Introduction: Self-watering Concrete Planter
I've been playing around recently with casting concrete and some friends of mine were getting married so I thought I'd take a stab at making a gift for them. There are tons of great Instructables and guides out there already specifically for making concrete planters so be sure to check out some of the other ones.
The idea with this planter is that you pour water into the section holding the rocks and a hidden channel allows the plant to pull water from the reservoir as needed. That's the theory anyway, I'll have to wait and ask my friends as to whether or not this actually works out.
The actual materials cost is pretty low and it doesn't require many tools so even starting from scratch wouldn't be too bad. The project only uses a small percentage of the concrete and foam board so these can be reused to make multiples or can be saved for other projects. Here's the price breakdown for materials:
- 50 lb. Fast-Setting Concrete Mix from Home Depot - $4.97
- FOAMULAR 1 in. x 2 ft. x 2 ft. R-5 Insulation Sheathing from Home Depot $5.98
- Plastic bowl from Dollar Tree - $1.00
- Oval-shaped plastic storage container from Dollar Tree - $1.00
- Decorative rocks from Dollar Tree or free if you head on down to the closest creek/river - $1.00
That brings the total to less than $15.00 and it's even less if you only consider what was actually used.
Step 1: Getting Started
Get an idea for the general shape you want the planter to take and then head off to gather the materials or just start looking around the house for ideas.
The oval storage container has a great shape so I decided to use this as the basis for everything else. I knew I wanted to include two compartments (one for the soil and one for the water) so I found a bowl that I thought was appropriately sized to use for the plant/soil compartment and then I knew I wanted the other compartment to be shaped around these two objects which meant I was going to have to make it.
I originally started by making the reservoir with wood, but it took a lot of sanding to make the edges conform uniformly to the shape I wanted and it ended up being difficult to remove from the concrete. The foam board is apparently pretty popular among hobbyist and after using it I can see why - it was super easy to work with and it took way less time to get the shape I wanted compared to using wood.
Since the sides of the oval are tapered, I traced the bottom of the oval container onto the foam and then did the same with the bowl. I marked the top so I could focus on keeping these edges cleaner since that will be exposed the edge.
Step 2: Planning for Casting
Now that I have the reservoir shape cut out, I had to figure out how to get everything set up for casting.
I drilled a hole in the bottom-center of the bowl so that I could use a nail to keep the bowl in place during casting. To give myself enough space for the water channel, I cut out a dumbbell shaped piece that the bowl and reservoir would sit on. To bring the height of the reservoir closer to the bowl, I cut out another smaller kidney-shaped piece.
Next, I did a dry fit and made a mark on the dumbbell shape so that I could push the nail for the bowl through the foam to keep the bowl in place when casting. Everything looked good, so I hot-glued the foam together.
Now I just had to hot-glue the foam rig to the oval container and I'm ready to cast. When fitting the test piece, make sure that there is enough space for the concrete to fit between everything - the bigger the gap, the stronger the concrete will be but try to make the gap consistent everywhere so that the final piece looks proportionate. One of my walls ended up being a little bit thicker on one side, but overall it isn't too noticeable.
Step 3: Casting With Concrete
The Quikcrete fast-setting formula is pretty nice because you only have to let it cure for a few hours before removing the forms. I'll use the heavy duty stuff typically but I only had a few days to get this together before the wedding so the fast-setting formula will have to do for this. It gets a little tricky since you only have about 20 minutes after mixing the concrete before it starts to cure so you have to work fast.
One thing I like to do is pull out the aggregate so that the concrete finish is smooth and consistent. This is going to the reduce the overall strength of the concrete but for something like this it shouldn't matter. I just use a cheap dollar store colander and sift through a few bags at a time so that I have plenty.
There are a bunch of different references out there for what the consistency of the mixture should look and feel like but just make sure you mix enough to actually fill the form! Nothing is worse than running out of concrete mix when the form is only half full.
Before I cast the concrete, I cover everything with a ton of non-stick cooking spray which helps to release the form once the concrete cures. I started with the bowl out and filled in the bottom first. Then I pushed the bowl back into place and filled in around the sides, tapping the whole thing as I went to release any air pockets.
Step 4: Prep the Form and Cast Again
After a few hours of curing I started getting ready for the second pour. At this point I have to remove all of the foam and that mostly involves chipping away with a screw driver. It would be nice to be able to reuse the foam to make multiple copies, but it's so easy to shape that it isn't that big of a loss.
The underside shows where the water channel is going to be - I cut a new piece of foam with slight angle so that water will move towards the bowl side. I don't think this was really necessary but oh well. I taped over the hole in the bowl and reinserted it into form. I plugged the other side with foam then hot-glued the channel down. Then I mixed another smaller batch of concrete and filled in around the channel again tapping on the sides to release air bubbles.
Step 5: Sealing the Concrete and Finishing Touches
After another few hours, the second pour had set and I was able to push the foam out of the channel. At this point, I waited overnight to let the concrete continue to cure before sealing it. I probably would have waited longer normally, but time was starting to run out.
Since the concrete was going to have to hold water, I used some concrete sealer that I had in the garage - I used BEHR Premium 1 gal. Protector and Waterproofer and applied it with a foam brush. I poured a bunch into the channel so that it was completely full to make sure that the insides would be adequately sealed. Hoping this also protects the plant from anything leeching out of the concrete but then again it's now exposed to whatever might be leeching off of the sealer. Probably wouldn't use this for herbs or anything that I'd be eating.
I used some mesh tape for tiling that I had to protect rocks and soil form blocking the water channel and attached it with hot glue. At this point I poured in some water to start testing everything and it seems like it should actually work! I'm hoping that the soil and roots (once they're established) will pack down and block the water from being able to just flood the plant compartment.
That's pretty much it - I really lightly sanded the top to get rid of some burrs and could have used an orbital sander with 60 grit to get the top smoother, but I like the rough-ish look of the pockets on the top especially with how smooth the sides are.
Thanks for reading!