You cast a concrete planter using two moulds that fit inside each other, with about 4 to 5 cm of space between them. You fill that space with concrete in a ratio of 1 part cement, 2 parts sand and 3 parts gravel, with a water amount of roughly half the weight of the cement. After 24 to 48 hours you take the planter out of the mould, but concrete only reaches its full strength after 28 days. Don’t forget the drainage hole, which you make with a piece of PVC pipe or a cork in the mould.
Sounds simple, and it is. But between “a planter that lasts 20 years” and “a planter that falls apart after the first frost” there’s a lot of detail. Below you get the whole story.
Why cast your own concrete planter?
Concrete is dirt cheap, seriously strong and you can make any shape you want with it. A 25 kilo bag of concrete costs a few pounds, and that’s enough to cast a solid planter in no time.
A decent ready-made concrete planter at the garden centre easily costs 40 to 100 pounds. Casting your own costs you a tenner in materials, fifteen at most for a medium-sized planter.
And you decide the size, the colour and the shape yourself. Round, square, low, tall, with a rough or a nice smooth surface. You won’t find that anywhere in the shop.
The one downside: concrete is heavy. I’ll come back to that later.
Which type of concrete do you choose for a planter?
Not all concrete is the same. The choice determines how strong, how fine and how heavy your planter turns out.
Ordinary concrete (with gravel)
Cement, sand and gravel. Strong and cheap. Ideal for larger, robust planters. The gravel makes the surface a bit coarser, so for a sleek, smooth result this isn’t the best choice.
Fine concrete or mortar (cement and sand, no gravel)
Finer in texture, a nicer smooth surface. Slightly less strong than concrete with gravel, so suited to smaller planters up to about 40 cm.
Rapid-set concrete
Hardens in a few hours instead of days. Handy if you’re in a hurry, but you have less working time and it’s harder to work out the air bubbles. For your first planter I wouldn’t use it.
Glass-fibre reinforced concrete (GRC)
Concrete with glass fibres mixed through it. Much stronger with thin walls, so you can make light, thin-walled planters. More expensive and a bit trickier to work with. Something for once you’ve had a practice run.
Concrete cire (microcement)
Not a casting concrete but a thin finishing layer you spread over an existing surface. Handy for giving an existing planter or pot a concrete look, but you don’t cast a planter with it.
For your first project: ordinary concrete with gravel for a large planter, or fine concrete for a smaller, smoother one.
| Type of concrete | Surface | Strength | Suited to |
|---|---|---|---|
| Ordinary concrete (gravel) | Coarse | High | Large planters |
| Fine concrete/mortar | Smooth | Medium | Small planters |
| Rapid-set concrete | Medium | High | When you’re in a hurry |
| Glass-fibre concrete (GRC) | Smooth | Very high | Thin-walled planters |
What is the right mix ratio for concrete?
The classic ratio for ordinary concrete is 1 part cement, 2 parts sand, 3 parts gravel
. Measure with a bucket or a scoop, not by guesswork.
Want a finer, smoother surface without gravel? Then you use 1 part cement and 3 parts sharp sand. That’s called mortar or fine concrete.
Then the water. This is where most people go wrong. You need roughly half the weight of the cement in water. That’s called the water-cement ratio, and 0.5 is a good rule of thumb.
In plain terms: too much water makes your concrete weak and porous. It seems handy because it mixes nice and smooth, but your planter becomes more brittle and soaks up more moisture. So go slightly short on water rather than over.
The right consistency is like wet, sticky earth or thick peanut butter. It should just flow when you poke it, not run off your scoop like soup.
Mix the dry parts thoroughly first. Then add the water in stages, not all at once. That way you keep control of the thickness.
Which mould do you use to pour concrete into?
You always need two moulds. An outer mould and an inner mould. The space between them becomes your concrete wall.
Keep about 4 to 5 cm of space between the two moulds. That’s the thickness of your wall. Thinner than 3 cm gets too weak, thicker than 6 cm is usually a waste of concrete and weight.
These are the mould options that really work:
Two buckets or tubs inside each other
The easiest. A big bucket as the outer mould, a smaller one as the inner mould. Fast and cheap, but your shape is limited to what you have around the house.
Two cardboard boxes
A big box inside a small box, reinforced with tape. Disposable, so after demoulding you just tear the cardboard off. Line the inside with plastic so the cardboard doesn’t soak through.
Wood and plywood
For sleek, straight shapes. Plywood is smooth, so your planter comes out smooth. More work to build, but you can make exactly the size you want. Screw the mould together so you can take it apart again without wrecking it.
Polystyrene (EPS)
Easy to cut for round or unusual shapes. Downside: polystyrene is brittle, so after one cast it’s often done for.
Silicone mould
For when you want several identical planters or a lot of detail. Flexible, so easy to demould. More expensive and more hassle to make yourself.
Always rub the inside of the mould with oil before you pour. Sunflower oil, cooking oil or a proper release oil. Then the concrete releases cleanly afterwards.
Make sure your inner mould doesn’t float up or sink in crooked in the wet concrete. Weigh it down with stones, water or sand, and fix it in place with a batten across the top edge if needed.
How do you make a drainage hole in the mould?
People forget this constantly, and then their plant drowns. A planter without drainage becomes a bathtub.
Before pouring, stand a piece of PVC pipe or a cork upright in the bottom of your mould. At the spot where you want the hole. The concrete flows around it and after demoulding you push or drill it out.
For a planter up to 30 cm wide, one hole of about 2 cm is enough. Give larger planters two or three holes.
Did you forget the hole after all? No disaster. You can drill a hole after curing with a masonry or concrete drill bit. But it’s more work and there’s a risk of cracks, so it’s better to get it right straight away.
Do you need reinforcement in your concrete planter?
For small planters up to about 30 cm tall, no. Concrete is strong enough on its own.
For larger and taller planters, yes. Concrete is strong under compression but weak under tension, and it’s precisely with large planters that it can crack. Reinforcement takes up those tensile forces.
Use chicken wire or a piece of steel reinforcing mesh that you hide inside the wall. Important: the steel needs at least 2 to 3 cm of concrete around it on all sides. If the steel sits too close to the outside, it starts to rust and the rust cracks your concrete apart.
In practice you do it like this: pour a layer of concrete first, lay your wire in it, then pour the rest over the top. Make sure the wire doesn’t touch the mould anywhere.
For planters taller than 50 cm or longer than 60 cm, reinforcement isn’t a luxury but a necessity.
Step-by-step: how to cast your concrete planter
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Set up the moulds. Choose your two moulds with 4 to 5 cm of space between them. Rub the contact surfaces with oil.
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Place the drainage. Stand a piece of PVC pipe or cork upright in the bottom of the outer mould.
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Mix the concrete. 1 part cement, 2 parts sand, 3 parts gravel. Add water until you have thick peanut butter. Mix thoroughly for at least a few minutes.
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Pour the base. Pour a 4 to 5 cm layer into the bottom of the outer mould first. That becomes the base of your planter.
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Place the inner mould. Press the inner mould into the wet concrete, right in the middle. Weigh it down so it doesn’t float up.
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Fill the walls. Scoop the concrete into the space between the moulds. Work evenly upward all around so the inner mould doesn’t sink in crooked.
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Release the air. Tap the outer mould with a rubber mallet or a batten, and poke into the concrete with a stick. That way the air bubbles rise to the top and you don’t get holes in your wall. This is called vibrating, and it’s genuinely the step that makes the difference.
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Smooth it off. Smooth the top edge with a putty knife.
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Cover it up. Lay plastic film over the planter. Concrete that dries too fast cracks. The plastic holds the moisture in so it cures slowly and strongly.
How long does concrete need to cure before you demould?
After 24 to 48 hours the concrete is hard enough to carefully take out of the mould. With rapid-set concrete, after just a few hours.
Take it out too early and the edges crumble away. Be patient, one extra day does no harm.
Important: out of the mould doesn’t mean fully strong. Concrete only reaches its full strength after 28 days. It’s already usable in the first week, but treat your planter gently for the first few days.
Keep the concrete moist during that first week. Spray it wet now and then or keep it under plastic. Concrete cured wet ends up stronger than concrete that dried out quickly. Sounds odd, but that’s how it works chemically: concrete doesn’t cure by drying, but by reacting with water.
| Stage | Time | What you can do |
|---|---|---|
| Demoulding | 24-48 hours | Carefully take out of the mould |
| Usable | 5-7 days | Set up the planter, fill it |
| Full strength | 28 days | Fully load-bearing and frost-proof |
How do you finish a concrete planter?
Freshly demoulded concrete often has sharp edges and small imperfections. You remove those with sandpaper or a sanding block. Wet sanding gives the least dust and a smoother result.
Small holes from air bubbles you fill in with a bit of mixed cement paste, then sand once more.
Want to protect the colour and the concrete? Then you impregnate or seal the planter. You do that with a concrete impregnator or a matt concrete varnish. This matters more than it sounds: untreated concrete soaks up moisture, and that moisture works its way through to the outside. Set a planter like that against a wall and you get damp patches. Impregnating keeps the moisture inside the planter.
Impregnating also makes your planter more resistant to frost and lime bloom (that white haze that appears on old concrete).
Can you add colour to concrete?
Yes. You buy concrete pigment or oxide powder in powder form, in colours like anthracite, black, ochre and terracotta. You mix that dry through the cement before you add water.
Reckon on about 2 to 5 percent pigment relative to the weight of the cement. More pigment gives a deeper colour, but too much weakens the concrete.
Mix the pigment through very thoroughly, otherwise you get patches and streaks. And keep the ratio the same between batches, otherwise one planter turns out lighter than the other.
Don’t fancy pigment? You can also paint the planter afterwards with concrete paint, but colour mixed right through is nicer because the colour can’t flake off.
Is a concrete planter frost-proof?
Well-made concrete is frost-proof, provided it’s fully cured (those 28 days) and doesn’t soak up too much water. That’s why impregnating is so important.
The danger with frost is water sitting in the concrete that freezes. Ice expands and cracks the concrete apart from the inside. Porous concrete mixed too wet is prone to this.
Make sure your planter can always drain outside, so that drainage hole, so no water stays sitting in it that could freeze.
Move sensitive plants somewhere dry in winter, or lift the planter onto feet or tiles so it isn’t standing in a puddle. A planter sitting on cold ground draws in more moisture and frost.
How heavy is a concrete planter and how do you move it?
Heavy. Reckon on roughly 2.3 kilos per litre of concrete. A planter with 5 cm walls measuring 40 by 40 by 40 cm easily weighs 50 to 75 kilos empty, and a fair bit more with potting soil and plants in it.
So work out before casting where the planter is going to stand, and ideally cast it in the spot where it stays. Move it with two people, using a sack truck or a platform trolley.
Want to keep it lighter? Then glass-fibre concrete (GRC) with thinner walls is an option, or you make the planter shallower. For large planters you can also put a layer of light filler in the bottom (polystyrene, old pots) so you need less potting soil and the planter is less heavy when it’s full.
What do you plant in a concrete planter, and how?
Build the planter up from the bottom:
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Drainage layer at the bottom. A layer of clay pebbles or gravel a few centimetres deep over the drainage hole. That way excess water drains away and the roots don’t drown.
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Fabric on top. A piece of root fabric or fleece over the drainage layer. That holds the potting soil back so it doesn’t clog the drainage hole.
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Potting soil. Fill up with potting soil that suits your plant. For Mediterranean plants and succulents, mix in some extra sand or gravel.
Fresh concrete planters can give off some lime, which makes the soil alkaline. Rinse a new planter through well with water a few times before planting, or impregnate the inside. Lime-sensitive plants like hydrangeas are better set in an impregnated planter.
Common mistakes when casting concrete (and how to avoid them)
Too much water in the mix. Seems handy, makes your concrete weak and porous. Stick to a 0.5 water-cement ratio, so thick peanut butter, not soup.
Not releasing the air. Skip the tapping step and you get ugly air holes in your wall and weak spots. Always tap and poke.
Letting it dry too fast. Concrete in full sun or a draught cracks. Cover with plastic and keep it moist for a week.
Demoulding too early. Edges crumble away. Wait at least 24 to 48 hours.
Reinforcement too close to the outside. The steel starts to rust and cracks the concrete apart. Keep at least 2 to 3 cm of cover.
Forgetting the drainage hole. The plant drowns. Always put a little pipe or cork in the mould.
An inner mould that floats up. Your wall thickness ends up uneven. Weigh the inner mould down well.
Not oiling the mould. The concrete sticks and you wreck your planter when demoulding. Always oil it.
What does it cost to cast a concrete planter?
Little. That’s exactly the appeal.
| Item | Cost |
|---|---|
| 25 kg bag of concrete | 4 – 8 pounds |
| Concrete pigment (optional) | 5 – 15 pounds |
| Impregnator | 10 – 20 pounds |
| Moulds (buckets/cardboard) | 0 – 10 pounds |
| Chicken wire (large planter) | 5 – 10 pounds |
For a medium-sized planter you’re looking at around 10 to 20 pounds in materials. A comparable ready-made planter easily costs three to five times that.
Frequently asked questions
How long does concrete need to dry for a planter?
After 24 to 48 hours you can demould. The planter is usable after 5 to 7 days and only reaches its full strength after 28 days. Keep the concrete moist the first week and it turns out stronger.
Which mix ratio do I use for concrete?
For ordinary concrete: 1 part cement, 2 parts sand, 3 parts gravel. For a finer, smoother surface: 1 part cement and 3 parts sharp sand. Water is roughly half the weight of the cement.
Does there need to be a hole in the bottom?
Yes, always. Without a drainage hole, water stays sitting in the planter and the roots drown. Before pouring, put a piece of PVC pipe or a cork in the mould.
Is a home-cast concrete planter frost-proof?
Yes, if the concrete is fully cured and you’ve impregnated it so it doesn’t soak up water. Make sure the planter can drain and isn’t standing in a puddle in winter.
Why is my concrete planter cracking?
Usually from drying too fast or too much water in the mix. Cover the concrete with plastic while it cures, keep it moist and don’t use too much mixing water.
Can I colour concrete?
Yes, with concrete pigment or oxide powder that you mix dry through the cement, about 2 to 5 percent of the cement weight. Colour mixed right through is nicer than painting afterwards, because the colour can’t flake off.
Get started
Casting a concrete planter is one of the most rewarding weekend jobs there is. Cheap, strong, and made entirely to your size. Do it right once and it’ll still be standing there in twenty years.
Want a sleek planter with a wooden mould to an exact size, or a bigger garden project that a planter like this fits into? At fredsdiyplans.com you’ll find clear build plans with dimensions and a materials list, so you know straight away how to build it. Have a good look around and you won’t have to work anything out yourself.