Make Concrete Garden Boxes PART 1 – Precast Form Build

First Cuts

I had my lumber supplier cut a 3/4″ plywood sheet into 4 strips, each 14″ x 48″. I have a small portable table saw so working with a full sheet is challenging. You can use a circular saw with a guide as an option too.

cast garden boxes - Cut down base
Cut down base

I’ll start by cutting these 4 bases down to the length of 43″. Then I lay out the design on each base. I’ll used a black marker to darken these pencil lines. I cut the rest of the 4×8 sheet roughly in half to make it more manageable.

Layout panel outline on base
Layout panel outline on base

Cut the Side parts

Once this is done start ripping the 8 Side Walls from the balance of the plywood sheet. Eight pieces are required. Then cut to length and label them part “B”.

Cut the SIDES to length - man about tools cast form mold concrete garden boxes
Cut the Sides to length

Now rip the eight Side Hold-Down strips and also cut them to length. There’s going to be a lot of pieces to this build so label the parts as you go along. On the drawing you will find a scheme that I came up with that works well. For instance, all the Sides are B, the Hold-Downs for the Sides are labeled C, and so on.

Pre-drill pilot holes on inside face of side wall
Pre-drill pilot holes on inside face of side wall

The Sides and Hold-Downs are attached to form a right angle “L” shape with glue and screws. I pre-drill the Sides on the inside face, 3/8″ up from the bottom edge with a countersink drill bit. Then draw a line and layout the hole locations approximately spaced 8 inches apart. A clamp as a stop on my drill press speeds this up. I also mark pencil lines on this stop as a spacing guide. I want the screw heads to be recessed into the hole. I’ll fill these holes with wood filler later after assembly.

man about tools cast form mold concrete garden boxes
Drill pilot holes for side hold-downs

I’ll also pre-drill the Side Hold-Down strips with a one eighth inch bit along the center line spaced about 8 inches apart. Later, during assembly, I’ll use pan head screws to attach the Sides to the Base, (part A on the drawings).

Drill 5/8" hole with Forstner Bit for pipe support dowel - man about tools cast form mold concrete garden boxes
Drill 5/8″ hole with Forstner Bit for pipe support dowel

Mark the sides and drill a 5/8″ hole with a Forster bit in each for the Pipe Support Dowel.

Set aside parts B and C.

Cut the End parts

Now I’ll rip and cut to length the End Walls. They are 2.5 x 8 inches and labeled E. These have their own Hold-Downs. Rip and cut these to length and label F.
The Ends and End Hold-Downs get pre-drilled in the same manner as the sides and their accompanying Hold-Downs.

Rip a 4x4 down to size
Rip a 4×4 down to size

I make the Pipe Holder End Blocks, part G, from a fir 4×4. Then rip this on my saw using several passes from both sides down to 2.5 x 2.5 inches. I then cut these to 4 inch lengths using a stop on my miter saw.

Mark center of block
Mark center of block

I inspect these blocks, label the UP side then with a square I mark the center of one end.

Center punch block
Center punch block

And use a punch to make a small divot that will held guide the drill bit.

Drill block for pipe support dowel pin
Drill block for pipe support dowel pin

I chuck a 5/8 Forstner bit in the drill press. I have stop blocks clamped to the table to help secure the block during this processes. You can also use a vice to hold the block if that works better for you. Drilling into end grain like this can sometimes cause the bit to wander so go slow and allow the chips to clear. Drill this hole 1 inch deep.

Continued on the Next Page

14 thoughts on “Make Concrete Garden Boxes PART 1 – Precast Form Build”

  1. I wonder if these molds, or some variation, would work for forming hypertufa into lego-style blocks that are also insulating, functionally. How great it could be to build a mobile greenhouse from them! End less possibilities! Your videos and written, illustrated instructions are excellent and inspire many variations, says this retired tech writer and lifelong gardener.

  2. How would these go stacked?

    I came across your youtube video when looking at options for building an aquaponic landscape for my back yard – looking at all different build types but ideally I want one that is unique and DIY for that extra sense of accomplishment!

    It would be quite large scale though so am just curious how much pressure they could cope with.

    I believe I would probably make mine a little thicker and put some extra rebar slots along the internal part of the panel to ensure they can hold their own but not sure how I would go with the stacking aspect…

    1. Hi,
      I have not considered that application for the panels. So you may have to do some tests to see how that would work out.

  3. I like the construction of the molds, it is simple and straight forward. Have you ever tried to build the panels taller? Or was 8″ the best height?

    1. Hi,
      8″ is about as deep a soil depth required to grow almost anything. So I decided that would be good for me. I have not built them taller.

  4. Steve Christensen

    Would you consider making this with a Melamine board? Would the melamine surface be more resistant to the acid corrosion and release easier?
    This looks like a great project and I look forward to doing it myself.

    1. I would consider it as melamine has a very smooth, waterproof, and non-stick (like) surface. However, it’s usually on a particle-board base and that is why I prefer solid wood or plywood. For one-off’s melamine is great. But if you want to do multiple castings then sealing all exposed wood is a must.

    1. The error was with the building supplier selling plywood marked as 3/4″ that’s actually undersized. My concern is that this might be more and more common. That’s why I mentioned it in the video and make mention of it on the plan instructions.

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