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The planter is made from 2x6 pine lumber and assembled with deck screws. If you want the planter to last longer, consider using redwood or cedar (but the cost will go up by 4 times or so.) Pressure treated is not recommended because chemicals may leach into the soil.
You'll also need two 2x4s for feet under the box and for side supports. I recommend a pressure treated 2x4 (not shown) for the feet since this will not be in contact with the soil but will be subject to lots of moisture.
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The width of the planter base is determined by four 2x6 boards, plus a little extra to allow for expansion.
I've inserted a nail between each board to provide this spacing. For this design the total width is 22 3/8"
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Now that I know how wide the bottom is, I can cut pieces for the planter sides using the remaining 2x6 lumber. For a more finished look, I mitered the corners.
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Time to assemble the first side frame. I applied glue to both surfaces of the joint and used framing clamps to secure the right angle joint.
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Once all of the sides are clamped and glued, I measured diagonally from top-left to bottom-right, then from top-right to bottom-left. If you have square joints, then both measurements will be the same; meaning that you have a perfect rectangle.
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While the clamps were still attached I fastened the sides with a nail gun, then with deck screws. Let the glue dry before moving this assembly.
This completes the first side frame. Repeat these steps to construct the second side frame, which will stack on top of this one to provide an 11" deep planting box (9.5" of soil).
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Because this raised garden bed will sit on a deck, it requires a wood bottom to hold the soil.
As noted earlier, the bottom will be constructed from four 2x6 boards, spaced a nail-width apart.
The bottom fits within the side frame, which was customized to fit the width of the boards. But the 2x6s need to be cut down in length to fit within the frame.
The long dimension within the frame is 92 3/4" so I cut the 2x6s to 92 1/2" (this allows a bit of wiggle room.)
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Place the boards within the side frame (remember to leave the nail-width spaces between boards). This verifies that everything fits correctly, and allows us to easily assemble the bottom and add the side braces.
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If the 2x6s have a flawed side, that side should be facing upward (this will ultimately be the feet side and face downward.)
Cut a pressure treated 2x4 into 22" lengths--these will be the feet.
Space the first foot about 2" from the end of the side frame. Insert nails to add a gap between boards, then screw the foot into place.
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 Repeat for the remaining feet, spacing them about 24" apart.
This completes the planter bottom assembly.
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Now it's time to attach 2x4 vertical braces that serve two functions:
- Allow the two side frame assemblies to be stacked and secured.
- Hold the sides of the box on top of the bottom assembly.
It doesn't matter that the bottom assembly is upside down while we attach the braces, as you'll see in a bit.
Screw the braces into place, using two screws for the short face board and two screws for the long face board (a total of four screws per brace.)
Add a brace to each corner.
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You can now lift the side frame off of the bottom assembly.
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I elected to stain the planter so it will match the deck and hopefully last a few extra years.
If you stain the planter, consider whether you want to stain the interior. There is some speculation that paints and stains can contaminate the soil and then get into the plants.
I will add a 4 mil plastic inside the planter to separate the solid color stain and the soil.
For the final coat of stain I assembled the units first; this elevates the side assemblies off of the ground and makes it easier to apply the stain.
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The black plastic that I used as a tarp will also serve as the inside liner (non-stain side up.)
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This is the empty planter stained to match the deck. It measures about 2' x 8' and is constructed as two separate pieces; a framed-side assembly and a bottom assembly.
The framed-side sits on top of the bottom, but is not fastened. This allows you to easily move the planter or remove the soil.
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Here's the planter box lined with 4 mil plastic and attached it with a staple gun.
I used a utility knife to place small slices in the bottom of the liner so water can drain. Make sure that you provide adequate drainage so that the plants don't get wet feet (and so that the planter does not get too heavy...)
Time to make the planting medium!
To calculate how much medium you need, multiply the planter width x length x soil depth. For example, for these planters: 8' x 2' x .6' = 9.6 cubit feet.
I recommend using a bagged soil-less potting mix (preferably without fertilizer) in the bottom half of the planter. Compost mixtures work best if used as a top-dressing--this is how nature fertilizes plants. This also helps to keep the weight down.
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Mel Bartholomew recommends a mix of 1 part compost, 1 part perlite (or vermiculite*) and 1 part peat in his All New Square Foot Gardening book.
So if you're using three 1 cu ft bags of compost, you want to add three cubic feet each of perlite and peat for a total of 9 cu ft of mix. Check the pH of the mix**--with all of this peat moss you probably need to add some lime to reduce the acidity.
For best results, use a variety of compost sources to get a good mix of nutrients.
NOTE: If you are placing these planters on a deck, weight is important: use Perlite in place of the vermiculite and make sure there are plenty of drainage holes.
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Place the compost in the center of a tarp. To mix, pull on an edge of the tarp to tumble the compost. Repeat a few times to mix well.
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Add the perlite and mix in well.
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Now mix in the peat and mix well.
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Once you have a good mix, it's time to move it into the planter.
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Now it's time to add a row cover frame and fabric. These steps are optional, but let's look at the benefits of adding a row cover:
- Wind protection
- Frost protection (down to 28 degrees)
- Pest protection (the pests can't see the plants)
These benefits should just about eliminate the need for me to harden the broccoli or onions before transplanting.
Both of these plants are very cold hardy and should perform well in the coming weeks.
The metal components used for the cover frame are all available in the electrical conduit section of your local hardware store.
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Start by attaching sleeves to the frame. The elbows and rails of the frame will fasten to these sleeves.
Notice that I used a deck screw and a rigid clamp mounted with a 1/4" bolt to mount the sleeve. You want to make sure that the wind will not lift the frame.
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Secure elbows to the sleeves with the screws.
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Cut the metal conduit to size and secure to the elbows using sleeves.
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Cut the row cover fabric to size and attach with clamps or clips. I purchased this fabric from Johnny's Selected Seeds.
This fabric allows water and sun (85%) to pass through to the plants. You can buy these fabrics in various thicknesses, depending on how much cold protection or sun transparency you need.
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 **Checking the pH of the mix
The soil mix that I used in the planters has a lot of peat moss, which is highly acidic.
About three weeks before planting I applied lime to the mix, then tested the pH. The result? Alkaline neutral., which is great for most vegetables, including broccoli, cabbage, mesclun and lettuce.
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