How deep does a planter need to be?

Depth questions usually arrive with a specific container already bought — a trough, a raised planter on legs, a windowsill box — and the real question is what will survive in it. The chart in the diagram gives working minimums. Everything on it does better with more, and nothing on it does well in a container that is deep but only 15cm wide.

Step by step

1. Measure the usable compost depth, not the container height.Subtract the watering gap at the top, about 25 to 40mm, and any bulk fill or drainage layer at the bottom. A 30cm deep trough with a 5cm layer of gravel in the base is a 22cm planter.
2. Match the depth to the group of plants, not the individual species.Shallow, 15cm: cut-and-come-again salad, radish, most annual herbs, spring bulbs, summer bedding. Medium, 20 to 25cm: bush tomatoes, dwarf French beans, spring onions, garlic, strawberries, chard, most patio tubs.
3. Go to 30cm for anything that will carry a crop all summer.Cordon tomatoes, courgettes, peppers, chillies, aubergines, beetroot, short-rooted carrots and potatoes in a bag. Below 30cm these plants can be grown but they will need watering twice a day in July and they will stall.
4. Go to 40cm for anything permanent.Shrubs, roses, climbers, ornamental grasses, maincrop carrots and parsnips, and any perennial expected to stay put for several years. Below that, the plant survives but does not thrive and needs repotting far sooner.
5. Go to 45 to 60cm or more for a small tree or a large evergreen.Fruit trees on dwarfing rootstock, bamboo, olives, bays and box balls of any size. At this point the practical limit is weight — a 60cm cube of wet compost is a two-person lift and is never moving again.
6. Then check the width, which usually matters more.Most plants root sideways far more than down. A wide shallow container beats a narrow deep one of the same volume for everything except long root crops, and it is also far more stable in wind.

Tips

• Depth alone does not make a container drain. A deep pot with no hole is a sump; a shallow pot with a clear hole and pot feet drains fine.
• Raised planters on legs are almost always shallower than they look — the leg height is not compost depth. Check before planting a shrub in one.
• For a hedge or screen in a trough, depth is what stops it drying out and width is what stops it blowing over. You need both, and 40cm minimum in each direction is realistic.

Common mistakes

• Filling the bottom third of a shallow planter with drainage material — It reduces the compost depth to below the working minimum and raises the saturated zone into the roots. Bulk-filling only makes sense in a planter that is far deeper than the plant needs.
• Buying a tall narrow pot for a shrub — Narrow pots hold less than they look, dry out faster because of the surface-to-volume ratio, and go over in the first autumn gale.

Questions people ask