Raised planting beds have been our workhorse for a number of years, for several reasons:
1) raising the beds slightly above grade, even only a few inches, allows for better oxygenation, better drainage, and warmer soil temps earlier in the year. For our cool, damp, Northern growing climate, those improvements can mean the difference between seeds that geminate on time, versus rotting in cold soil. It can also keep the warm-season plants happier during cool, damp turns of weather in mid-summer. We must hang onto whatever soil heat we can generate here, and raised beds help us do that.
2) raised beds go hand-in-hand with looser soils with good tilth. There are a number of ways to raise beds, from installing boards or stone borders along a small garden bed, to setting up a cultivator to scallop soil out of furrows and into a central track. Even standard plows can form raised beds if the field is plowed to turn either towards or away from each other, thereby forming raised beds with channels on either side. The method of forming beds is therefore dependent upon your planting area, but the principle is the same. Get those soils up off grade, and you've incoporated more breathing area into your root zone.
3) By creating raised beds, via whatever means, we also almost by default create walkways alongside the beds. These walkways can become heavy traffic areas without trampling the roots. By deliberately leaving room between beds for such walkways, we can even lay down either low-growing cover crops or landscaping fabric, such that we have smooth, level, non-weedy walkways between beds.
4) While there's no guarantee on this next point, we have generally found better yields in raised beds. That may be due to better oxygenation at the root zone, warmer soil temperatures, less root crowding, and/or the fact that we usually combine raised beds with other high-density and deep soil preparation methods (see our other pages on double-dug beds, lasagna gardening, mulch planting, and square foot gardening). The fact that these methods all dovetail so nicely is what matters to us. Teasing out the factor or factors responsible for the improved growth would be an interesting academic question but would probably not give us much better performance.
5) One of the things that we appreciate more and more about raised beds is that we don't have to lean over so far to work in them. That reduction in kneeling starts to add up as the body gets older, and/or as the number and size of beds gets larger. For those folks who don't get around as well as they used to, or are on crutches, canes, walkers or wheelchairs, that raised work surface can mean the difference between continuing to grow their own food or having to watch from the sidelines. Whatever the reason, it's a nice feature that helps make gardening easier on the body.
As the above photos indicate, raised beds can be almost any size which
is practical and functional for the individual grower. The length and
width of the beds is in part dependent upon what you're growing, and
what you're using to form the beds. Most raised beds we've either seen
or worked with are at least 24" across, but no more than 60" across.
That maximum width is defined by the distance you can reach into a bed
from the side, without stepping in the bed in the process. Most folks
can only reach about 30" into the bed from either side, hence the 60"
maximum width. Maximally wide beds, and minimal walkways, will also
maximize the amount of plantable space in any given area. That may seem
like a minor difference, but compare the number of plants you can grow
in a single densely planted bed, versus in rows, and very quickly you'll
see how much area is saved by not having spaces between the rows.
However, there is a tradeoff in comfort when you have to strain to reach
the middle of those beds and you have hundreds of bed length to cover.
Many growers have yielded to a certain amount of comfort versus
efficiency, and use 48" wide beds. They are much easier (and therefore
less fatiguing) to reach across. Similarly, a slightly wider walkway
(at least 24") is a lot easier to use when steering wheelbarrows,
driving small scale implements, and pushing/pulling harvesting bins.
Raised beds may not be wise to use in certain growing conditions. For instance, if your soils are hot and dry to begin with, and/or have difficulty holding onto moisture, then raised beds would magnify those issues. If that is only a seasonal problem, addition of plentiful organic matter in the form of plowed-under cover crops, applied compost or heavy mulch may allow for the best of both worlds - warm dry soils in spring and fall, yet shaded, moist soils in the heat of summer.
For more information about using raised beds in either small-scale or large-scale applications, check out the following resources:
Colo State Univ Raised Beds
Univ of MO Raised Beds
Texas A&M Raised Beds
Univ of Alaska-Fairbanks Raised Beds
UC Davis Raised Beds
Univ of Tenn Raised Beds
Wash State Univ Raised Beds
Books about Raised Bed Gardening
The boundary is very fuzzy between raised beds and double-dug beds, along with intensive planting in either type. Accordingly, the books that talk about any of these approaches will often jump back and forth amongst the various traditional and modern sources of information about these approaches. Thus there is no one book about raised beds, per se. On my double digging/intensive planting page, I list one of sustainable gardening's classic reference books, John Jeavon's How to Raise More Vegetables.... He certainly has done a lot of work on promoting that approach. Yet there are other writers who have also done considerable service to the farming and gardening communities with their own works. Here I list a few of them, in no particular order. All of them use raised beds and intensive planting in some way shape or form. All of them deserve recognition for the work they've done, helping folks feed themselves and others.
The New Self Sufficient Gardener by John Seymour
This is another one of my favorite books of all time. Where John Jeavon's How to Grow More Vegetables... is a veritable encyclopedia of information, John Seymour's New Self Sufficient Gardener is a bible. Seymour speaks not merely to our growing minds and hearts but to our grower souls as well. His writing, and the gorgeous illustrations throughout the book, remind me why I love growing things. When I'm laying out my garden plan for the year I'll pull How to Grow More Vegetables... off the shelf for all the data it has. But when it's cold and raining and dreary outside and the spring can't get here soon enough, I'll pull out Seymour's book and slowly page through it, stoking my faith in the growing year to come. Happily, it is also chock full of information, some of which is good-old-fashioned growing hints from a man whose garden was his primary source of nutrition for many years. I wish John Jeavons taught at the nearby college. But I wish John Seymour lived next door so that we could visit over the fence. His The New Self Sufficient Gardener is the next best thing.