There are lots of reasons why people like a raised garden bed.

The thing that isn't so likable about them is that they can be rather expensive. This is the design we came up with to use readily available Cedar lumber and other supplies. At nearly a foot and a half tall, it's taller than most raised beds while still being quite inexpensive. Cost to build the frame is around $50... and it will take another $50 worth of garden soil to fill it. Then there are all sorts of optional things. It's around $120 per bed with soaker irrigation, fencing, and various wildlife deterrents -- as we built ours.

Supplies

To build one 6x3 foot bed, approximately 18" tall, you'll need:

One 8-foot 4x4 Cedar post (you could substitute a pair of 2x4" screwed together)
One 4-foot 2x4 Cedar (half an 8-foot)
Nine and a half 6-foot 5/8" x 5 1/2" Cedar fence picket (either flat top or dog-ear is ok)
One box of 1 5/8" exterior screws (need approx. 162 screws)
Landscape fabric (need approx. 20x2 feet)
A stapler and staples for the fabric (could be tacked instead)
One cubic yard of soil (e.g., a garden mix of topsoil, compost, and peat moss)
Optionally: a soaker irrigation kit and tubing or hoses
Optionally: bird netting and T-Posts fence with plastic lattice gate
Optionally: solar lights, fake owl, wind chime, and/or other things to deter wildlife

It should be noted that the limiting factor here was availability and pricing of Cedar that could be used for the side panels. The only other Cedar lumber available locally was the 8-foot 2x4 and 4x4, so I was very happy to find the Cedar pickets for under $2.50 each.

Other material choices would include using untreated or treated Pine, Redwood, or lumber made of recycled plastic. Pine is about half the cost of Cedar, and is available as longer decking boards, but I don't trust it to hold up unless it's treated. The treated lumber is now supposed to be safe for garden use, but I still don't like the idea of chemicals potentially ending-up in our vegetables (this and cost is also why I avoided landscaping timbers). Redwood wasn't available locally, and really wouldn't be much different from Cedar. Plastic composite boards are available as decking, etc., and they certainly will not rot, but they deform under load rather easily, so I'm not sure they're good for holding back dirt... and they're an order of magnitude more expensive.

The various optional materials listed are explained later in this Instructable.

Step 1: Building the Frame

Building the frame is not very difficult, nor does it require very high precision. However, you will want to do the initial assembly someplace with a flat, hard, surface -- i.e., not in your garden. A garage, driveway, or patio near your garden should work well.

Start by cutting the lumber:

Cut the 8-foot 4x4 into four pieces each approximately 24" long
Cut the 2x4 to make two pieces each approximately 24" long
Cut four of the fence pickets in half, making eight 36" long boards (of which you'll need seven)

Now assemble the frame (to be as shown in the photo) using the screws:

Lay two of the 4x4 posts on their sides parallel to each other such that the outside edges are about 1/2" more than 6 feet
Lay one of the 6-foot pickets across the posts and square-up the angles so that what will be the top surface of the picket as a side-wall of the frame is about 1 1/2" below what will be the top of each 4x4
Screw the picket in place using at least 3 screws on each post; the dog-ear clipping of the corners shouldn't interfere with the screws, but don't place any screws too near the picket edges (to help avoid splitting the picket)
Using the first picket for alignment, put the second picket below the first and screw it in
Using the second picket for alignment, put the third picket below the second and screw it in
Stand the side panel you just built upside down
Measure the center, place a 2x4 standing up against the center of the side panel (a couple of clamps would be handy to hold it in place), and attach it to the pickets with screws
Repeat 1-7 to make the other side
Now position both sides standing upside down parallel to each other with the 2x4 and 4x4 pieces on the inside and clamp one of the 3-foot pickets on each end 1 1/2" off the floor; this ensures that the top edge on the frame sides will all align
Before screwing-in the clamped 3-foot pickets, use a tape measure to measure diagonally from corner to corner of the 4x4 posts; the box isn't a rectangle unless the measurement on both diagonals is the same, so adjust positions until it is within 1/2" or so
Screw-in the clamped pickets and remove the clamps
Using the already screwed-in 3-foot pickets as guides, place and screw two more 3-foot pickets to build the narrow side walls of each end of the frame; this completes the frame, but we want to reinforce it a little
Lift one of the 3-foot sides to sit the frame on the other end
Place one 3-foot picket across the two 2x4 pieces and aligned with what will be the bottom picket on the long sides and screw it into the 2x4 on each side; this piece will keep the long sides of the frame from bending outward as the soil pushes against it

I know that sounds like a lot, but it's really pretty easy.

Why are we using screws and putting the slats on the outside of the posts? Because that way if a slat (picket) should start to fail years later, we can easily replace the slat without tearing the whole thing apart. The screws should be strong enough to hold things together, but they're still easy to remove.

Step 2: Level Where the Bed Will Go

The ground where you put the raised bed doesn't have to be level, but the top of the raised bed should be.

The bed is intended to have the bottom edges of the side panels rest on the ground with the 2x4 and 4x4 posts set into the ground. If the ground was already flat, you just need to dig a little hole for each post. If it wasn't flat, spend some time leveling it first. Our garden happens to be on a fairly significant slope, so we cut into the dirt a little on the high end and built-up the low end.

Once things seem approximately level and holes have been dug for the posts, carry the frame over and place it in the desired spot. Again, use a tape measure to check the frame is still square. Use a level, like the one shown in the photo, to check the tops of the side panels for level. Where they are too low or too high, add or remove dirt. Also add/remove dirt to make the ground level match the bottom of the sides. You're done when all four side panel tops are level and there is no light showing through between the bottoms of the side panels and the the surrounding dirt.

Step 3: Line and Fill the Bed

The landscape fabric liner serves three purposes:

Prevent soil from washing through any small gaps between the Cedar pickets in the side panels
Protect the Cedar a bit, hopefully extending the life of the raised bed
Make it possible to replace a picket without all the soil falling out

In truth, we're really not sure the liner is necessary, nor are we sure it wouldn't be better to use a liner that isn't water permeable (e.g., a sheet of plastic). Our liner is a landscaping fabric that we've used for years to block weeds, and it's pretty durable. It also is four feet wide, so we actually folded it in half and double-lined the beds. We used a stapler to tack the liner to the sidewalls and let it overlap the ground under the raised bed by about 6" to help prevent soil blowing out under the bottom edge of the sidewalls, but we didn't completely seal the bottom. The liner also is cut to allow for the center bracing piece.

The frame gets filled with your choice of garden soil mix. We used a shovel to break-up the ground in the center of the frame a little to get a smoother transition to the soil mix in the frame. The actual soil mix we used was primarily 50% topsoil and 50% compost, but we augmented that with some peat moss to make the mix a bit lighter and aid in water retention. Each frame holds one cubic yard of soil, which literally weighs a ton, so we had a local contractor deliver five cubic yards to a pile near where we wanted our six raised beds and then used a shovel and a cart to move the soil into the frames, where we mixed-in the peat moss. You want the pile as close to the garden as you can get it, but dump tracks can't go on unstable land and can easily tear-up lawn, so plan carefully where you want a load dumped.

You might also have noticed little black marks on the tops of the sidewalls. Those were placed at one foot intervals using a permanent marker to facilitate "square foot" gardening -- placing plants in equally-spaced square foot areas. Obviously, you can make such markings any time with whatever spacing (or other information, such as plant type) is useful to you. Typical spacing would be either three rows of 12x12" or two rows of 18x18" for things like tomato plants.

Step 4: Plumbing

You really don't want to be sprinkling raised beds in a way that gets the paths around them (and the wooden frame) wet. For that reason, and to save water in general, a soaker irrigation makes a lot more sense.

There are various soaker hoses and kits available. We decided to go with a 20 piece, 100 foot, soaker kit branded Flexon. It cost less than $30 and is enough to reasonably handle six beds... although we have our beds planted denser than usual, so we actually used 150 feet of soaker hose: 25 feet per 6x3 foot bed. This kit isn't really intended to water multiple disjoint beds without using a garden hose to each. However, that would have been awkward. Instead, we bought clear vinyl tubing of the same dimension (1/2" ID, 5/8" OD), which is compatible with the same fittings used in the kit, and used it to span between beds. It was surprisingly easy to cut the soaker hose and vinyl tubing to the desired lengths with scissors, push the cut ends into the plastic fittings in the kit, and route water to every plant. In case you're wondering how such wimpy push connections can possibly work, the answer is that soaker hoses require low pressure. In our kit, the pressure is dropped using a little disc with a hole in it to restrict the water flow where the kit is connected to a water supply -- hooking it up without that will do bad things.

Ah, but where does the water come from? Well, in our case, one garden hose running about 125 feet from our house. The catch is that we didn't want to have to run another hose for spot watering and we also wanted the raised beds to be in two separate "zones" that could be turned on/off independently. That's how we decided to create the rig shown in the photo.

The supply hose is connected to the input of a brass "4-way hose faucet connection" or "4-outlet hose faucet manifiold" that allows up to four hoses to share one supply, each with an independent cut-off valve. This is pinned with a few bolts to the top of one of the 4x4 posts to keep it in position. Before pinning it there, I covered the top of the 4x4 with aluminum duct tape to prevent any water drips from going directly into the wood's end grain. Two of the four outlets are used to independently control the supply for each of the two rows of three beds, a third outlet has a hose connected with a manual head attached for spot watering and/or rinsing vegetables picked, and the fourth outlet is unused (left for future expansion).

Step 5: Who Let the Dogs Out?

Not us. We don't have a dog -- but one of our neighbors apparently does, and it's regularly hanging around our yard pooping and tearing-up things. We also have a lot of wildlife because there's just a 200-acre farm between us and a nature sanctuary. Basically, we need to do more than just raise the bed 16" in order to stop critters from destroying the garden.

The first thing we do is to surround the garden with a fence. We've found that the fine mesh of bird netting works quite well to keep even little vermin out. It's also minimal visually -- we still see the garden and the garden still sees full sunlight. The netting isn't very durable, but it generally lasts 1-2 seasons and is cheap and easy to replace. Of course, the netting doesn't make a good gate, so we use a 4x8' plastic lattice trapped between several sets of metal posts. We simply slide it to one side to get access to the garden.

The second thing we do is to add some features that might discourage critters. Solar lights are a good start. We've also tried a fake owl, wind chimes, etc. None has been magically effective, but each has had some beneficial effect at some time. It seems the critters learn things are harmless over time, so changing things regularly is probably best....

Step 6: Done... But Gardening Work Is Never Done

We think this raised garden will be easier to maintain that our previous flat garden... but we don't really know yet. We just finished building this.

For example, my wife has put grass cuttings on the paths between the beds to try to minimize weed growth there, but we have no idea if that will really work. We also don't yet know how many weeds we'll get in the beds, if tomato blight will be hindered or worsened by this, etc. I'll post updates as we learn more....