Within the past six years, there have been four serious fires in my apartment complex. As in, burn to the ground, rebuild from scratch, type fires. You see, the buildings were constructed from wood back in the late '70s, so any flame retardant materials typical of more modern apartments weren't used. On top of that, being located nearby a university, it's filled with young people who, at risk of generalizing too much, tend to be less careful than someone... with more life experience.

At least three of the four fires were started by people lighting wooden planter boxes on their balconies on fire by dumping burning ashes (from say, a hookah) into them. Long story sort, the HOA has banned the use of any wooden planters on balconies.

Seeing as I just became interested in gardening due to some strange I'm-becoming-a-boring-adult instinct, I began looking for solutions. Everything available on the market was either a) not very cool looking, or b) too expensive, so I figured that I'll just have to make my own. Could be fun. FILDI!

Here's what I did:

Step 1: What You'll Need

The materials and equipment you'll need for this project are:

Materials:

Quickrete countertop concrete -- I chose this because the aggregate is small (coarse sand, as opposed to pebbles), so it is easier to mix and pour. It's a little more expensive than regular concrete, but it makes life easier.
Melamine boards -- The melamine surface won't allow water to soak into the form and the concrete won't adhere to it at all once it has hardened, making removal easy
Concrete pigment -- Optional. I'm not sure if it was intended to be used with the countertop concrete... the effect I saw was minimal.
Caulk -- I used what I had sitting around in my garage, but I would recommend something silicone-based as it won't react with the concrete at all.
Mineral Spirits
Extruded polystyrene sheets (otherwise known as pink insulation foam) -- They come in a range of sizes so choose what's best for the size of your project. I used 1" thick sheets.
Foam-safe spray adhesive -- Spray adhesive is an easy way to stick the foam sheets together. I used a general-purpose 3M spray adhesive, however there are foam-specific ones as well. Just be sure the adhesive doesn't melt your foam!
Finally, you'll need some kind of sealer or waterproof paint to coat the inside of the planter with to prevent water from seeping into the concrete and causing long-term damage. I haven't done this yet with mine, but I'll update the Instructable with what I use, when I use it!
Screws -- Used for assembling your form

Equipment

Table or circular saw
Clamps (optional)
Utility knife
Straight edge
Metal file (optional)
5-gallon bucket (or something to mix concrete in)
Shovel (or something to mix the concrete with)
Pry bar (to remove foam after concrete has hardened)
Sand paper -- 100 grit
Sanding block
Handheld sander, or something that will produce vibrations (a hammer can work)
Rubber gloves (optional)
Large plastic trash bag

You'll also notice that I have a small bag of fiberglass in the picture. That was part of a failed experiment while trying to make DIY GFRC. It probably would have worked, but it wasn't necessary for the particular geometry I chose for the planter.

Step 2: Planning

I knew I wanted to create something with a modern look, so I opted to incorporate a lot of clean, straight edges into my design. My materials were limited to some remnants I had in my garage, so that more-or-less dictated the size. You can follow along with my design, or improvise your own!

I decided to cast my planter upside-down since that would ensure all of my visible surfaces and edges were smooth and crisp. All of those features would be created by the mold, and not by trying to fudge a flat surface by hand. The bottom of the planter (which won't be visible when it's sitting on my patio) is open to the air and can be as ugly as it decides to be.

Remember to design in a couple of drainage holes to avoid drowning your plants.

You can design features into the outer surface by sticking sculpted pieces of foam onto the inside of the form.

As for a couple of basic idiot-proofing principles to follow, I suggest:

Design your walls to be at least an inch thick if you're making a larger planter. You can go thinner if you're making a small planter. Go what feels right when it comes to proportions, your intuition is probably pretty good.
Keep in mind which surfaces will be visible and decide how to cast your planter accordingly. The mold will be much more precise than anything you can do by hand.
Double-check your thinking! Designing a shape is one thing, designing a MOLD for that shape is another. It's easy to get turned around when flipping and inverting, so be thorough.

Step 3: Assembling the Outer Form

Now comes the fun part of actually making this thing! I started by ripping the melamine boards to size with my table saw. You can see in one of the pictures that I had to improvise a table saw sled to make the horizontal cuts. I could have made those cuts with a circular saw, but I wanted to make sure everything was being cut at right angles.

Once I finished cutting the pieces, I had:

1x base board (bottom) -- 6" x 30"
2x long side boards -- 8" x 30.75" (the extra 0.75" is accounting for the thickness of the material)
2x short side boards -- 8" x 6.75"

Then I arranged the boards into the configuration shown above and drilled pilot-holes for several screws. This ensures that the screws won't cause the board to split/deform. The screws were mainly used to align everything. They do provide some structure, but for the sake of easy disassembly (once the concrete is cast), I avoided using too many.

I clamped any weak spots together, and finally wrapped some tape around the outside to prevent the form from bowing outward due to pressure from the concrete.

Step 4: Prepping the Outer From

With the outer form assembled, the next step is to prep the outer form by sealing the corners with caulk. Do this by running a bead of caulk along the inner corners of the form and remove any excess using a wet paper towel. Once the caulk has dried, clean the inner surface of the form with mineral spirits. This will help ensure the concrete will not bond to the melamine.

Step 5: Prepping the Inner Form

Next up is assembling the foam to create the inner form. This will be the cavity of your planter.

Mark out the correct dimensions onto your foam using your straight edge. Then cut the foam by first scoring it with a SHARP ultility knife. Repeat this several times until the cut is about 3/4" deep. It should slice smoothly. If the blade catches, stop and gently slide the blade over that spot a few times until it begins to cut smoothly again. The blade may not cut all the way through, but now you can fold the foam and it will break cleanly along the line.

I ultimately needed an inner form that was 28"l x 4"w x 5"h with a circular cutout in one corner to accommodate the design on the outer surface

The foam I bought was unfortunately only 24" wide, so I had to cut a few extra short pieces to get the full 28". Foam is pretty forgiving though, and gluing a bunch of pieces together should go pretty smoothly.

Spray a thin coat of adhesive on each side and press the pieces together. Eventually you'll have a chunk of foam to use as the inner form.

Because of my design, I had to cut out a circular feature into one of the sheets. I did this by scoring the shape with the utility knife, leaving plenty of extra space (since the foam only breaks cleanly along straight lines), and breaking the piece off. Then I used a metal file to get right up to the line.

After everything is glued together, use the sanding block to smooth out any uneven surfaces.

Step 6: Prepping the Surface Design

To mold in a design into the outer surface of the planter, I used remnants of the insulation foam. I wanted the features to be set into the surface, so I glued some features onto the inside of the form using spray adhesive. This way, the concrete will not fill in those areas and once it has hardened, those features will show up on the surface after removing the foam. You can also embed things into the surface -- wood, glass, metal, whatever. The sky's the limit, really. Get creative!

My design involved a 3" circle and a long, thin strip protruding from the circle. I marked, cut, and filed the circle to shape, then chopped off some sides so it would fit in the corner of the form. Then I cut the whole thing in half, reducing its thickness from 1" to 1/2". Leaving it at the full 1" thickness would have caused difficulties when pouring in the concrete. I used the same basic method of scoring/folding/breaking the foam to create the long strips. Because the foam is pretty delicate, I had to be a bit more careful, but it worked out pretty well.

Step 7: Getting Ready to Pour Concrete

Once you're finished with any exterior design work, now you need to insert your inner form into the mold. I used spray adhesive to stick my chunk of foam to the bottom of the form. Then, glue some small pieces of foam that stick straight up to the top of your inner form. These will protrude from the concrete and end up as drainage holes.

Once this is done, your form is ready for concrete!

Step 8: Mixing, Pouring, and Vibrating

Now, find a good place outdoors (preferably a place that gets minimal direct sunlight all day) that you'll be able store your form for a few days while the concrete hardens. If possible mix up your concrete in the same place so you don't have to move your form after pouring.

Estimate how much concrete you'll need for your planter and pour that amount of dry concrete into your 5-gallon bucket. I eyeballed the amount I needed and added a little more for good measure. I used a little under half a bag (roughly 35lbs of a 80lb bag) for my planter.

Follow the instructions on the bag for mixing, otherwise the strength or longevity of your planter may suffer.

If you're mixing with a shovel, consider this a fair warning: mixing concrete is hard work! Make sure you mix the concrete completely, as any dry concrete will cause serious issues with your pour.

Once you've mixed the concrete thoroughly, slowly and carefully pour the concrete into the form. Do your best to make sure that concrete makes it into every nook and cranny, especially if you have under-cut features. If you have rubber gloves, you can carefully stick your hand into the form and squish the concrete into tight corners. Just be sure not to bump the foam!

After filling the form all the way to the top (if you've done your math right, your planter will have a good 1"-1.5" thick base), then use your handheld sander/hammer/fists to vibrate or lightly tap the walls of the form. You'll start to see air bubbles rise to the surface and pop. Continue doing this until the bubbles stop rising to the surface. This will minimize the number of voids and pinholes in the planter's surface the concrete has hardened.

Step 9: Cover and Cure

After you've poured your concrete, cover your form with a plastic trash bag. This will help prevent the water from evaporating too quickly, which can cause cracking.

Now leave the planter to cure in a nice, shady area for at least two days before removing it from the form. The concrete will continue to strengthen over the course of 28 days, so the longer you can let it cure, the better. But the next step is the most exciting part, so I'm sure you'll be eager to crack it open as soon as possible!

Step 10: Extracting Your Planter!

After waiting for two days, now comes the best part! I don't expect you'll need much guidance here -- take apart your form and remove the foam with a pry-bar. 'Nuff said.

Step 11: Finishing

Finally, clean up your planter by wet-sanding with 100-grit sand paper. This will remove any foam residue or any crud that was left on the inside of the form before you poured. You can use this as an opportunity to smooth out any blemishes.

As you can see, despite pounding/vibrating the sides of the form, I still wound up having some voids in the concrete. It's possible to get rid of these after the fact by mixing up a small slurry of concrete and filling them in. I chose to leave mine, since I kind of liked the added texture.

The last step will be to seal the inside of the planter to prevent any long-term water damage. I haven't done this to mine yet, and I'll update this page once I do. There are special sealers/paints available at most hardware stores. I've also seen people smearing a thin layer of caulk all over the inside as well, however I'm not sure how durable or long-lasting that technique is. I'd love to hear some ideas!

Anyway, once you've done that, you're ready to plant some things! I'm intending to use mine as a small herb garden starting next summer.

Thanks for looking!

P.S. I've entered this into a contest. If you liked it, please vote for me! I'll be sure to use the prize to create some more cool Instructables!