DIY How-to: Concrete Planter

Greetings. And welcome to another DIY project. In this video I show you, very roughly, how we built these cool and rugged concrete planters.

I don’t go into too much detail in the video, its more of an overview of our process, but I have plans and measurements and materials available to download here.

Below are semi-detailed instructions and I cover the following steps:

1. Assess & Design
2. Materials & Tools
3. Building the Outer Form
4. Building the Inner Form
5. Final Preparations
6. Mixing the Concrete
7. Filling the Form with Concrete
8. Removing the Form
9. Curing the Concrete

Assess & Design

So, what was the objective for this project? We wanted something modern and simple, so we went for the square shape. And concrete is such a versatile material; its cheap, it can look old or new, and its fairly easy to work with. Its also heavy so its unlikely to be stolen, so there's that too. 

We wanted something fairly large to take up some of the space on the curbside verge, and with some scale to contrast with the rusty pipe arrangement. We settled on a 17” x 17” square by 24” tall with a 2” thick wall.

Materials & Tools

So lets get started on the build. What you need first, obviously is to buy your  materials. You will need;

• 4’ x 8’ sheet of plywood or melamine*
• Some general purpose 1½” screws (I used some leftover deck screws and drywall screws I had lying around)
• 140” (about 14’) of 1” x 2” or similar size wood (Again, I used some scrap wood I had leftover)
• Some wood glue
• 3” of PVC 1” pipe
• Various scrap wood - 1x2, 2x4 etc
• Shims
• 90° brackets

*plywood is cheaper, but can be in quite poor shape. Birch plywood is a much smoother finish but is double the price. Melamine is quite heavy to work with and absorbs water more, so is prone to swelling, but it is very smooth and produces a lovely finish. Its also cheaper than the birch plywood. We opted for ½” birch plywood, but in hindsight I wish we would have used the 3/4"

The tools we used on this project were:

• Table saw
• Circular saw
• 1” barrel saw
• Drill with various bits and drivers
• 90° Corner guides
• Hammer
• Quick clamps or C-clamps
• 24” Woodworkers clamps
• Small cement mixer
• Large plastic masonry tub
• Various trowels and scrapers

Build The Outer Form

First thing we did was cut the outer form pieces. Again, measurements and plans are in the link in the description, but to summarize we had 2 sides cut to 17” x 24” and 2 sides cut to 18” x 24” which accounted for the overlap on each end. I used some scrap 1x2 wood as a straight edge while cutting with the circular saw.

With the 4 sides cut I used the corner guides and clamps to assemble the outer form, screwing the sides together at the top and bottom. 

For the base, I cut a roughly 24” x 24” square out of the plywood. Using the assembled box as a guide I attached 4 retaining strips on the base to hold the form in shape. 

For the middle bracing, cut 2 strips of 1x2 to 18” and attach to the outer form by drilling pilot holes, counter-sinking and screwing from the inside of the form.

This bracing and the retaining strips are a very important step - especially if you opt for ½” material like we did - as screws alone will not hold the from together under pressure from all the concrete.

Build The Inner Form

With the outer form complete, next we cut the pieces for the inner form. As the measurements for these are smaller I was able to use the table saw. Much easier. We had 2 sides cut to 15” x 22” and 2 sides cut to 14” x 22”, again to account for ½” overlap each end.

Its important to get the measurements accurate as you want the inner corners to be perfectly square and smooth. Otherwise it makes removing the inner form more difficult. I learned that the hard way, as you will see in the video.

Next we cut 8 corner braces from 1x2 material - roughly about 4” long - with 45° miters on each end. I drilled pilot holes and a deep counter sink, trying not to split the wood. We assembled the inner form using the corner guides and clamps as before, then secured the corner braces to the inside of the form.

For the inner base, I cut a 16” x 16” square out of the plywood, as this would need to fit inside the box. I then marked the center point and drilled a pilot hole. Using the 1” barrel saw I cut a hole in the center of the base for the PVC pipe to fit.

With the PVC pipe cut to size I tapped it into place in the base board, leaving 2”  protruding from the bottom. This will ensure the planter has a drain hole, and that the inner form will leave a 2” space to fill with concrete.

To hold the base in position simply glued a small wooden tab ½” from the bottom of the inner form on each side. For convenience I also added a couple of brackets I had lying around to use as handles.

Final Preparations

Before I begin mixing the concrete I had to  prepare the wooden surfaces. To help release the form once the concrete is dry, I used some spray cooking oil on every surface of the inner and outer forms. It’s a lot cheaper than professional concrete release agent and it does a pretty good job. 

I use a putty knife to work the oil into the wood grain, to make sure its really well coated. Just to create a water barrier to stop the wood getting wet from the concrete, as well as helping it release when we remove the form.

I set the base up on two ‘rails' of scrap 2x4. That way I’m sure it’s not sat on an uneven surface. I use a spirit level to make sure the planter is properly level, raising one side of the rails with shims to make any adjustments.

Check the bracing is nice and tight, so the form holds together under the weight of all the concrete.

With the inner form built I just insert it into place. To keep the inner form centered within the outer form, I attach a small section of 2x4 to the inside of each wall of the outer form just using a 1½” screw. I just screw these wedges to the sides and that creates a good spacer to hold the inner form in position, keeping the 2” wall on all sides.

For additional support to the wood bracing around the middle, I used four long 24” woodworkers clamps to help hold the sides together. 

Mixing Concrete

I can only mix one bag at a time in our mixer, using about a gallon of water per bag. It took a several minutes of churning and some manual labor to get all the dry material at the back of the mixer loose and combined with the water. Getting a good mix is essential to getting strong concrete so I made sure it was not too wet but still well mixed. I mixed a bag at a time, then poured the mixed concrete into a large tub, where my wife would shovel it into the form. I continued to mix the next bag so there was a constant supply of wet concrete ready to pour.

Filling the Form with Concrete

With the inner form set aside for now, I put a plastic cap from a spray paint can in the middle at the bottom of the form. This is not critical but it helps keep concrete out of the area where the PVC drain hole form will sit.

After transferring the concrete from the mixer into a large plastic tub, we begin shoveling concrete into the bottom of the form until about it’s about 2” deep, making sure to agitate the mixture well into the corners and removing any air bubbles. Then we had to insert the inner form. One of us quickly removed the spray paint cap while the other quickly but carefully slid the inner form into position. We had to wiggle it down into the concrete a bit, but once seated I wedged it tightly between the spacers using the shims.

We continued to fill the sides the now narrow walls of the form making sure to tap and level the mixture inside the form, packing it tight into all corners and removing any air bubbles. 

When the concrete reaches the level of the 2x4 spacers, we removed the 4 screws holding them in place and carefully pulled out all 4 of the spacers. We carry on filling up the form with concrete until it is to the top of the outer form, making sure the bracing and clamps are holding the form together.

Once it is full, I used a small trowel to level the top of the planter, smoothing the concrete to a flat finish. I top up or remove excess mixture as necessary, making sure any large aggregate is worked well down below the surface.

With the hard work done, we covered it with plastic and left it to cure for a day or so to fully cure. In the video, I leave it to cure for 2 days. This allowed the concrete to fully harden, which made it more stable but also made the inner form harder work to remove, as you will see. The first planter we built, I removed the form after 12 hours and the concrete was still quite wet, which made it easier to remove but was still a bit sloppy and fragile on the edges.

Removing the Form

Removing the outer form was easy. The inner form was a bit more of a process. I first remove the inner bracing 45° wedges.

Then using a rubber mallet on a bit of scrap wood, I tap out the short-sides first. It can take a little bit of work if the wood swelled or bowed under pressure. And if the corners are not completely smooth and flush, that can make them a bit sticky to get out.

But with a little bit of work, no rushing, and just gently tapping it out, not trying to get it out in one go. They do eventually come loose. 

You can see in the video I had to resort to a little hacksaw and just shave a way enough wood on the edge to tap out the side.

Finally, I used pliers to pull on the handles to release the base. The PVC pipe released with not too much effort and worked its way out then the base was removed.

Curing

To finish the 7 day wet curing process I hose it down twice a day and cover with plastic to keep it wet for another 5 days. 

After the planter had fully cured we needed to move it to the front of the house. A standard rolling dolly made moving this 250lb planter quite easy, obviously taking good care not to drop and damage all that work. Once set in place we filled with gravel, soil and plants and cracked open a nice cold bottle of grown up soda.

Okay, I hope this gives you some inspiration, and you were able to follow along. Feel free to contact me with any questions if something is not clear or working out for you. Have fun making custom elements for your garden landscaping. See you next time.