How to make a DIY planter box

Sometimes the planters at your local hardware store just aren’t enough. They are missing something. Gravity. Prestige. The hearty scent of cedar, perhaps. Fear not—making your own cedar fence post planter box is a simple afternoon project and can add a renewed sense of style to your landscape decor.

While you can make a planter box out of almost any wood (pine is popular because it’s affordable), cedar is great for a few reasons. First and foremost, it’s rot-resistant, so it will withstand weather, dirt and bugs for years and years, even untreated. Second, cedar fence posts are relatively inexpensive and easy to work with. Finally, cedar actively repels many types of bugs, which is why this wood is so often used in wardrobes. While the scent won’t keep away all garden pests, it can deter some, including houseflies, ticks, mosquitoes, termites and some types of ants, according to the U.S. Department of Agriculture’s Agricultural Research Service.

This particular DIY planter box is a simple and versatile project. The box I made is 12 inches square and 16 inches tall, but you can easily make one with a 16 by 16 inch or 24 by 24 inch footprint by modifying the dimensions below. Any longer than that, and you’ll probably need to upgrade to a long pot box design.

Statistical data

  • Year: 2 to 3 hours
  • Cost of materials: $30 to $50
  • Difficulty: moderate

Materials

How to make a cedar planter box

1. Cut the fence posts to rough length. Before squaring the wood properly, always cut it to rough length first. You can use a miter saw to cut the fence posts into sections, but I did it on my table saw with a crosscut sled because I already had it set up. Cutting the wood into shorter lengths before milling reduces the amount of material you will need to remove to get flat surfaces.

2. Join the ends and cut the pips to the final length. For this project, I decided not to level the faces of the fence posts. They were pretty flat to begin with and I didn’t want to lose the rough, rustic look of the boards by mistake. I butted the ends using a plane on my table saw and then cut the boards to final length with my cross cut sled.

The final cut list for this project is:

  • 4 wide side frame feet (2.5 by 16 inches)
  • 4 narrow side frame feet (2 by 16 inches)
  • 8 sideboards (3 by 11 inches)
  • 8 narrow side boards (3 by 10 inches)
  • 4 top miter frames (1.5 by 15 inches)
  • 3 base boards (3.5 by 11 inches) (you will cut them to fit exactly once you assemble the box)

3. Sand all the boards. As I mentioned, I didn’t want to lose the rustic look of the fence posts. However, they were still pretty rough and I was afraid that if I left them untouched, people would get splinters when planting. So I spent some quality time with my orbital sander and 120 grit sandpaper smoothing both sides of all the boards. How long you scrub is entirely up to your aesthetic preference, but I only did a minute per face to remove any loose or jagged material. But I sanded the top boards and legs a little more than the other boards—they’ll be the ones people touch the most.

4. Separate the boards for the four side panels. 11-inch panel boards are paired with 2.5-inch wide legs and 10-inch boards with 2-inch legs. Don’t forget this or you’ll end up with a box with a rectangular footprint instead of a square one.

[Related: Benefit your neighborhood bats with this DIY bat box]

5. Build the side panels. Using a piece of ½-inch plywood and a speed square, you can create a basic jig to align the boards. First, lay one leg on your assembly table, with the long edge lined up with the flat edge of the plywood. Align the speed square along the same plywood edge, touching the top of the leg and extending the plywood. Next, apply wood glue to the leg, leaving about an inch on the edge furthest from the plywood.

The large sheet of plywood on the left serves as an alignment jig, helping you make sure the leg of the side panel (right) is lined up with the top board of the board (above). The upright piece here is the spacer for the hinge where the other side panel will connect to it. Jean Levasseur

Place the top panel board on the plywood, extending over the glued part of the leg. You’ll need to leave enough room for the leg of the other frame, since the panel legs nest together when assembled, so use one of the extra boards as a spacer—place it edge-to-side along the outside edge of the board feet into the area you left clear of glue. Align one panel board on the speed square, line up with the top of the leg and push it up so it just touches the spacer board. Once it’s in place and square, pin it in place to hold the orientation. Then add the three bottom boards using the board and top panel spacer for alignment and nail them in place. Finally, turn the almost finished side panel over and glue and nail the second leg to the other end of the panel boards using the speed square and spacer for alignment.

Repeat this step for the remaining three panels.

6. Add protrusions to the inside of two side panels. Before assembling the planter box, add small overhangs to either the two wide frames or the two narrow panels. It does not matter which panels have these protrusions, as long as they are equal in size. The tabs will hold the boards for the bottom of the box, so they need to be opposite each other. It’s much easier to do this before assembly, so don’t skip this step.

I used the scraps I cut when I cut the legs to width. To glue, I cut the scraps to 9 ½ inches long, then glued and Brad nailed them in place.

  • Note: I added two rows of overhangs so I can adjust the depth of my planter box if needed. The first is 6 inches below the top of the board and the second is 9 inches below.

7. Assemble the box. This is an easy assembly, but be careful. The most important thing to remember is that the narrow panels sit inside the wide panels. This is what makes the planter box square—if the wide panels sit inside the narrow, you’ll end up with a rectangle.

To assemble, apply glue to the inside edge of the legs where you placed the spacer in Step 5. Then, lock all the panels together by lifting them up on a flat surface such as the assembly table or table saw. This will help keep the bottom of the legs level so the planter box doesn’t wobble.

Once the boards are interlocked, add angle clamps to the top to keep everything square and in place. Then wrap the strap clamps around the outside of the box and tighten. I used two straps, one about an inch from the top and the other about an inch from the bottom of the panel boards.

Make sure all the corners of your planter box are square. Measure from one corner to the opposite and then between the other two—both diagonals should be the same length.

  • Note: If you don’t have belt clamps and/or corner clamps, use regular clamps.
A DIY cedar planter box assembled, glued, clamped and sitting on a table in the middle of a woodworking shop.
The complete glued and clamped assembly. Jean Levasseur

8. Cut the boards for the base. Exact measurements will vary somewhat depending on how thick the boards were to start with and how much material you sanded. My boards were just under 10 inches long. Two I left 3.5 inches wide and cut the third to about 2.5 inches. The boards should fit snugly, but not so tight that they can’t slide in and out.

9. Cut the miters for the top frame. This step is basically like making a frame to fit over the box and hide all those ugly seams. The trick is to make it slightly smaller than the inside dimensions of the box. I left about a ⅛-inch overhang on all four sides so I didn’t have to achieve perfect alignment, which is hard to do with unsanded wood—why, it’s hard to do with perfectly sanded wood.

[Related: Every woodworker should know how to mill their own boards]

Cut the corners with a miter saw set to 45 degrees or with a miter saw on the table saw. I used the latter option, again because I was already working with my table saw. First, I cut a 45 degree miter on one end of each board. I then used a stop block to set the finished length and cut the counter miter for all four boards. This is a better way to ensure that all four are the same length than trying to measure and mark them individually.

Once all four boards are cut, attach them with a strap clamp if you have one or regular clamps if you don’t, again making sure the frame is square.

10. Glue the top panel to the box. Apply glue along the top edges of the box and line up the top frame. Try to keep it square on the sides and the inside sticking out steady. Clamp the frame in place to allow the glue to dry.

11. Sand away any unsuitable adhesive. Chances are you have some glue to clean up. Use 120 grit paper on the orbital sander and/or your hands to finish sanding any areas of the box that need it, especially around the joints. Don’t worry about the inside of the box—it will get covered in dirt or pots.

12. Install baseboards. Slide the baseboards into place. I didn’t glue or nail them because there is no way they will fall off. This allows me to move them in the future if I need to change the depth of the box for any reason.

An empty DIY cedar planter box on a blue rug in front of a white door inside a house.
The empty planter, showing the mitered top frame and baseboards at the bottom. Jean Levasseur

13. (Optional) Line the inside with horizontal fabric. Cedar is a rot-resistant wood, but rot-resistant does not mean rot-resistant. If you plan to fill the box with dirt directly, consider adding some horizontal fabric to the inside of the box to add a barrier between the dirt and the wood. Cut the horizontal fabric to size with scissors, then secure in place with a staple gun.

And you are ready to plant! We plan to put existing pots inside the planter box because it’s easier and faster, but you can definitely plant directly into the box itself—we might end up doing that in the future.

Now all you have to do is figure out which plants will enhance your curb appeal the most. Fortunately, the folks at PennState Extension have put together a guide to some best practices for container gardening. Please come out and garden.

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