DIY Modern Minimal Concrete Planter Boxes

There’s something about simple, straightforward concrete planter boxes that appeals to many people regardless of their style preferences. However, the price tag of a solid-form concrete planter can be a deterrent. If you can relate to this dilemma, you’ll be happy to learn that this simple DIY idea is not only fast, but it’s quite cost-effective and easy. That’s a win-win-win. Let’s get started, so you’re ready for the planting season.

DIY Level: Beginner

Materials Needed:

• Four (4) equal-sized, straight-edged concrete pavers (example shows 12” squares)
• Landscape adhesive
• Ardex feather finish (not shown)
• Concrete sealer
• Potting soil & plants

Begin your project on a clean, flat surface outside. I recommend some concrete steps if possible, as the rise of the nearest step will provide some 90-degree angle support while the adhesive on your planter box dries. Set your first concrete paver (Paver A) on its side, propped next to the step for support.

Apply landscape adhesive to the side edge of your second concrete paver (Paver B). A zig-zag application is helpful to solidify the adherence and the method I recommend.

Bring the glued edge of Paver B close to the side face of Paver A. Straighten their alignment to as close to a 90-degree angle as possible.

Attach the edge and face together, pushing them tightly into each other.

Try to keep a 90-degree angle here, but don’t stress about it yet. You’ll finalize the angles a little later. This is one reason, however, why it’s helpful to utilize any nearby vertical faces – it’ll help the planter stay upright solidly without having to worry about its falling over while the adhesive is still fresh.

Apply landscape adhesive to the side edge of your third concrete paver (Paver C), and push it firmly into the side face of Paver B. Again, do your best to maintain 90-degree corners, but don’t be too precise quite yet. Note: If you want to end up with a true square planter box, it’s important to pay attention to the attachment of the side edges to the side faces, so all sides end up equal.

Grab your fourth concrete paver (Paver D) and apply landscape adhesive to one side edge and also to the opposing side face. Keep the zig-zag of adhesive on the side face about the same size/width as the adhesive you put on the side edge – this shouldn’t be hard, as you’ll likely have a feel for it by now.

Line up Paver D so that (1) the glued side edge will press into the side face of Paver C, and (2) the glued side face will press into the side edge of Paver A.

When all sides are pressed firmly together, square off each corner to a perfect 90 degrees. When the planter box is squared off and secure, leave it alone for at least 24 hours, or until the landscape adhesive cures and dries completely.

Optional: carefully transfer the concrete planter boxes to a gravel workspace so the Ardex feather finish won’t mar any existing concrete surfaces.

Mix up the feather finish as per package instructions.

For each concrete planter box, I used a mixture of about 3 cups of powder…

…with about 2 cups of water.

Stir it up.

This is slightly more water than the instructions call for; play around with your ratios to get a good peanut butter-like consistency.

You’re going to apply the feather finish freely to the exterior faces, tops, and upper half or third of the interior faces of your concrete planter box.

Start by slathering some feather finish to the inside corners. Your goal here is to cover the seam and the adhesive.

Load your spreader with feather finish.

Apply to the inside wall’s top half (or top third).

Repeat for other inside corners and walls, then move onto the four top edges of the planter.

On the outside walls, first fill the adhesive crack.

To do this, work your loaded spreader horizontally over the crack, from top to bottom.

Work out excess toward the center of the wall.

When the surfaces near the adhesive crack are covered, work your spreader vertically to make the adhesive crack smooth. If you go horizontally over this, your spreader will always dip slightly into the crack. Your last passes should be vertical to make the crack disappear.

Continue to work feather finish around the outside wall, adding more to your spreader as needed.

Add additional feather finish to the corners; you can always sand these down a little later. It’s better to have more, with a solid covering on the corners, than less.

Smooth the entire wall to your taste. Some people prefer a really raw concrete finish (very industrial), while others prefer more subtle ones.

Run your spreader in multiple directions to decrease bubbles and increase interesting spreader lines. (If you like that kind of thing…which I do.)

Loosely check your corners and edges from multiple angles here; if you want to smooth them out a bit, go ahead. Keep in mind, though, that too much messing around with the feather finish can make things look worse. You’ll be able to sand away rough corners and edges after it dries, too.

You can even smooth out corners with your finger, if you find that helpful.

Of course, it’s up to you and your preference, but I would recommend that you don’t be too careful about smoothness here.

Leaving in some of the trowel lines will give the concrete planter a little more character and that industrial aesthetic we tend to love.

Let the feather finish dry thoroughly. Touch up as needed.

Sand with coarse to medium grit sandpaper if you want; this isn’t necessary, though.

It simply depends on your preferences for raw industrialism aesthetics – the rougher your concrete planter box is, the more industrial it will likely look.

Choose a concrete sealant that is designed for exterior use.

The sealant will probably have a bluish-white tinge to it, resembling skim milk or similar. Although perhaps disconcerting, don’t worry about this; when it dries, the sealant will be completely clear and glossy.

Sealing is rather easy; however, this stuff tends to dry fairly quickly, so your application time might be limited. I recommend a system that will allow you to seal the entire planter box strategically and eliminate, or at least minimize, unnecessary drips of sealant. Start with a generous swipe across the top surface of one side. Ignore the dripping down the sides for a minute.

Seal the inside of the planter box on that side, beginning with the two affected corners and then covering the inside wall between them.

Then move to the outside wall on that same side of your planter box, and seal the entire thing. Look briefly on both adjoining sides for noticeable drips and wipe them away before moving onto the next side.

Repeat this method for all four sides; then do a final once-over for any lingering drips of sealant.

Let the whole thing dry thoroughly. Then flip it over and seal the bottom edge – not for the concrete paver itself, but more for the very edge of feather finish on the bottom edge that may not have been effectively sealed yet. This will help to prevent moisture’s creeping up through the feather finish from the ground when it rains or whatever.

Place the concrete planter boxes where you want them.

In this instance, two concrete planter boxes flank the ramp up to a backyard shed.Loving the way that they look, however, we are planning on making four more to spread around the backyard landscape.

Fill with potting soil and add your plants, whatever they may be. Example shows a single tidal wave petunia in each concrete planter box, which will fill the top space beautifully in just a few weeks.

Be sure to add a water source, such as a sprinkler or drip system, or take care that you water your plants every day. Any growing thing planted above ground will dry out faster than plants in the ground itself, due to increased wind and heat effects on the pot itself and, consequently, the soil.

It’s such a cool, crisply industrial look for such little cost and effort.

We love the contrast of a bright flowering plant with the grey concrete of the simple planter box. Beautiful.

And, if you’re making one, I highly recommend making at least two at the same time. Because you’ll want more of them. Trust me.