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How to fill a plastic raised bed

Updated: October 2026

The frame is on the slabs, or on a weedy corner, and the boot is still empty. How to fill a plastic raised bed is a question of mix, base and litres. The Royal Horticultural Society fills most beds with topsoil, organic matter and horticultural sharp sand, then leaves the level to settle. Compost on its own breaks down, and the surface drops. A patio makes that worse, because roots cannot borrow depth from the ground. If you are still choosing the frame, use the [plastic raised bed comparison](/hub/raised-beds-greenhouses-and-compost/raised-bed-plastic). Peter F. mapped these steps from RHS guidance, with no soil laboratory test. A qualifying purchase can earn a commission at no extra cost to you.

The frame is on the slabs, or on a weedy corner, and the boot is still empty. How to fill a plastic raised bed is a question of mix, base and litres.

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On this page
  1. 1.Fill with soil, not a bed of compost
  2. 2.Set the base for soil or for paving
  3. 3.Count the litres before you buy the bags
  4. 4.Match the depth to the crop, then let it settle

Fill with soil, not a bed of compost

A plastic side does not change what the roots sit in. RHS advice is to fill mainly with soil, because soil holds its volume and compost does not. The mix is seven parts topsoil, three parts organic matter and two parts horticultural sharp sand, by loose volume. Organic matter means garden compost, leafmould or well-rotted manure. Builders' sand is a different product, and John Innes is a loam for pots, not this recipe. Alpines, bulbs and herbs take double the sand. A closed reservoir is on the self-watering plastic raised beds page.

Set the base for soil or for paving

On garden soil, lift the turf and lightly fork the ground. For no-dig on grass, lay a double layer of plain cardboard, overlapped, with tape and staples removed. February or early March is the RHS window if the soil is not waterlogged. Cardboard does not make contaminated ground safe. On a patio or tarmac, roots cannot leave the box, and RHS school guidance wants at least 50 cm of height. Thompson & Morgan, for the Garden Gear bed, says use a liner and about 10–20 mm of gravel on a patio or deck. Follow that when your kit prints it. Do not lay cardboard on slabs. If the slab is above a room, get a structural view before wet soil goes in.
DIVCHI DIVCHI planter with lid and legs

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DIVCHI DIVCHI planter with lid and legs
8.1/10about £29.99
View offer

Count the litres before you buy the bags

Litres are internal length times width times fill depth, in metres, times 1,000. Stop about 5 cm short of the rim. A 1 m by 0.8 m bed filled to 25 cm is 200 litres, close to the Garden Gear figure for a 100 × 80 × 30 cm bed. Measure your own inside. Split 200 litres as a rule of thumb: about 117 litres of topsoil, 50 litres of organic matter and 33 litres of sharp sand. On paving, the sandier mix is roughly 100, 43 and 57 litres. A 40 or 50 litre sack is the usual UK bag.

Match the depth to the crop, then let it settle

Thirty centimetres is enough for salads, strawberries and dwarf bulbs. Most other plants want 45 cm or more. On paving, a 21 cm panel or 28 cm of soil in a legged planter is the whole root depth. If the height is wrong, change the frame in the plastic raised bed buying guide, not the compost. Rubble in the base is only for beds deeper than 50 cm. Fill to about 5 cm below the rim and do not stand in the bed. Keep the width under about 1.5 m. Whether plastic is worth it against timber is in plastic versus wood raised beds.

Common questions

Can I fill a plastic raised bed with compost only?
You can grow something for a season in multi-purpose compost, but the level drops as it breaks down. Fill mainly with soil: seven parts topsoil, three parts organic matter and two parts horticultural sharp sand. Let compost be that organic share, or a thin feed later. On a patio the slump matters more, because you cannot borrow depth from under the slabs.
What sand goes in the mix?
Use horticultural sharp sand, the washed gritty sand sold for potting mixes. Builders' sand is a different bag. On open soil the share is two parts in every twelve. Double it for alpines, bulbs and herbs. On paving the hard-surface recipe is already four parts sand to seven of topsoil and three of organic matter. Do not stack both changes.
Should I put cardboard under the bed?
On soil or lawn, yes if you are starting no-dig. Use a double layer of plain cardboard, overlap the joins, and take off tape and staples. February or early March suits this if the ground is not waterlogged. On paving or a balcony, cardboard stays wet and does not replace drainage holes. It also does not seal off contaminated ground.
How many litres should I buy?
Multiply internal length, width and fill depth in metres, then by 1,000, and leave about 5 cm at the rim. A 1 m by 0.8 m fill at 25 cm is 200 litres, close to the Garden Gear figure for a 100 × 80 × 30 cm bed. Split the total by the RHS ratio and buy a little extra for the top-up after a fortnight. Measure your panels before you copy another kit.
How deep does the soil need to be?
Thirty centimetres is enough for salads, strawberries and dwarf bulbs. Most other plants want 45 cm or more. On open soil, roots can continue below a shallower open bottom. On tarmac they cannot, and school guidance looks for at least 50 cm of height. A 21 cm kit stays 21 cm, however carefully you mix the fill.
When should I stop and ask someone else?
Ask for a soil test if the ground may be contaminated. Ask for a structural opinion before a balcony takes wet soil, because the filled weight far exceeds the empty product. A long run that raises the ground by more than 30 cm can be a planning question, which is the Planning Portal threshold. A small movable bed is often outside that. A cracked panel is a retailer matter under the Consumer Rights Act 2015.

Related pages

Best plastic raised bed

Plastic raised bed

Plastic raised bed buying guide for depth and type

Self-watering plastic raised bed overflow and patio

Plastic vs wood raised bed for rot, cost and bowing

Sources

Public UK sources from the research digest, last checked on 20 October 2026. Amazon.co.uk listing lines are the price and review source for the six ASINs. No weathering or grow-out test was run.

  1. RHS — How to make a raised bed (retrieved 20.10.2026)
  2. RHS — Growing veg in raised beds (retrieved 20.10.2026)
  3. RHS — Creating raised beds (retrieved 20.10.2026)
  4. RHS — No-dig gardening (retrieved 20.10.2026)
  5. Consumer Rights Act 2015, section 9 (retrieved 20.10.2026)
  6. GOV.UK — returns and refunds (retrieved 20.10.2026)
  7. British Recycled Plastic — raised garden beds (retrieved 20.10.2026)
  8. Gardeners' World — self-watering planters (retrieved 20.10.2026)