Make Concrete Garden Boxes PART 1 – Precast Form Build

Assemble the SIDES

Now assemble the Sides with glue and screws.

Attach side walls to side hold-down
Attach side walls to side hold-down

I attached a stop strip to my workbench and onto it I attached two hold down clamps. The clamps quickly and securely hold part C to the table. I also attached an end stop to align the ends of B and C.

Hold down clamps - man about tools cast form mold concrete garden boxes
Hold down clamps

Then apply a small bead of exterior wood glue to the side and press it into place. I drill a pilot hole into the hold-down strip then run in a screw.Then drill pilots thru the side all the way down and secure it with screws. Check each assembly that the L shape is 90 degrees.
Set these aside for the glue to set up.
The Sides have a small square Stop Block to hold the ends in place. Rip and cut these to length.

Drill pilot holes in small square stop block - concrete garden boxes
Drill pilot holes in small square stop block

Then pre-drill these as well.

Assemble the ENDS

Now assemble the ends parts E and F.

Attach block to end assemblies
Attach block to end assemblies

I marked the center of each to help with alignment and also used the hold down clamps to secure the pieces during assembly. Like before, apply glue, drill, and attach with screws.
Drill pilot holes into E and attach the Blocks with glue and screws.

Attach stop blocks to side assemblies
Attach stop blocks to side assemblies

Mark the side assemblies and attach the small square Stop Blocks with glue and 1″ screws.

Make the INSET

Next we will make the angled Inset.

Rip Inset at 30 degrees on table saw
Rip Inset at 30 degrees on table saw

I used 5/8″ material, and beveled the sides 30 degrees on the table saw. And cut to length on my miter saw also at 30 degrees.

Attach inset with glue and screws
Attach inset with glue and screws

I attach the Inset part to the base by pre-drilling with a countersink, then attaching it with glue and short screws.

Reduce end of dowel to fit into pipe
Reduce end of dowel to fit into pipe

Lastly, I’ll cut the 5/8″ Dowels to length and chamfer the end on the disc sander, or you can round off one end with coarse sandpaper.

Glue dowels into block and sides - man about tools cast form mold concrete garden boxes
Glue dowels into block and sides

Glue these into place and let the glue fully setup.
Cut a short test piece of half inch PVC electrical conduit.

Test fit pipes on dowel pins
Test fit pipes on dowel pins

Use this to test fit the Dowel Pins. Use some sandpaper to slightly reduce the diameter of these dowels so the pipe slips on easily. The inside diameter of the pipe is bit less than 5/8″.

Fill screw head holes with wood filler
Fill screw head holes with wood filler

Fill the screw holes with wood-filler, let it dry completely, then sand smooth. Also fill any gaps in the plywood and add some filler to the inset where it meets the base. Give everything a light sanding to break any sharp edges.

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

DISCLAIMER: The majority of these are affiliate links, so if you purchase through these links, I make a little bit of return from it. And a big THANK YOU If you choose to support me in that way!

25 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.

    1. Bob, looks like I need to get the follow up video for the project ready soon. I’ll try aircrete and see what happens.

  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.

  5. It is a great idea. I used to work in tilt-up construction. This is right up my alley! I just don’t have a yard to build in, yet.

    1. I have been using cedar, spruce, or plywood. All work fine. The best results are when you apply multiple coats of polyurethane to wood before casting.

  6. On later projects you used mineral oil to seal the forms. Which works better – polyurethane or mineral oil – for multiple re-use of the forms?

    (I am using southern yellow pine 2x’s for form material)

  7. I bought the plans a while back, made about 15-20 forms of the various sizes indicated (2′, 3′, and 4′). Haven’t poured any yet this spring but am about to start (have to service my concrete mixer). I use regular bag-mix concrete with the fiber in it; solid concrete is heavier but much more impact resistant when mowing around the beds, and once set I don’t plan on moving them in my lifetime. I have tried several other options (structural fibers instead of steel reinforcing doesn’t seem to work well, too much air loses strength, too much water cracks, etc.) and am using reinforcing for 8″ block wall from the masonry supplier here. When they come out well, I love them – even started using a bit of color in the mix to get a terra cotta panel. Even when they don’t come out well, we use them laid flat to make borders around perennials like rhubarb and asparagus. Next I want to carve patterns in the relief panel to get more artsy.

    One problem I have found is the dimensions do not make for modular panels. 24″, 36″, and 48″ SHOULD be the dimension from center-to-center of the sleeves for the rebar pins (I use #3 bar as it gives more wiggle room when building 12′ long beds). If you build your forms that way, then the panels can be added or subtracted depending on what sizes I have available; right now they are not. For example, I was building a bed and rand short of 4′ panels so I figured I could use 2-2′ panels for each 4′. Nope. Can’t use 4-3′ panels to make a 12′, either. So PLEASE put that out there and put a note on your drawings. I realize you are conserving lumber length, but for reusable forms wouldn’t it make more sense for modular panel sizes? After all, I have to rip a 2×10 down to get a full 8″ width anyway! I will use these forms until they are no longer suitable, but the next ones I make will be truly modular.

    1. Chris, thank you for your thoughts here and letting me know what you have done with the panels. I am aware that the sizes are not modular and that they should have been designed with multiple sizes from pin to pin. I think somewhere that was mentioned in a blog post but, I will address that again. I will look to add a note to the plans going forward. I appreciate the feedback!

  8. Hi From Switzerland.

    Super project and comprehensive plans, thank you kindly for sharing.
    I thought to let you know that I picked up an small error with the metric measurement of parts F on Page 3. The correct conversion for 6.25 inches = 158 mm (195 mm is incorrect)

Leave a Comment

Your email address will not be published. Required fields are marked *

Shopping Cart
Scroll to Top