Fill a deep planter with nothing but bagged mix and you find out very quickly how much soil a box really holds. A box two metres long and 50cm wide and deep takes half a cubic metre. That’s most of a small trailer load. Spring is when a lot of raised planter boxes get filled or topped up, and it’s worth knowing what belongs down in the bottom before you start, along with what quietly causes trouble a season or two later.
The gravel-for-drainage habit
A layer of gravel or broken rubble in the base is probably the most common shortcut, and it’s usually done with good intentions. The idea is that water will run straight through it and keep roots from sitting wet.
In practice it often works the other way.
Raised planters drain well on their own anyway, since they sit above the surrounding ground. Water also doesn’t move from fine soil into coarse stones as readily as people expect. It hangs around in the soil just above where it meets the gravel, and that band can stay soggy right where the roots are heading.
Filler that earns its place
Very deep boxes are a different story. The bottom third can be filled with old logs and garden waste instead of soil, as long as it’s the right kind. In a planter much shallower than knee height, there’s rarely enough room to bother, and the whole depth is better given over to good good soil.
- Untreated logs and thick branches. They break down slowly over a few years and hold moisture well while they do. The bed will sink a little as they go, so expect to add more soil.
- Coarse prunings and old leaves, packed down hard so they don’t collapse all at once.
- A layer of plain cardboard on the base if the box is going over lawn. It smothers the grass and breaks down within a season.
Builders’ rubble and treated timber are better left out altogether, and so is anything plastic. None of them improve the soil, and some can leach into it.
Getting the soil mix right
The top half of the box is where plants actually live, and that’s where the money should go. Iowa State University’s extension service suggests a soil mix for a raised bed of roughly 70% topsoil to 30% compost, or equal parts topsoil, compost and coarse sand. The more useful part of their advice is to mix as you fill rather than tipping in one material after another, because distinct layers slow water down and discourage roots from pushing deeper.
Bagged potting mix on its own is light and easy to carry, and that’s half the appeal. A deep box filled entirely with it dries out fast once summer arrives though. A decent topsoil blended through gives the mix some body and helps it hold moisture for longer.
Topping up each spring
Every raised planter settles over time, especially one with organic filler in the base. Before adding anything in spring, check how far the soil had dropped below the rim. A small drop is normal. If it’s more than a few centimetres, the filler underneath is probably breaking down and the box will need a bit extra.
Compost is usually enough to bring the level back. Fork it lightly into the existing soil rather than leaving a fresh layer sitting on top, then spread a layer of mulch over the surface to slow evaporation through the warmer months.
Give the box a good soak before planting. Dry mix can shed water for the first few days, and seedlings planted straight into it often struggle before they’ve had a chance to settle in.
Fill it properly once
A planter box filled with some care at the start is mostly a matter of topping up from then on. Filled in a hurry, it tends to get dug out and redone, usually in the middle of a hot summer when the plants have started sulking.
