Using Cardboard Under Mulch to Suppress Annual Weeds
Old cardboard delivery boxes
If you have borders full of annual weeds or large numbers of weed seedlings, using cardboard under a mulch is a simple, low-cost, and very effective solution. This method works particularly well for weeds such as chickweed and is ideal when creating new borders or improving untidy areas.
This technique is often referred to as sheet mulching and is excellent for reducing maintenance while improving soil health.
How Cardboard Mulching Works
Cardboard acts as a biodegradable weed barrier. By blocking light, it prevents annual weeds from photosynthesising and growing. Over time, the cardboard breaks down naturally, feeding soil organisms and improving soil structure.
Covering the cardboard with organic matter such as compost, bark, manure, or wood chips improves its appearance, keeps it in place, and further benefits soil health.
Cardboard Mulching and Weed Types
Annual Weeds
Cardboard mulching is particularly effective against annual weeds such as chickweed and other fast-germinating seedlings. These weeds rely on light to grow, and cardboard prevents them from establishing.
Perennial Weeds (Important)
For perennial weeds such as bindweed, couch grass, nettles, or ground elder, cardboard on its own is usually not strong enough. These weeds have extensive root systems and can often push through as the cardboard breaks down.
In these situations, it is better to use woven landscape sheeting. This should be laid over the soil, cut neatly around existing plants, and then covered with compost, bark, or wood chips. Woven landscape fabric provides long-term weed suppression while still allowing water to penetrate.
What Do We Mean by “Compost”?
In this article, the word compost refers to organic soil-improving materials rather than household compost.
- For most border plants, compost normally means spent mushroom compost.
- For acid-loving plants (such as rhododendrons, azaleas, camellias, and blueberries), compost normally means well-rotted horse manure or farmyard manure, which is slightly acidic.
Choosing the right compost helps ensure plants grow well and soil conditions remain suitable.
When Cardboard Mulching Works Best
Cardboard mulching is ideal for:
- Borders with lots of annual weeds or seedlings
- New planting beds
- No-dig gardening systems
For borders dominated by perennial weeds, woven landscape sheeting is the better option.
Step-by-Step Guide to Cardboard Mulching
1. Prepare the Area
- Cut back tall weeds.
- Remove perennial weeds where possible.
- There is no need to dig — this method works well in no-dig gardens.
2. Lay the Cardboard
- Use plain brown cardboard only (no glossy or printed surfaces).
- Remove all plastic tape, labels, and staples.
- Overlap sheets by at least 10–15 cm (4–6 inches).
- Water the cardboard thoroughly once laid to help it settle and start breaking down.
3. Add the Mulch Layer
You can use one material or a combination of the following:
Compost (5–10 cm deep)
- Spent mushroom compost for most border plants
- Well-rotted horse or farmyard manure for acid-loving plants
This layer is ideal for planting directly into and quickly improves soil fertility.
Bark or Wood Chips (7–10 cm deep)
- Longer lasting and lower maintenance
- Excellent for shrubs, trees, and ornamental areas
A combination works very well: compost on the cardboard, with bark or wood chips on top.
Feeding Before Mulching (Important)
As organic mulches break down, soil bacteria use nitrogen, which can temporarily reduce nutrient availability.
To avoid this:
- Apply a general fertiliser (such as Growmore) before laying the cardboard.
- Use around one handful per square metre.
More detail can be found here:
https://gardenadvice.co.uk/gardening-tips/mulching-your-garden-borders/
Planting Through Cardboard or Sheeting
- Cut a cross or circle in the cardboard or woven sheeting.
- Fold it back slightly.
- Plant into the soil or compost below.
- Make sure plant roots reach the soil beneath the barrier.
Benefits of Sheet Mulching
✔ Excellent suppression of annual weeds
✔ Reduces garden maintenance
✔ Improves moisture retention
✔ Encourages earthworms and beneficial soil bacteria
✔ Improves soil structure over time
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