Make Concrete Garden Boxes PART 1 – Precast Form Build

Test Fit and Assembly

With pan head screws, assemble the form using the layout lines as a guide. Check that everything is square, fits tight, and there are no gaps.

Test fit and square all part assemblies
Test fit and square all part assemblies

If there are any variations with each assembly, you may need to try different part combinations to see what fits together best.

Label parts - garden boxes
Label parts

Label each mould assembly one through four. And also label each part assemblies with the corresponding base so you reassemble them the exact same way each time you use them.

Apply Finish

I apply several coats of Polyurethane Finish, lightly sanding between coats.

Apply 4 coats of polyurethane finish

I used four coats of this durable flooring finish. Wet cement is very corrosive so you want to protect the wood to get the most life from your concrete forms.

Pipes and Wire

Cut the PVC conduit down to length.

Wrap wire around pipe to reinforce concrete - cast garden boxes
Wrap wire around pipe to reinforce concrete

Also cut a 30″ length of stiff wire and wrap it loosely around the pipe. This will reinforce the concrete and give it strength where it is the thinnest.

Final Assembly

Cut a section of Concrete Reinforcing Mesh to size with small bolt cutters and have it ready to lay in the wet cement.

Cut wire mesh with small bolt cutters
Cut wire mesh with small bolt cutters

This will make the concrete panels very strong.

Shims to adjust overall panel length to 36\"
Shims to adjust overall panel length to 36″

This is where I remembered that not all 3/4″ plywood is fully 3/4″ thick. Most is now slightly undersized and I laid out the parts for actual 3/4″ plywood. It’s an easy fix by adding a few slim shims.

Add wire and pipes on final assembly - concrete garden boxes
Add wire and pipes on final assembly

Once assembled, the molds need to be made water tight. I use a small bead of Latex Caulking to do this.

Apply Caulking

Latex caulking to waterproof forms
Latex caulking to waterproof forms

Latex works well as it’s not overly strong and will allow you to disassemble the mold easily once the concrete sets up. Don’t use regular silicone for this. It’s too strong and will be difficult to remove later. Latex caulk is all that’s needed.

Level Forms

Level forms in both directions on secure surface - man about tools cast form mold concrete garden boxes
Level forms in both directions on secure surface

Place the moulds on a strong and secure surface and level them in both directions.

Spray with vegetable oil cooking spray
Spray with vegetable oil cooking spray

Once the caulking is dry, spray the mould with a releasing agent to keep the concrete from sticking. I used Vegetable Oil Cooking Spray for this.
UPDATE: Cover the pipe and pipe wrap wire with a cloth to keep the oil spray away from it during this step.

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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