Planning a Cardboard Mulch Layer?
Cardboard under mulch is one of the most effective weed barriers you can create for free. Here’s when it works, when it doesn’t, and how to do it properly.
Quick Answer: Cardboard Under Mulch
Laying cardboard under mulch — sometimes called “sheet mulching” — is a technique used by no-dig gardeners, allotment holders and professional landscapers alike. It works on a simple principle: cardboard blocks 100% of light, which kills existing weeds and prevents new seeds from germinating. The mulch layer on top holds the cardboard in place, looks presentable, and provides additional weed suppression as the cardboard slowly breaks down.
It’s particularly effective against persistent perennial weeds that would push through ordinary mulch. Combined with a good bark mulch layer, cardboard creates a weed barrier that lasts a full growing season with minimal maintenance.
How to Lay Cardboard Under Mulch
The method is straightforward, but the details matter:
- Choose the right cardboard. Plain brown corrugated cardboard is ideal. Remove all tape, staples and labels — these are plastic and won’t decompose. Avoid glossy printed cardboard and waxed boxes (often used for produce). Single-layer card works but corrugated is thicker and more effective
- Prepare the area. Mow or strim any tall weeds down to ground level. You don’t need to remove roots — the cardboard will smother them. For heavily weedy areas, consider applying weed killer first and waiting for it to take effect before laying cardboard
- Lay flat with generous overlaps. Place sheets directly on the soil, overlapping each piece by at least 15cm. Work from one end to the other, stepping on laid sheets to hold them down. For areas with aggressive weeds like bindweed or couch grass, double the cardboard layer
- Wet the cardboard thoroughly. Soak each sheet with a hose or watering can as you lay it. Wet cardboard moulds to the ground surface, stays in place and begins decomposing immediately. Dry cardboard blows around and can repel water initially
- Cut holes for existing plants. If you’re mulching around established plants or trees, cut X-shaped slits in the cardboard and fold the flaps around the stems. Leave a 5cm gap from trunks and woody stems
- Cover with 8–10cm of mulch. Apply bark, wood chip, hay or compost on top. The mulch layer hides the cardboard, adds weight to keep it in place, and provides additional weed suppression
Why Cardboard Attracts Earthworms
One of the biggest benefits of cardboard mulch is what happens underground. Earthworms are strongly attracted to decomposing cardboard because:
- Moisture retention: Cardboard creates a consistently damp layer that worms thrive in
- Food source: The cellulose in cardboard feeds soil fungi and bacteria, which worms then consume
- Temperature regulation: The insulating layer keeps soil temperatures stable, protecting worms from extremes
- Darkness: Worms are photosensitive — the complete light block encourages them to work near the surface
More earthworms means better soil structure, improved drainage, faster nutrient cycling and healthier plants. This is why no-dig gardeners are such enthusiastic advocates of cardboard mulching — it actively builds soil quality while suppressing weeds.
When NOT to Use Cardboard Under Mulch
Cardboard mulching isn’t suitable for every situation:
- Waterlogged soil: If your soil already has poor drainage, an impermeable cardboard layer can make it worse. On heavy clay that sits wet in winter, use a thinner mulch layer without cardboard and focus on improving drainage first
- Around shallow-rooted plants: Plants with surface-feeding roots (like raspberries) can struggle if thick cardboard blocks air and water exchange near the surface. Use single-layer card rather than corrugated for these areas
- Annual flower beds: If you replant beds each season, cardboard creates an awkward barrier when digging. It’s better suited to permanent plantings and perennial borders
- Slopes: On steep ground, water runs off the cardboard surface before soaking through. Mulch on slopes is better applied directly to soil
Cardboard vs Landscape Fabric
For most home gardens, cardboard is the better choice. It’s free, effective for a full season, and actually improves soil quality as it breaks down. Landscape fabric has its place in commercial settings or permanent gravel paths, but in planted beds it causes more long-term problems than it solves — weeds eventually root into the fabric surface, making removal extremely difficult.
Best Uses for the Cardboard Method
Cardboard under mulch works particularly well for:
- New bed creation: Converting lawn or rough ground into garden beds without digging. Lay cardboard in autumn, cover with compost and mulch, and plant in spring
- Perennial weed suppression: Dealing with bindweed, couch grass, mare’s tail or other invasive weeds. The cardboard blocks their growth while mulch keeps conditions dark. For the worst weeds like dandelions and docks, combine with a weed killer treatment first
- Allotment paths: Cardboard under wood chip creates durable, weed-free paths between growing beds
- Around established trees and shrubs: Creates a clean, weed-free mulch ring that feeds the soil gradually
- Seasonal vegetable beds: Lay cardboard between crop rows to suppress weeds in the walking areas. At season’s end, dig the decomposed cardboard into the soil
Frequently Asked Questions
Does cardboard under mulch attract termites?
In the UK, termites are not present, so this isn’t a concern for British gardeners. In countries where termites are an issue, keeping cardboard away from building foundations is sensible. For UK gardens, the only creatures attracted to decomposing cardboard are earthworms and other beneficial soil organisms.
Can I use newspaper instead of cardboard?
Yes, but you’ll need multiple layers (8–10 sheets minimum) to match the weed-blocking ability of a single sheet of corrugated cardboard. Newspaper works well in a pinch, but it’s thinner, tears more easily and decomposes faster. Corrugated cardboard is the better option when available.
How long does cardboard last under mulch?
In most UK conditions, cardboard decomposes in 6–12 months depending on moisture, temperature and earthworm activity. Corrugated cardboard lasts longer than single-ply. In damp conditions with active soil biology, expect the lower end of that range. Reapply annually for ongoing weed suppression, ideally in autumn or early spring.
Will cardboard prevent water reaching plant roots?
When properly wetted during installation, cardboard allows water to pass through. However, if cardboard dries out completely it can temporarily become water-repellent. This is why soaking the cardboard during laying is important, and why a mulch layer on top helps maintain consistent moisture. In heavy rainfall, water eventually penetrates even dry cardboard.
What about the ink and glue on cardboard?
Modern cardboard printing uses soy-based inks, and the adhesives in corrugated board are starch-based. Both are safe for garden use and break down harmlessly in soil. Remove plastic tape, shipping labels and any glossy printed inserts — these are the only components that don’t decompose naturally.
Can I plant through cardboard?
Yes. Cut X-shaped slits and fold back the flaps to create planting holes. For transplants and established plants this works perfectly. For direct sowing from seed, remove the cardboard in the planting row and mulch between rows only. As the cardboard decomposes, plant roots grow through it easily.
You might also find it helpful to read about tough weeds like ground elder.
You might also find it helpful to read about a systemic herbicide.
You might also find it helpful to read about safe for pets and wildlife.
Tackling Established Weeds First?
For the best results, clear perennial weeds with a strong weed killer before laying cardboard. This combination — chemical clearance followed by a physical barrier — is the most effective long-term weed control strategy.
