows. The soil in raised beds not only drains better, preventing rot, but actually warms up sooner in the spring and stays warmer later into the fall, extending your growing season. Soil in raised beds doesn't get walked on, so it stays less compacted, improving oxygen and water flow that creates a healthier underground environment for roots and organisms.

Raised beds are more accessible to people who have a hard time bending over or squatting down to ground level. Edges of raised beds built at 15 to 19 inches high provide comfortable seating while working in the garden.

Some P-Patch locations do not allow framed raised beds, but those who are allowed to build them may enjoy their benefits.


Beds can be any length, but standard lumber dimensions and the size of your garden plot will help you decide how long to make your beds. Most P-Patch plots are 10 feet by 20 feet. You must remember to accommodate narrow paths between your plot and your neighbor's – so you don't really get to build out to the full 10-foot or 20-foot dimensions.

Lumber is sold in a few standard lengths: 8, 10, and 12 feet are the most common. Designing your beds based on standard lengths will help reduce the number of saw cuts you must make (or pay for). Though I've built beds longer than 14 feet, transporting the lumber is challenging, and butting shorter boards end to end creates wobbly joints. I prefer to build 8-foot long beds since that doesn't require cutting down standard lumber and leaves room for paths within a 10-foot plot dimension.


Optimal raised beds are anywhere from 2 to 4 feet across. If you can only access one side, a 2-foot wide bed makes sense. That's about as wide as your kitchen counters at home. Typically you have access to all sides, where widths of 3 or 4 feet give ample access.

Four-foot wide beds only require one cut of an 8-foot board, however more than a few 4-foot wide beds don't fit nicely into a standard P-Patch plot. Some 3-foot wide beds are inevitably required, though that means two cuts of an 8-foot board. If you don't have a power saw (and don't want to spend hours with a hand saw), you can usually get a home store or lumber yard to make one cut for free, but you may end up paying for other cuts.

(For those of you paying too close attention: Yes, if you follow the construction diagram below, the outside dimension of the beds will be 3 inches wider than the short board (two 2-by widths). That doesn't really affect the general layout options at the end of this article.)


Here's where standard lumber sizes get tricky! The end measurement of a 2x8 is really 1 1/2 inches by 7 1/2 inches. A 2x10 is really 1 1/2 inches by 9 1/2 inches. A bed made with one 2x8 or 2x10 high is adequate, but doesn't provide comfortable seating. 2x10s also get quite heavy. I prefer walls made of two 2x8s high, which means I'll have a low seat at 15 inches.

The long sides of your bed will require some vertical bracing. Many folks use rebar, but pounding rebar into the ground can damage water or drainage pipes, and become obstacles that attack your knees and shins! Use one short piece of 2x4 (cut as long as your walls are high) for every 4 feet of bed length. Also use short 2x4 sections to strengthen the corners of your beds rather than just attaching the edge of one board to the end of another. You will have to make six cuts to get six 15 inch sections from a standard 2x4x8' (with a little scrap left over).

To prevent the sides from bowing out after the beds are filled with soil, you can set the beds a few inches into the ground, or nail a length of plumber's tape (that anodized metal strapping with holes) from side to side across the bottom of your bed.

Use three-inch anodized nails or deck screws to join the lumber. Use two per board at each of the positions shown below. If you use screws, pre-drill holes in the 2x8s (but don't pre-drill the 2x4s). The drill should be slightly larger than the bore of the screw you're using. I like to pre-fabricate the short sides at home since it makes the work at the garden easier.


The best lumber to build your raised beds from is Douglas fir, which will last from four to six years in the garden. Cedar is very brittle, tends to splinter easily, and doesn't stand up to garden tools well. Commonly available cedar fence boards are not strong enough to contain the weight of soil. Treated lumber contains a variety of harmful chemicals including cyanide and should never be used to build garden beds.

You will need three or six 2x8x8' boards (three for single height beds, six for double height beds) and one 2x4x8' for each individual bed you build. You will need 56 nails or screws per bed (plus a few extras to make up for those you bend or lose in the dirt.) Nails and deck screws are typically sold by the pound. A store employee can estimate how many screws or nails you'll get in a pound.

If you don't have a vehicle suitable for transporting lumber, most lumber yards offer delivery for a fee, or even free for large orders.

Sample Layouts

Here are some example plot layouts that fit a few standard raised beds into a typical 10-foot by 20-foot P-Patch plot with room left over for paths. Line your paths with a layer of compost material, wood chips, or even pavers.

Fill your newly built raised beds to a few inches from the top with soil from your garden, or mound up compost materials as high as you reasonably can to create next season's soil. No matter which direction you orient your planting beds, the old rule of planting rows in a north-south direction will still expose your plants to the maximum amount of light as the sun moves overhead.

Copyright © 2005 Brian Ballard. All rights reserved.


Juliet said...

Hi Brian,
Any thoughts about using the WPC timber (ie Trex) for building raised beds? Saw one at the Bellevue Demo Garden that seemed decent, built well.

Brian said...


Thanks for leaving a comment - I think you're my first one!

I think Trex would be a good alternative. It would certainly last longer than fir (I had to start shoring up my fir a little after 4 seasons...) It's supposed to come in the same dimensions as regular lumber, so the plans I described should still work.

The only drawback I can think of is that it might be pretty expensive. I haven't priced it out though.