Here's one way to make a large lightweight pot that could pass for being made of concrete on casual inspection.
Step 1: Why Bother?
Well sure, it's a lot less effort to just go buy a pot, but I always wondered if I could mold things out of spray foam insulation, and after a few tries, I found out that I could.
It remains to be seen how long it will hold up to the elements and whether plants will tolerate being in a pot made of polyurethane foam.
Step 2: Warnings, Disclaimers
Likewise, spray paint is employed at the final step and that too has its risks to be mindful of.
Wear a vapour mask, safety glasses and gloves and wear clothes you don't care about if they get anything on them.
The spray foam insulation material is extremely sticky, if you get it in your hair, you pretty much have to cut off your hair to get it out, so if you have long hair, you might want to pull it back out of the way. Same thing for clothes, if you get it on your clothes, your best bet is to let it cure completely, about 24 hours and then you stand a chance of pulling it off the fabric. Don't attempt to wash it out right away, you will only succeed in embedding it into the fibers and then it simply will never come out.
If you get it on your skin, you can wait for it to cure. They say that acetone can be used to remove it, though the times it has happened to me, I just put up with it till it cures and then you can just peel it off for the most part.
Note that foamcore does not require much pressure to cut if the blade is sharp. Always use a straight edge when cutting to help protect your fingers, don't try to freehand it.
Step 3: Materials
ruler or straight edge
two cans of Great Stuff Spray Foam Insulation or similar: $3.00 - $3.50 /can
spray paint in some color $3.50 - $5.00 /can
a big cutting surface, such as a large board
cutting plans (attached)
Step 4: Cut Out the Mold Pieces
Four outer side panels
four inner side panels
I used a craft cutter to cut templates out first, and then just traced those onto the foam core before cutting them.
Step 5: Tape Mold Pieces Together
Apply tape to one of the outer edges, fold it in and then fold in the trapezoid on the opposite side on top of the exposed tape to complete the main shape.
Step 6: Leave Room for a Drainage Hole
Step 7: Fill the Mold With Spray Foam
User Cambenora adds: do this outside, with no flames anywhere nearby!
Begin by shaking the spray foam as indicated on the packaging. Also, be sure to check the expiration date on the bottom of the can. The first time I tried this, I used some that had been expired for six months, so it didn't expand very much and I had to start over.
Spray some in around the drainage hole shape, then place the inner trapezoidal box shape (actually called a "frustrum") on top of that, inside the larger frustrum shape.
Fill in the remaining void with spray foam, but leave room for expansion, otherwise you'll probably have a seam failure somewhere.
If the spray foam is new and within its use by date, you can probably expect it to expand at least 50% of the volume you put in, so if you fill the container 2/3 full with the stuff, hopefully you won't be overfilling it when it is done expanding. However you don't want to underfill it or you will have voids in it which you would have to try to fill in later.
Insert the inner frustrum if you haven't already and then center it using the top frame shape.
Tape everything together good with duct tape, reinforce all the seams and then set the whole thing aside for at least 48 hours.
The package tells you that it fully cures within 24 hours, but I found it takes longer, possibly because the stuff does not get as much air exposure in the mold as it would usually.
Step 8: Tear Away the Mold
I probably ought to figure out some sort of mold release step to make this easier...
Step 9: Paint It!
Unlike styrofoam, this foam takes spray paint well and doesn't deform from it.
I painted these with some gray and brown outdoor grade spray paint to give them a look something like stone.
Step 10: Progress Report
Geraniums are nigh unkillable, so that will be the canary plant. I usually kill lobelias first, I added some of those anyway. And some other random plants.
One thing I did notice was that the drainage hole was too large, so I added a sheet of plastic netting at the bottom to keep the soil from falling out.
I watered thoroughly and did notice a tiny bit of seepage here and there, but nothing major.
I'll try to remember to post another progress report in a week.