Picture of DIY CONCRETE:: Planter Box
This Instructable will show you how to make concrete boxes using cardboard and duct tape. This example is a planter, but they could also be useful as storage cubbies. This project is nice because it's cheap, quick, and easy.

You could cast concrete in a cardboard box... except for one thing - it's not water tight. The cardboard will suck the moisture out of the concrete, leaving a chalky finish at best. At worst the box will blow out or fall apart before the concrete has cured.

The solution to this problem is, of course, Duct Tape. Taping the inside of the box will make it water tight, and also give the concrete some texture. Taping cardboard edging to the corners will help strengthen the box and minimize the bowing under pressure that happens when casting.

Materials + Supplies: Concrete Mixing:
  • 5 Gallon Bucket
  • Shovel / Trowel
  • Particle Mask
  • Rubber Gloves
Concrete Finishing:
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Step 1: Tape the Boxes

Picture of Tape the Boxes
What makes a good cardboard box for this project:
  • Heavy Duty
  • Rigid / Relatively New
  • Not too big (12"x12"x12" of concrete weighs about 135+ pounds, think about the pressure)
Less than Ideal:
  • Flimsy
  • Dirty
  • Thin cardboard
Find two boxes that fit inside each other with a 1" gap on all sides. More than 1" and the box could be quite heavy, less than 1" and it won't be as strong.

On the larger box (the main form):

1. Fold the flaps outward and tape them down.
2. Tape the inside edges of the box, especially the corners.
3. Tape the entire inside of the box, with some overlap, in any pattern you want.
4. Tape cardboard edging on the outside wherever possible to help strengthen the box.
5. Add any decoration, like leaves, rubber stamps, pieces of tile, with spray adhesive.

On the smaller box (the knockout):

1. Fold the flaps inward.
2. Tape the outside of the box completely.

Place the smaller box inside the larger box. Lift up the smaller box until there is a 1" gap at the bottom. Make a mark on the small box, even with the rim of the larger box. When casting, you'll use this line as a reference so you know how deep to push the smaller box down.
jguardiani11 months ago

How much does the finished product weigh?

CHENG Concrete (author)  jguardiani11 months ago

The weight depends on the thickness - In this case it's right between 30 and 40 pounds. Heavy for a planter, but very stable!

40 lbs for what dimensions? The box pictured looks to be about 14" cubed with a 1" thick wall. Is that right?

peets10 months ago

i hav a box and plants!1!!

ideaknock1 year ago
good idea!
Tom 71 year ago
very nice!
you could use wax to seal the cardboard, or a varnish.
just a thought, You could wax or varnish a heavy texture fabric onto the mold as a stripe or spot.
I would also suggest to anyone making this that they either make them 2 inches thick(Manufactures don't pay that extra money to still make them thick for no reason) Or, go to the hardware store and get a 3 foot tall roll of some cheap metal fencing and embed a piece into each side.
static1 year ago
I agree that a square shaped planter is not an original idea. However the instructable detail how to use cardboard and duct tape to create a mold. I believe votes determine contest winners, clearly many thought this was a winning entry. I suspect many who post instructables, also market what the build. They could decide to just market, and to to share
dana-dxb2 years ago
i sent this link to my friends too coz just like they where to looking for a solution so thanx again
dana-dxb2 years ago
awesome job iv been looking for something like this for ages thanx for the share
Ericc8152 years ago
This article is true self serving duplicity in the part of Cheng with a very old topic, he has plastered YouTube with his multitude of concrete countertop videos as well, selling his commercial products. This is not original and very commercial, outside the spirit of Instructables.
Sreyo antioch2 years ago
I agree with Eric, It's completely unfair how an unoriginal idea, of a concrete box, can win a contest. So creative.
He shared, I learned, Instructables spirit upheld.
I LOVE this project and would like to use it for my summer camp art class with my kids!

danzo3212 years ago
Never touch cement powder or wet concrete mix with your skin.
May as well set the inner box on the mix in the bigger box and start adding weights, rather than pressing it down, which will distort it and move it sideways.
"Never touch cement powder or wet concrete mix with your skin"

Seriously? This stuff isn't anthrax. Wash your hands when done.
It's the lye in the mixture. Ggloves are always a good idea.
Boys! There is no lye, but it is a strong caustic, Aleph. It will ruin cuticles, then fingertips and skin between fingers.
Truly, it is caustic, but statements like "never touch" only scare people. Mildly caustic. I've worked with cement my entire life on different projects (sculpture, fencing, the occasional walkway) and I have yet to do any harm. Sometimes we can take things too far.
A few months back I started on concrete. As a first time case... I worked with my bare hands as I found it easir... Later I found my hand felt weird .... However I applied a good dose of lotion and it was fine the next day.
You sound like I normally do.. but I've worked with concrete 40 years and pros take pride in never touching it.. Meanwhile, bad sculpture teachers encourage kids to 'be one with your materials' and it tears up their hands. You have the caustic and the abrasion. I've told people to relax about plenty of other things where you'd have to swim in it to see any effect.
sandrak3202 years ago
My grandmother used to use two large plastic bowls. She lined the larger bowl with wet newspaper, pour in some cement mix, push a cork in the bottom of the bowl (if it was to be a planter, no cork if to be a birdbath, dog water bowl), place the smaller bowl on the cork and continue filling with cement mix. I don't remember if the smaller bowl had wet newspaper wrapped about it's outside; or what she used to weigh it down until set. Got to remember to let it age before using it as a birdbath or waterbowl. The lye in the cement mix is toxic.
And she never sealed these bowls. I have one I use for a dogwater bowl (had it over 25 years) and the evaporation through the cement helps cool the water.
Very nice but there are two odd things about your instructions that make me wonder.

1. Why do you seal the thing? Unglazed clay pots are considered the best pots, because they breath. Unsealed concrete breaths similarly. While I imagine sealing it helps the pot last longer, being concrete means it will last a long time regardless.

2. Why did you remove the cardboard? It is just cellulose.
CHENG Concrete (author)  JohnnyMorales2 years ago
Hi Johnny, I haven't sealed this piece, but you make a good point and it wouldn't be necessary to seal an outdoor planter. However, when we seal our projects, the sealer isn't just protecting the concrete, it also makes the colors more rich and brightens the finish (not as dull). If I were using this piece indoors as a storage cubby for example, I might seal it just to make it look nice. The cardboard is removed because it isn't necessary, and there's also that layer of duct tape...
ladybgood2 years ago
very cool but personally I'd go with hypertufa instead of plan cement
CHENG Concrete (author)  ladybgood2 years ago
Plain cement is very brittle by itself. When you add sand and rock aggregate to cement, it is called concrete! Cement is like the glue that holds the rock matrix of concrete together. A hypertufa mix might work very well for this project!
danzo3212 years ago
A cu ft of crete is more like 155 lbs., in case you were interested. Sometimes at the supermarket you will see dark cardboard boxes, meant for fruit.. this is wax-flooded corrugated. It is assembled with staples, as glue won't stick. Anyhoo if you wax your boxes maybe the ducttape forest will not have to be clearcut.
CHENG Concrete (author)  danzo3212 years ago
Hey danzo, thanks for the comments. very useful!
danzo3212 years ago
A beautiful blue, and duct tape gives subtle striping. I would go to plywood for the outside mold but it seems you needed to go with corrug and duct tape so well done.
danzo3212 years ago
Tool commonly used for smoothing concrete is a "truing stone," a block of silicon carbide grains I think. Small ones look like butter sticks.
bighunk2 years ago
I think what you did was great.
I do think that plastic sheeting could
be part of your project to make
the cardboard waterproof.

Thank You - bighunk
CHENG Concrete (author)  bighunk2 years ago
Yes! Lining the box with a plastic trash bag would be a very quick way to make it water tight.
jorricks2 years ago
How do you maintain "a 1" + gap" on the bottom?
A rock, a guesstimate, when inserting and pushing 2nd box into 1st box.
Somehow I missed that point.
CHENG Concrete (author)  jorricks2 years ago
Sorry if that wasn't clear. I will usually make a mark on the 2nd box that is even with the rim of the 1st so I have a reference point and know where to stop. You could also glue some sort of drain hole knockout on the bottom of the 1st box that is 1" thick that will act as a stop. The actual wall thickness of the blue box is 3/4" on the sides and probably 1.5" on the bottom. I recommended 1" to be safe but 3/4" is fine, 1/2" is probably pushing it with traditional concrete but you can do it. Thanks jorricks for the comment.
lowky2 years ago
did you add a colorant, or is that brand of concrete usually that blue? thought for future could you include a piece of pvc or a cardboard tube for a drainage hole?
CHENG Concrete (author)  lowky2 years ago
The color comes from the CHENG Pro-Formula admixture, this blue is called Tahoe. The mix comes in sixteen colors and you add it to regular sacked concrete. Nice suggestion about the drainage hole! A wine cork work pretty well and is easy to remove. Just cut it to length and glue it to the bottom of the main form with spray adhesive. Nice ouroboros there too!
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