How to Use Cardboard to Kill Weeds: A Simple, Eco-Friendly Approach

Weeds are the bane of every gardener’s existence. Even with the best care and planning, they can creep into established beds, competing with your plants for nutrients, water, and sunlight. But rather than resorting to chemical herbicides or back-breaking weeding, there’s a better way—using cardboard as a natural weed suppressant.

garden established using cardboard

I’ll walk you through the steps to eliminate weeds from an existing garden using cardboard. This method is affordable, easy to apply, and gentle on the environment, leaving your plants happier and the soil healthier.

Why Use Cardboard to Suppress Weeds?

Using cardboard to control weeds offers several benefits:

- Blocks Light to Stop Growth: Cardboard cuts off the sunlight weeds need to grow and photosynthesise.

- Breaks Down into Organic Matter: As the cardboard decomposes, it enriches the soil and attracts earthworms.

- Reduces Weeding Time: A single application can suppress weeds for months, giving you more time to enjoy your garden.

- Eco-Friendly and Cheap: Cardboard is biodegradable, and reusing old boxes keeps waste out of landfills.

- Gentle on Your Garden: No chemicals needed—just simple, natural suppression!

When is the best time to Use Cardboard to Kill Weeds ?

The best time to tackle weeds with cardboard is in early spring or autumn, when plants are dormant, and weeds are less aggressive. However, you can apply cardboard mulch at any time of year, especially after a weeding session or when preparing for a new planting season.

Materials You’ll Need

1. Cardboard: Use plain cardboard without glossy coatings, tape, or staples. Boxes from deliveries or stores work perfectly.

2. Mulch: Choose straw, wood chips, shredded leaves, or compost to layer over the cardboard.

3. Water: To wet the cardboard and mulch thoroughly.

4. Optional: A small garden knife or scissors to cut holes for existing plants.

Step-by-Step Guide to Using Cardboard to Kill Weeds

1. Prepare the Garden Bed

- Weed the area as much as possible. Removing large, established weeds with deep roots (like dandelions) or weeds around the base of established plants, will make this method more effective. Don’t worry about getting every tiny weed—most smaller ones will die under the cardboard.

- Prune your existing plants if needed to give you easier access to the soil surface.

2. Gather and Prepare the Cardboard

- Flatten the cardboard boxes and remove any staples, tape, or plastic labels. Avoid glossy, coloured cardboard as it may contain harmful chemicals.

- Pre-soak the cardboard by dunking it in water or spraying it with a hose. Wet cardboard will mold better to the soil surface and begin decomposing sooner with less chance of weeds pushing the cardboard up.

3. Lay the Cardboard Around Plants

- Cut slits or holes in the cardboard to fit around existing plants, shrubs, or perennials. This ensures the cardboard lies flat on the soil without smothering your desired plants. Alternatively, lay the cardboard up close to the stems of established plants (as shown below)

- If you’re covering large open areas, overlap the cardboard pieces by at least 10 cm (4 inches) to block light and prevent weeds from sneaking through the gaps.

4. Add a Layer of Mulch

- Spread 5–10 cm (2–4 inches) of mulch on top of the cardboard. This layer helps insulate the soil, holds the cardboard in place, and improves the look of your garden bed and sunlight getting to the weeds.

- Use straw, wood chips, compost, or shredded leaves, depending on what you have on hand. For vegetable or flower beds, compost or straw is ideal because it breaks down quickly. For paths or less-maintained areas, opt for wood chips, which last longer.

5. Water the Layers Thoroughly

- Soak the entire area after placing the cardboard and mulch. This ensures the cardboard stays in place, promotes decomposition, and helps the soil below retain moisture.

- Water regularly over the following weeks, especially during dry periods, to maintain the moisture needed for decomposition and weed suppression.

6. Monitor and Maintain the Bed

- Keep an eye out for weeds creeping in along the edges of the cardboard or between plants. Use spot weeding to remove these before they become a problem.

- Add more mulch as needed over time. Mulch will naturally compress and decompose, so it’s helpful to top it up to maintain an effective barrier.

- If you see any cardboard poking through the mulch, add more mulch on top.

How Long Does the Cardboard Last?

In most climates, cardboard will break down within 3–6 months, depending on the amount of moisture and heat it receives. As it decomposes, it becomes part of the soil, feeding earthworms and improving the soil’s structure.

Can You Plant Immediately After Applying Cardboard?

Yes! If you want to add new plants to your bed, simply:

1. Cut a hole in the cardboard where you want to plant.

2. Add compost or soil to the hole, and plant your seedling or transplant as usual.

3. Cover the area with mulch to blend it into the rest of the bed.

This method makes it easy to plant flowers, vegetables, or herbs while still keeping weeds under control.

Tips for Success

- Double up the cardboard in areas with aggressive weeds like couch grass to create an extra-strong barrier.

- Use thicker mulch if your garden is prone to weeds—10–15 cm (4–6 inches) of wood chips or straw works well.

- Refresh the mulch yearly to keep the weed barrier intact and your garden looking its best.

- Watch the edges of your cardboard layer. Weeds often sneak in from the sides, so extend the cardboard slightly beyond the edges of your planting area.

Troubleshooting Common Problems

- Persistent Weeds Breaking Through: This can happen if the cardboard layer was too thin or not overlapped properly. Add another layer of cardboard and mulch on top.

- Cardboard Shifting or Exposing Soil: In windy areas, cardboard may shift if not weighted down. Make sure your mulch layer is thick enough to hold it in place.

- Plants Look Stressed: If existing plants seem stressed, they may be getting too much water or not enough airflow. Check the soil moisture and adjust your watering routine if needed.

Final Thoughts:

Using cardboard to control weeds in an established garden is a low-maintenance, sustainable solution that benefits both your plants and the environment. With just a few layers of cardboard and mulch, you can smother weeds, improve soil health, and free up more time to enjoy your garden.

Whether you’re maintaining a flower bed, vegetable patch, or ornamental garden, this method ensures that weeds stay under control with minimal effort. Plus, as the cardboard breaks down, it enriches the soil, giving your plants a natural boost.

So next time weeds start taking over, skip the chemicals and give cardboard mulching a try. Your garden—and the planet—will thank you!

Have you tried using cardboard to tackle weeds? I’d love to hear how it worked for you! Share your tips and experiences in the comments.