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There are a number of reasons to grow vegetables in a raised bed:
1. In the Northwest, veggies like the drainage.
2. In the Northwest, veggies like the warmth (sun beating on the side of the bed)
3. It is easy to build custom cloches that fit the bed perfectly, thereby extending your growing season.
4. In urban yards, a raised bed is tidy and attractive, and may fit in better with ornamental plantings.
5. In urban areas, our soils are often contaminated with arsenic and lead. A raised bed that is high enough will keep your plants' roots out of the contaminated ground soil.
6. If you don't enjoy stooping, you will find that a high raised bed is easier to work.
There are a few drawbacks to raised beds as well. A permanent bed of any sort means your veggie garden will be in the same place every year, and many experienced Northwest gardeners say this can result in a nasty build-up of soil-borne plant disease. A raised bed also involves a greater amount of initial work, and raised beds made of wood won't last all that long in this climate.
That said, for the urban gardener, there are probably more ups than downs to using a raised bed. If you want to play it safe in regards to soil-borne plant disease, consider using one (or a couple) of your beds for several years, and keeping an equal number of beds planted with grasses. In a farm setting, these would be grazing grasses, but I imagine you could find some ornamental grasses to do the job in the city. After a few years, switch the beds.
Building a raised bed is a perfect do-it-yourself project (though if you want a particularly attractive bed, are short on time, or just want to support your local craftspeople, you could hire a professional). Assuming you are going to be involved at least in the planning process, there are some considerations:
First of all, you want to consider placement. A raised bed should go in the sunniest part of your yard. If this means digging a rectangular patch in the middle of your lawn, I recommend doing so.
Secondly, you want to consider materials. How much money do you have to spend on materials? The simplest raised beds don't involve any materials at all; the bed is "raised" simply by mounding the soil up a few inches above the general soil level. I waxed romantically about a mounded bed in a prior blog post. These are the easiest, the oldest, and in many respects the best kinds of raised beds. However, they are very farm-like (not what many people would consider pretty, though I love it), and they don't address the issue of contaminated soil.
To find out for certain if your soil is contaminated, you can test the soil and send it to a lab.
If you are going to build a higher raised bed, you might use cedar boards, brick, stone, cinder block, or other materials you find lying around. My housemate recently made a 7" high bed with our old cedar decking boards, which isn't going to last long but was free.
Bricks are long-lasting and often found lying around. However, it takes a lot of bricks to build a bed, and you'd want to use mortar if you were building it high. Moreover, if you don't find used ones for free, they can be a bit expensive.
Be careful about using certain materials as not all materials are safe. For information on this--and for the closest thing to the definitive word on whether or not it's safe to use pressure-treated lumber for building raised veggie beds--refer to this King County report. Interestingly, the report debunks the idea that store-bought pressure-treated lumber is unsafe.
I've seen very nice beds built out of large cedar planks. My guess is that they last anywhere from 4 -7 years in the Northwest before they completely fall apart (anybody have any data on this?), though if you treat them with a natural product like boiled linseed oil you may get an extra year or two out of them.
I've seen many wooden beds built by screwing or nailing the planks into 4"x4" corner posts. I don't recommend this. I would recommend what my brother did: using metal "L" brackets inside the corners, with holes for bolts that run all the way through the planks.
I recommend this because my understanding of rot is that it generally begins at the ends of boards and around screws or nails. Moisture condenses on metal quicker than on most materials, and will rot the wood out around the hardware. The only question is whether the wood will rot quicker than the screw will rust. A bolt, on the other hand, will provide a secure connection until the plank rots out completely.
Another thing to consider with wooden beds is that a bed of any length, especially the standard 8 feet, will tend to sag out in the middle as the soil settles over time. You can address this by pounding a post (or rebar) into the ground outside the boards at the midpoint of their length, but this will not necessarily be pretty.
You might, instead, pound a nail stake into the ground on the inside of the boards at the midpoint of their length, and attach them to the stake with skinny bolts.
I built a raised bed out of cinder blocks last year, and am mostly happy with it. Stacked three blocks high, it is two feet off the ground, which keeps the plant roots completely out of any contaminated ground soil.
The process is pretty simple: lay the first run of blocks out in a perimeter, lift them up, dig a shallow trench (only an inch or two deep, and barely wider than the blocks), pour in some sand, level the sand, and lay down the first run of blocks. Use a level to level them from one to next and build up the wall. Cinder blocks are heavy enough that you can just stack them dry, without mortar. Unless you back into them with a car or kick them ferociously, they will hold their shape.
One thing to be wary of is keeping the bed square, as an un-squared bed will come together poorly, resulting in a corner that is not flush. Though not the end of the world, it's unnecessary and annoying. Square the bed ahead of time by running string between posts to mark the perimeter. Measure the length of the diagonals between opposite corners ("kitty corner"). If the measurements of both diagonals are the same, you're good to go.
Finally, if you've made a high raised bed, you will need to import a lot of soil. For my 4'x9' cinder block bed that is 2' high, I imported a yard and a half of topsoil from Pacific Topsoils, and brought in nearly a full yard of compost. The veggies performed swimmingly.
If you're the solo weekend-warrior project type, making a raised bed can be a great way to spend time outside and play with practical and aesthetic design considerations. If you're more of the group-project type, or want something the whole family can be involved in, a raised bed is of sufficient scale to occupy a handful of people for half a day or more. Remember: it doesn't have to be perfect and it's not rocket-science.
It's spring! Get out there and get dirty!
