A container’s stated size is its outside volume, and the compost goes in less than all of it. There is a drainage layer at the bottom, there is a gap at the top for watering, and the roots want the middle. Fill a two-hundred-litre planter to the brim with compost and you have used a third more than you needed, spent money on material that is sitting below the waterline, and made the container heavier than it had to be. This splits the container into what drains, what grows and what is left for the watering can.
Container, drainage and compost
The container is bigger than the compost. Some of it has to be drainage.
50 mm of gravel or broken crock at the bottom; more in a deep container.
So watering does not run straight off the rim.
Result
Three zones, and only one of them is compost
A container from the outside looks like a box of soil. From the inside it is three zones. At the bottom there is a drainage layer — gravel, broken terracotta, expanded clay — which keeps the compost out of standing water at the base. In the middle is the rooting zone, which is the expensive part. At the top there is a gap, which is what lets you water without the can overflowing onto the patio the moment you pour.
On a deep container those two non-compost zones are a modest share. On a shallow one they dominate: a window box 150 millimetres deep with 50 millimetres of drainage and 40 millimetres of gap has 60 millimetres of compost, which is forty percent of the container and not much root room. This is why shallow containers need watering twice a day in summer and deep ones do not.
Why the drainage layer is not optional
Roots sitting in water die, and they die quickly — a container with blocked holes is a death sentence for most plants within a fortnight, and the symptoms look exactly like under-watering, which is why people respond by watering more. The drainage layer is cheap insurance against it, and so are enough holes: a large planter wants several, not one, and they want to be clear of the ground so they can actually drain.
The old advice to put a layer of gravel at the bottom is sometimes criticised on the grounds that it raises the water table rather than lowering it. That is true in a sealed container with no holes; with proper drainage holes, which every outdoor planter should have, the layer's job is to keep the holes clear of compost and to stop the fine material washing down into them. Broken crock over the holes does that job better than gravel.
A filled planter is far heavier than an empty one. Compost at around 700 kg per cubic metre plus drainage material at around 1,600 is a serious load on a balcony or a roof terrace.
Fill containers where they will stand, not where it is convenient. A large planter filled before it is moved is a two-person job at best.
Peat-free, and what that changes
Peat-free compost behaves differently from peat-based, and the difference that matters for volume is that it holds less water and needs more frequent watering. It is also lighter and it shrinks as it breaks down, which means a container filled to the brim in spring is visibly short by autumn and needs a top-up.
That shrinking is worth budgeting for: a container loses around ten percent of its volume over a season as the organic matter decomposes, and the usual treatment is to top it up in autumn or to replace the top third each spring. Either way the bag count here is the first fill, not the last.
| Planting | Mix | Drainage | Note |
|---|---|---|---|
| Summer bedding | Multipurpose peat-free | 50 mm | Feed fortnightly |
| Shrubs and small trees | Soil-based, John Innes 3 | 75 mm | Heavy, stable, slow release |
| Herbs | Gritty, half compost half grit | 50 mm | Drainage matters more than feed |
| Vegetables in pots | Compost plus slow-release feed | 50 mm | Tomatoes need volume |
| Acid-loving plants | Ericaceous compost | 50 mm | Not just the feed — the pH |
| Succulents | Half grit, half compost | 50 mm | Almost no water retention wanted |
A worked example
- Container volume1.2 by 0.45 by 0.5, times two, is 0.54 m³ of container.
- Take out drainage and gapFifty millimetres below and forty above leaves 0.41 m of compost depth.
- Compost volumeThat is 0.443 m³, or 443 litres.
- BagsAt forty litres a bag that is 12 bags, at $7 each $84.
- And the restThe drainage layer needs 54 litres of gravel or crock, and the filled planters will weigh around 396 kg.
Three hundred and ninety-six kilograms is the number to check against where the planters are going. On a balcony, that is a structural question, not a gardening one.
Watering and aftercare
Containers dry out far faster than open ground, because they have a large surface relative to their volume and because the sides are exposed to sun and wind. A black planter in full sun can need watering twice a day in high summer, and the gap at the top is what makes that possible without flooding the patio.
Feed, because containers have a finite supply. A slow-release granular feed mixed in at planting plus a liquid feed through the season is the usual regime, and it matters more in a container than anywhere else, because every watering leaches a little more out of the holes.
If the planters are being irrigated rather than hand-watered, the drip line figure sizes the system, and the irrigation run time turns the container volume into minutes. Where a group of planters is being arranged, the walkway width figure decides whether there is room to walk between them.
Do
- Leave a gap at the top for watering, and drainage at the bottom.
- Check that the holes are clear and off the ground.
- Use a soil-based compost for permanent planting, peat-free multipurpose for bedding.
- Fill containers where they will stand, not where it is convenient.
- Budget a top-up, because peat-free compost shrinks over a season.
Avoid
- Filling to the brim and watching water run straight off.
- Blocking the drainage holes with fine compost.
- Using a peat-based mix and expecting it to hold water like one.
- Moving a large planter after filling it.
- Forgetting that containers need feeding as well as watering.
Where the numbers stop being true
Containers are treated as rectangular boxes. Round pots hold less — about seventy-eight percent of the square container of the same nominal diameter and height — and tapered ones, which is most of them, hold less again. If the container is round, multiply the result by about 0.78.
The drainage and gap depths are yours to enter and they are the least standardised figures on the page. A deep planter with a water reservoir at the base has a much larger non-compost zone than the fifty millimetres used here, and those reservoirs are worth having on a large container.
Weight uses 700 kg per cubic metre for compost and 1,600 for drainage material, both of which vary with how wet they are. Wet compost is roughly twice the weight of the same compost dry, so a container that weighs 200 kg after filling can weigh 300 after a week of rain.
How the figures are produced
Length times width times depth times the number of planters gives the total container volume. The compost depth is the container depth minus the drainage layer and the top gap, and the compost volume is that depth times the planter's footprint times the count. Drainage material is computed the same way from the drainage depth. Bags are compost litres divided by bag size, rounded up, and weight uses 700 kg per cubic metre of compost and 1,600 kg per cubic metre of drainage material.
Questions people ask
Why is it not just length x width x depth?
Because some of the container is drainage and some is the watering gap. In a 0.5 m planter with 50 mm of drainage and a 40 mm gap, the compost depth is 0.41 m, not 0.5.
How much drainage material do I need?
The footprint times the drainage depth. Two planters at 1.2 by 0.45 with 50 mm is 54 litres — more than a bag of compost.
Do I really need a gap at the top?
Yes. Without it, watering runs off the rim before it soaks in, and you end up watering the patio rather than the plant.
Can I use garden soil in a planter?
Not on its own. It compacts, it drains badly in a container and it can carry pests and weed seeds. Use compost, or a mix with grit in it.
How heavy is a filled planter?
Around 700 kg per cubic metre of compost plus 1,600 per cubic metre of drainage material, and roughly twice that when wet. Check the structure before filling.
Should I use crock or gravel at the bottom?
Broken terracotta over the holes works better, because it keeps the holes clear without raising the water table the way a deep gravel layer can.