Concrete planter casting

Not ideally for small planters. General-purpose concrete contains coarse gravel aggregate that is too chunky for detailed or small moulds, leaving a lumpy, stony surface and weak thin walls. Use a fine sand-and-cement mortar or a dedicated casting concrete instead, which pours smoothly and gives clean edges. For larger, chunky planters with thick walls, a finer concrete still gives a much better result than gravel-filled mixes.

Usually too much water or drying out too fast. Adding extra water to make the mix pour easily weakens it badly, since the water leaves voids as it evaporates, so mix to a thick, just-pourable consistency. Letting the surface dry quickly also causes weak, cracking concrete, so cover the fresh pour with plastic and cure it slowly over a day or two. A stiffer mix cured slowly gives strong, crack-resistant pieces.

Use nested moulds. Place a smaller container inside a larger one, leaving a gap of the wall thickness you want, then pour concrete into that gap. Weight the inner container so it does not float up, and make sure it is centred. Once cured, remove both moulds and you have a planter with a hollow centre. Adding a drainage hole, by setting a small dowel or bolt in the base, is worth doing for real plants.

Yes, with basic precautions. Dry cement dust is harmful to inhale, so wear a dust mask when handling the powder, and wet concrete is alkaline and can irritate or burn skin over prolonged contact, so wear gloves and rinse off splashes. Work in a ventilated area and never pour leftover wet concrete down a drain, where it sets solid. With a mask, gloves, and sensible disposal, home concrete casting is perfectly manageable.