Building ProjectsGardening

Make Concrete Garden Boxes – Complete Precast Form Build

By May 26, 2018 January 21st, 2019 No Comments

Make your own Concrete Garden Boxes! Build the forms and precast your own reinforced concrete panels that lock together to make long lasting and durable concrete garden boxes. See the complete build video that shows step-by-step how to make the plywood molds for the precast garden boxes. An alternative to concrete garden edging.

See the Build Video and download the Plans.

Background for the Cast Concrete Garden Box Project

man about tools cast form mold concrete garden boxes

Hi is Kent from Man about Tools and today we are building plywood moulds to precast our own concrete garden box panels.

man about tools cast form mold concrete garden boxes

Concrete Garden Box Panels Link Together

These panels link together to create long lasting, rot proof concrete garden boxes.

Precast concrete raised garden beds.

If you’re like me then you’ve made many wooden garden boxes only to see them rot away over time. Now, I know wooden boxes have their place and they are cheap and easy to build, but they just don’t last.

plastic-lined wooden garden boxes

Plastic Lined Wood Boxes

I’ve used plastic liners, made them from longer lasting red cedar, bought a truckload of cheaper culled lumber and no matter what I do they still rot away.

So I decided to try making some boxes from concrete. One option is to build plywood frames, stake them into the ground and pour them in place. In a similar fashion as pouring building foundations or retaining walls. That does work well, and I like that option if you have the materials.

But instead, I want to make a more decorative, unique garden box, one that might be more suited for flowers, or closer to the house, instead of a large vegetable garden.

Poured concrete raised beds.

The panels that I make interlock in the corners in a straight line or at a 90 degree angle. I cast in plastic pipe so they lock together with rebar.

Pouring your own concrete panels gives you the flexibility to modify this simple design to your liking. You can easily change some of the dimensions to make them longer, shorter, taller, or thinner. You can also combine panels of different lengths as I’m going to do in this video.

I have two identical moulds that make a 48 inch long panel that is 8 inches tall x 2 1/2 inches thick.

48" plywood concrete garden box form

48″ plywood concrete garden box form

I’m going to make 4 new moulds similar to that original design but, I’m going to shorten the length to 36 inches. Having a box 3 feet wide can make it easier to reach in to plant and weed.

DIY concrete raised beds.

I have plans available for download and I also have a list of all the tools I use for each project. So please check those out.

  1. The PARTS
  2. FIRST cuts
  3. assemble the SIDES
  4. assemble the ENDS
  5. make the INSET
  6. TEST FIT and ASSEMBLY
  7. apply FINISH
  8. PIPES and WIRE
  9. final ASSEMBLY
  10. pour CONCRETE
  11. make the GARDEN BOX

Concrete Garden Box Sides – the Build

Here’s how I build the forms to make the concrete panels.

4 Concrete Garden Box Forms from 1 sheet of 4x8 plywood

4 Concrete Garden Box Forms from 1 sheet of 4×8 plywood

The Parts

One sheet of 3/4″ plywood is enough for 4 forms. I’ll also need a length of 5/8″ material 3″ wide for the decorative insets (104″ in total). And a 4×4 – 32″ long that I’ll rip into a 2 1/2 x 2 1/2 inch block to hold one of the pins. These pipe holder pins are made from 5/8″ diameter doweling (16″ required).

Wire Grid, Pipes, and Wire wrapped around the Pipes are embedded in the concrete panel

Wire Grid, Pipes, and Wire wrapped around the Pipes are embedded in the concrete panel

Here’s the main parts of the mould. A base, the inset, the sides, the ends, and pipe holder pins. Cast into the concrete there’s a reinforcing stiff wire grid, PVC pipe, and wire loosely wrapped around the pipe.

Base and Inset

Base and Inset

SIDES for the form

SIDES for the form

ENDS of the plywood mold

ENDS of the plywood mold

First Cuts

 

 

First Cuts to make plywood sheet easier to handle

First Cuts to make plywood sheet easier to handle

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.

man about tools cast form mold concrete 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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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.

Assemble the SIDES

Now assemble the Sides with glue and screws.

Attach side walls to side hold-down - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold 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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

Attach stop blocks to side assemblies

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

Make the INSET

Next we will make the angled Inset.

Rip Inset at 30 degrees on table saw - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete 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.

"<yoastmark

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 - man about tools cast form mold concrete 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 bold cutters and have it ready to lay in the wet cement.

Cut wire mesh with small bolt cutters - man about tools cast form mold concrete garden boxes

Cut wire mesh with small bolt cutters

This will make the concrete panels very strong.

Shims to adjust overall panel length to 36" - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold 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.

Latex caulking to waterproof forms - man about tools cast form mold concrete garden boxes

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 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 - man about tools cast form mold concrete garden boxes

Spray with vegetable oil cooking spray

Once the caulking is dry, spray the mould with a Releasent Agent to keep the concrete from sticking. I used Vegetable Oil Cooking Spray for this.

Pour Concrete for the Concrete Garden Box

The 48 inch mould I made takes one bag of cement to fill. These new shorter moulds 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.

Mix concrete with cold water in wheelbarrow - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

Fill form half way then add wire grid

Shovel the wet cement into a mould and fill it half way, lay in the Wire Mesh, and fill the rest of the way.

Tap with hammer to settle concrete and remove bubbles - man about tools cast form mold concrete garden boxes

Tap with hammer to settle concrete and remove bubbles

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

Cover wet cement with plastic sheet - man about tools cast form mold concrete garden boxes

Cover wet cement with plastic sheet

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

Remove screws and gently remove sides and ends - man about tools 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.

Lift panel from base - man about tools cast form mold concrete garden boxes

Lift panel from base

Properly formed concrete panel for garden box - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold 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 - man about tools cast form mold concrete garden boxes

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.

man about tools cast form mold concrete garden boxes

Level and square panels in garden

Pound in rebar to secure corners - man about tools cast form mold concrete garden boxes

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 - man about tools cast form mold concrete garden boxes

Secure corners with 12″ epoxy coated rebar

For deeper 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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