Answer: You will want to use cedar or redwood so the wood will last a long time. If you use untreated pine or fir, the planter boxes will need to be replaced within a few years. I'd recommend building your planter boxes at least 18" deep rather than only one foot tall. The extra room will accommodate the roots of most plants and the soil will remain cooler and moisture if it is 18" deep.
The easiest way to make a planter box is to use 6" wide cedar slats (sold for fencing). You can attach 3 together for each side of your box and end up with an 18" tall planter with virtually no rip cutting. Here's how I would build one:
Quantity 7, 6 foot x 6 inch Cedar Fence Pickets
Quantity 4, 6 foot x 3 inch Cedar Fence Pickets
1 pound of 5 penny galvanized nails
4 feet of 2 x 2 inch redwood or cedar
The next step is to measure and cut the pieces to length. The easy part about this is that this design has accounted for the average width of Cedar Fence Pickets (5 1/2 inches) and used these measurements to design and build the project. What this means for you is not complicated ripping cuts.
Long Sides: You may not need to cut if you purchase 6' long cedar slats.
Bottom: Same as above.
Ends: Cut 4 pieces at 36 or 48 inches (for your 3-4 foot width)
Side Supports: Cut 8 pieces
Bottom Support: Cut 4 pieces
Put the sides together, then the bottom, then attach them all together with nails. Good luck with your project!
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