Make Concrete Garden Boxes PART 1 – Precast Form Build

Pour Concrete for the Concrete Garden Boxes

The 48 inch mould I made takes one bag of cement to fill (66lb). These new shorter molds will take approximately 3/4 of a bag.
Have everything ready as you need to work quickly before the concrete sets up. I like to add a small additional amount of Portland Cement to my mix to give it a bit more strength (optional).

Mix concrete with cold water in wheelbarrow
Mix concrete with cold water in wheelbarrow

I mix the concrete in a wheelbarrow and use cold water.
Try to not make your mix overly wet. Too much water reduces the strength of the concrete and this extra water may soak into the wood forma and shorten their life.
It can take a while to get a feel for just the right mix.

Fill form half way then add wire grid
Fill form half way then add wire grid

Shovel the mix into a mould and fill it half way, lay in the Wire Mesh, and fill the rest of the way.
UPDATE: When I made this panel I placed the wire mesh too close to the edges. It should be buried deeper in the panel. Cut the mesh grid so there’s only one wire running the length of the form with approximately 2″ pieces of the cross members on each side.

Tap with hammer to settle concrete and remove bubbles
Tap with hammer to settle concrete and remove bubbles

Gently tap the mould with a hammer to help settle the wet concrete and allow bubbles to come to the surface. Also, any tool that vibrates can work well to settle the concrete. I have a drill with an impact setting that I have used for this.

Cover with Plastic to Harden

Cover wet cement with plastic sheet
Cover wet cement with plastic sheet

Now cover the wet concrete with a plastic sheet and leave for at least 2 days.

Remove screws and gently remove sides and ends - cast form mold concrete garden boxes
Remove screws and gently remove sides and ends

Remove the Hold-Down screws and gently pry off the Sides and Ends. Gently lift the panel off the base. The concrete is still “soft” and will not cure to full strength for a few weeks so be careful handling them. Keep the panel wet while it cures to full strength in a few weeks.

Lift panel from base - cast form mold concrete garden boxes
Lift panel from base
Properly formed concrete panel for garden box
Properly formed concrete panel for garden box

I use a paint scraper to remove the old latex caulking and a cloth rag to wipe down and clean the mould parts.

Scrape dried latex caulk with paint scraper - concrete garden boxes
Scrape dried latex caulk with paint scraper

Make the Concrete Garden Boxes

I cut rebar into one foot lengths with a cutoff blade in my angle grinder. I clamp the long length of rebar across sawhorses. A hacksaw will work but it takes longer.

Cut rebar with angle grinder
Cut rebar with angle grinder

I level a spot in my garden and overlap two panels, and check the corners for square then drive in the rebar thru the corner into the ground to secure them. It’s great to reach the point of finally assembling the concrete garden boxes.

assemble concrete garden boxes
Level and square panels in garden
Pound in rebar to secure corners
Pound in rebar to secure corners

I repeat this on each corner to complete the Garden Box. For this box I used 2 – 48″ panels and 2 – 36″ panels.

Secure corners with 12\" epoxy coated rebar
Secure corners with 12″ epoxy coated rebar

For deeper concrete garden boxes you can stack and alternate the panels if you like. You will need longer rebar pins.
We put a layer of cardboard down over the grass then fill the box with compost and soil before planting.
It’s pretty rewarding to remove the panels from the forms. I get a kick out of that every time. I hope this tutorial has informed and inspired you to give concrete forming a try!
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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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