How do I fill a very large planter without using ten bags of compost?

A 90cm cube planter takes about 700 litres of compost. Nobody needs a shrub to root through 700 litres, and nobody wants to buy fourteen 50 litre bags to fill it. The answer is to fill the part the roots will never reach with something else — but the reason it works is weight and cost, not drainage, and getting that distinction wrong is what kills the plant.

Step by step

1. Work out the root depth the plant actually needs.Bedding and salads, 15cm. Summer vegetables, 30cm. Shrubs, roses and perennials, 40cm. A small tree, 45 to 60cm. That depth of compost, measured down from the top, is untouchable.
2. Bulk-fill only the space below that.If your planter is 80cm deep and the plant needs 40cm, you have 40cm to fill with something cheap. If the planter is 45cm deep and the plant needs 40cm, you have nothing to fill and you buy the compost.
3. Use inert, permanent, light material.Upturned plastic plant pots are the best of all — strong, free, and they leave air space. Sealed empty plastic bottles with the caps on. Chunks of polystyrene packaging. Crushed drinks cans in a mesh bag. Anything that will not compress and will not rot.
4. Separate the fill from the compost with a permeable layer.A sheet of weed membrane or an old compost bag punched full of holes over the fill stops the compost trickling down into the voids and dropping the level. It must be permeable — a solid sheet creates the perched water table you are trying to avoid.
5. Fill the root zone with compost right through.One material, no gravel layer, no sand layer, no drainage layer. Any change of texture inside the root zone creates a boundary where water accumulates instead of moving on.
6. Leave the watering gap.Finish the compost 25 to 40mm below the rim. Without that gap, water runs over the side instead of soaking in, and every watering wastes most of itself.
7. Firm as you go, and water thoroughly when done.Fill in layers of 15 to 20cm, firming gently with your fists rather than treading. Then water until it runs out of the base, top up the level, and water again.

Tips

• Work out the volume before you shop. Width times depth times height in centimetres, divided by 1,000, gives litres. People routinely underestimate large planters by a factor of two or three.
• For a permanent planting, use a loam-based compost such as John Innes No. 3 in the root zone, or blend it half and half with a peat-free multipurpose. Pure multipurpose slumps and exhausts within a season or two.
• If the planter is against a wall and only the front is visible, you can build a false internal base from treated timber rather than filling the void at all.

Common mistakes

• Using rubble, broken brick or hardcore as fill — It is heavy, which defeats the purpose, it holds water, and old mortar and concrete leach lime that pushes the compost alkaline — fatal for ericaceous plants and bad for most others.
• Filling with woody prunings, logs or turf — It works for a season and then rots. The whole planting drops by 100mm or more, roots are left exposed, and nitrogen is locked up by the decomposition.
• Calling the bulk-fill layer 'drainage' — It drains no better than compost would, and by raising the base of the compost column it moves the permanently wet zone up into the roots. It is a weight and cost measure, nothing else.
• Using unsealed bottles — They fill with water and become dead weight, and any that stay part-full breed mosquitoes and go anaerobic. Caps on, always.

Questions people ask