How do I make a new bed on top of a lawn without digging?

This is the single most useful practical technique in modern gardening: a bed goes in over an afternoon, nothing is dug, and it is plantable the same day. The mechanism is simple — grass and annual weeds die in total darkness, and worms pull the compost and the rotting cardboard down into the soil below, joining the new layer to the old ground. The two things that make it fail are gaps in the cardboard and perennial weeds.

Safety

What you'll need

• Plain brown cardboard, more than you think
• Well-rotted compost or rotted manure, 100 to 150mm deep over the whole area
• A hose or watering cans
• A half-moon edger or spade for the edge (optional)
• A fork for perennial weeds

Step by step

1. Mark the bed and work out how much compost it needs.Area in square metres multiplied by depth in metres gives cubic metres — a 3m by 1.5m bed at 125mm needs 3 × 1.5 × 0.125, which is about 0.56 cubic metres, most of a bulk bag. Order before you start; running out halfway leaves a bed that fails in the thin part.
2. Mow the grass as short as the mower will go.Short grass rots faster and gives less bulk to settle unevenly. Leave the clippings where they fall — they are nitrogen and they go under the cardboard.
3. Deal with perennial weeds now, not later.Bindweed, couch grass, ground elder, mare's tail, docks and brambles all come through cardboard. Dig out what you can find, and if the area is badly infested, cover the whole thing with an opaque sheet for a full growing season before you build the bed. There is no shortcut here and no way to fix it afterwards.
4. Prepare the cardboard.Plain brown corrugated cardboard. Remove every scrap of plastic tape, staples, plastic labels and glossy printed sleeves. Bike shops, furniture shops and supermarkets will give you more than you can carry.
5. Lay it with generous overlaps.Every joint overlapped by at least 150mm, and the whole sheet run 150mm beyond where the compost will finish on every side. Light through a gap is a hole the grass grows through, and the edges are where new beds fail because grass creeps in sideways.
6. Wet the cardboard thoroughly before anything goes on top.Soak it. Dry cardboard under compost stays dry for months, repels water, and stops the layers joining. This step takes ten minutes and is the one most often skipped.
7. Spread 100 to 150mm of compost.Well-rotted garden compost, green-waste compost or rotted manure. This is the layer you plant into, so it needs to be finished material and not half-rotted. Rake it level and firm it very lightly with the back of the rake.
8. Plant straight into the compost the same day.A trowel makes a hole in the compost and stops at the cardboard — that is fine for most vegetables and bedding, whose roots go down through the softening cardboard within weeks. For anything with a large rootball, cut a cross in the cardboard at that point and plant through it.
9. Cut a clean edge and keep it cut.A half-moon edger cut around the perimeter, or a physical edging board, stops the surrounding grass running back in. This is a maintenance job for the first two seasons and then it settles down.
10. Top up with 25 to 50mm of compost each year.That is the whole ongoing regime. No digging, no forking. The level drops as material is taken down by worms, and the annual top-up replaces it.

Tips

• Sow-direct crops are harder in year one than transplants, because the compost surface is coarse and the cardboard is still intact. Plant modules and young plants in the first season and sow directly from year two.
• Autumn is the ideal time to build a bed like this — the cardboard has broken down and everything has settled by spring.
• Do not use compost that has not finished rotting. Immature compost robs nitrogen and can scorch. If it does not smell sweet and look like dark crumb, stack it for another few months.
• Green-waste compost from a local authority site is usually the cheapest way to get a cubic metre. Ask about herbicide residue before you buy — it is the one material class where the question is worth asking.
• Newspaper works instead of cardboard but needs eight to ten sheets thick, blows about while you are laying it, and breaks down much faster. Cardboard is better in every respect.

Common mistakes

• Laying the cardboard butt-jointed rather than overlapped — The joints open as the cardboard wets and settles, and grass finds every one of them. Overlap generously — cardboard is free.
• Building over bindweed or couch grass and hoping — They come straight through, and now they are growing up through a bed you have planted, where you cannot dig them out without wrecking it.
• Spreading only 50mm of compost — Too thin. Grass pushes through, annual weed seed germinates in it, and there is not enough depth to plant into. 100mm is the practical minimum and 150mm is better.

Questions people ask