Yes, plain cardboard can be placed beneath an open-bottom raised garden bed, particularly when the bed is being installed over an existing lawn or an area covered with ordinary annual weeds.

Cardboard temporarily blocks sunlight from reaching the vegetation below. When covered with soil, compost, or mulch, it helps smother grass while gradually softening and decomposing.

This makes it a convenient option for gardeners who want to establish a raised bed without removing every piece of turf.

However, cardboard is neither required for every raised bed nor suitable for every problem. It does not permanently stop aggressive perennial weeds, tree roots, or burrowing animals. It should also never be used to block the drainage holes of an elevated planter.

The decision should be based on the type of bed, the condition of the ground, and what you are trying to accomplish.

Quick Answer

Use cardboard when:

• Installing an open-bottom raised bed over ordinary lawn
• Smothering shallow annual weeds
• Creating a temporary biodegradable barrier
• Establishing a no-dig garden bed
• Covering bare soil beneath a new growing area

Do not rely on cardboard when:

• Bermuda grass, bindweed, nutsedge, bamboo, or another aggressive plant is present
• Tree roots repeatedly invade the bed
• Gophers or voles are the main problem
• The soil below is contaminated
• The planter has a solid bottom and needs drainage
• The cardboard is glossy, waxed, heavily laminated, or covered in plastic
• You need a permanent barrier

Benefits and Limitations

When Is Cardboard Useful?

Installing a Raised Bed Over Lawn

This is the most common and practical reason to use cardboard.

Instead of cutting out and removing the sod, mow the grass as short as possible, cover it with overlapping sheets of cardboard, and build the bed above it.

The cardboard restricts light and compresses the existing vegetation. Moisture, soil organisms, and time gradually break down both the cardboard and the grass below.

This works best for an ordinary lawn made up of relatively shallow-rooted grasses.

It is less dependable when the site contains aggressive grasses that spread through strong underground rhizomes or stolons. These plants may grow around the edges, enter through gaps, or survive until the cardboard decomposes.

Starting a No-Dig Garden

Cardboard is frequently used in no-dig or sheet-mulched gardens.

A typical system includes:

1. Existing soil or short grass
2. Overlapping plain cardboard
3. Compost or suitable topsoil
4. Organic surface mulch

The goal is to reduce disturbance to the soil while suppressing existing vegetation.

Cardboard is not essential to every no-dig bed. When a site is already free of grass and persistent weeds, compost and mulch may be applied without it.

Smothering Annual Weeds

Cardboard can suppress many shallow-rooted annual weeds by blocking light.

It is most effective when:

• Weeds are cut down first
• Seed heads are removed
• Sheets overlap generously
• The cardboard is completely covered
• Edges remain buried
• The area receives sufficient moisture

It is less effective against plants that store substantial energy in deep roots, bulbs, tubers, or rhizomes.

Recycling Clean Shipping Boxes

Reusing plain shipping boxes keeps a useful carbon-rich material in the garden rather than discarding it immediately.

However, cardboard used in a bed should be viewed primarily as a temporary physical layer—not as fertiliser. It contains carbon but provides far fewer nutrients than compost, aged manure, or a balanced soil amendment.

When Cardboard Is Not Necessary

A Raised Bed Over Clean, Healthy Soil

If an open-bottom bed is installed over healthy soil without grass or serious weeds, no bottom layer may be needed.

Direct soil contact allows:

• Water to drain naturally
• Earthworms and other organisms to move into the bed
• Plant roots to grow into the ground
• Soil layers to interact without an initial barrier

Loosen severe compaction before filling the bed, but avoid adding cardboard simply because other gardeners use it.

A Deep Bed Over Light Grass

A sufficiently deep raised bed filled with soil may suppress weak grass without cardboard, particularly after the sod is cut short.

Cardboard can provide extra short-term protection, but it is not always essential.

An Elevated Planter

An elevated planter with a built-in bottom functions like a large container. Cardboard should not cover its drainage holes.

If soil escapes through openings, use an appropriate permeable liner that retains the growing medium while allowing water to drain.

Cardboard inside an elevated planter may soften, collapse, and block drainage unpredictably.

A Bed Over Concrete

Cardboard beneath a bed on concrete does not suppress weeds and may interfere with drainage if it covers the available exit points.

Beds over hard surfaces require a clear drainage plan, adequate soil depth, and protection for the surrounding surface. Cardboard provides little benefit in this situation.

What Kind of Cardboard Is Safe?

Choose plain brown corrugated cardboard from a known, clean source.

Good options include:

• Plain shipping boxes
• Plain appliance boxes
• Uncoated moving boxes
• Simple brown corrugated packaging
• Boxes with minimal printing after labels and tape are removed

Prepare the cardboard by removing:

• Plastic packing tape
• Shipping labels
• Plastic windows
• Metal staples
• Foam
• Packing straps
• Wax coatings
• Glossy laminated areas

Small amounts of ordinary printing on standard corrugated boxes are generally less concerning than coatings, plastic films, or unknown chemical contamination. If the material has an unusual surface, strong smell, grease stain, or unknown industrial use, recycle or discard it instead.

Cardboard to Avoid

Do not use:

• Waxed produce boxes
• Glossy retail packaging
• Plastic-coated beverage cartons
• Frozen-food boxes with moisture-resistant coatings
• Foil-lined packaging
• Heavily laminated cardboard
• Grease-soaked pizza boxes
• Boxes contaminated with chemicals
• Cardboard treated with water-resistant products
• Material with mould or a strong chemical odour

Avoid using cardboard from a construction site unless you know it is clean and untreated.

How to Put Cardboard Under a Raised Garden Bed

Step 1: Inspect the Site

Identify the vegetation currently growing there.

Ordinary lawn and annual weeds can often be smothered. Aggressive perennial weeds should be removed as thoroughly as possible before installation.

Look for:

• Bermuda grass
• Bindweed
• Nutsedge
• Quackgrass
• Bamboo
• Japanese knotweed
• Brambles
• Tree roots
• Poison ivy
• Deep-rooted perennial weeds

Do not expect cardboard to eliminate these plants by itself.

Step 2: Mow or Cut Vegetation Short

Cut grass and weeds close to ground level.

Remove mature seed heads so they are not introduced into the new bed. Pull or dig out large woody stems, bulbs, rhizomes, and persistent crowns.

Leaving short grass clippings in place is generally acceptable if the plants have not been treated with a persistent herbicide.

Step 3: Prepare the Ground

Remove stones, branches, plastic, and other debris that would prevent the bed from sitting level.

If the soil is severely compacted, loosen it gently with a garden fork before covering it. This improves the connection between the raised-bed soil and the ground after the cardboard decomposes.

Do not pulverise wet clay soil, as this can worsen its structure.

Step 4: Remove Tape and Labels

Flatten the boxes completely.

Remove plastic tape from every seam. Check both sides for labels, staples, packing material, and plastic film.

This is easier to do before the cardboard becomes wet.

Step 5: Overlap the Sheets

Cover the entire interior footprint of the bed.

Overlap adjacent pieces by approximately 6 inches or more so that grass cannot immediately grow through the seams.

Extend the cardboard slightly beneath the sides of the bed where practical. Pay particular attention to edges and corners, as these are the most common entry points for grass.

One substantial layer of corrugated cardboard is often enough for ordinary lawn. Several thinner pieces may be overlapped where gaps occur.

Avoid creating an unnecessarily thick, compact stack.

Step 6: Wet the Cardboard

Soak the cardboard thoroughly with water.

Wet cardboard:

• Stays in position
• Conforms to the ground
• Is easier to cover
• Begins breaking down
• Reduces dry air gaps
• Helps create contact with the soil below

Do not leave large dry voids beneath the sheets.

Step 7: Install the Bed and Fill It

Place the raised-bed frame in its final position and add the appropriate growing mix.

The required soil depth depends on the crops. In general, provide more usable soil for tomatoes, peppers, cucumbers, and root vegetables than for lettuce or shallow-rooted herbs.

Do not count coarse logs, undecomposed filler, or compacted cardboard as usable root-zone depth.

Step 8: Water and Check for Settling

Water the filled bed so the soil settles into gaps.

Top up low areas before planting. Organic materials and cardboard may settle further during the first season, so additional compost or soil may be needed later.

How Many Layers of Cardboard Should You Use?

For ordinary lawn, one continuous layer of sturdy corrugated cardboard with generously overlapped seams is often sufficient.

The number of boxes matters less than complete coverage.

Too little cardboard leaves gaps where grass can grow. Excessively thick layers may:

• Take longer to decompose
• Temporarily restrict water movement
• Delay root access to native soil
• Create uneven settling
• Form a slippery compressed mat

Use enough to block light without building a deep stack of tightly compressed material.

For difficult perennial weeds, more cardboard is not necessarily the solution. Remove the weed roots or use a management method appropriate to the species.

How Much Soil Should Go Above Cardboard?

Add enough soil for the plants to grow before the cardboard decomposes.

Approximate usable depths include:

These are general guidelines. Plant varieties, climate, soil texture, and bed design affect the actual requirement.

A shallow layer of soil directly over fresh cardboard may dry quickly and restrict early root growth. For immediate planting, provide sufficient finished growing medium above it.

How Long Does Cardboard Take to Decompose?

There is no fixed decomposition schedule.

Cardboard may soften within weeks but take several months or longer to disappear completely. The speed depends on:

• Cardboard thickness
• Number of layers
• Soil temperature
• Moisture
• Microbial activity
• Earthworm activity
• Climate
• Soil contact
• Type of cardboard
• Material placed above it

Warm, consistently moist conditions generally accelerate decomposition. Thick, dry layers in cool climates may persist longer.

You do not need to remove ordinary cardboard after it begins breaking down. It can remain in place and become incorporated into the soil.

Does Cardboard Attract Earthworms?

Moist cardboard can create a dark, protected environment containing carbon-rich fibres that soil organisms can gradually break down.

Earthworms may gather around the layer when moisture and temperature conditions are favourable.

However, cardboard does not create earthworms or guarantee healthy soil. Earthworm activity also depends on:

• Local species
• Soil moisture
• Organic matter
• Temperature
• Soil chemistry
• Pesticide exposure
• Available food

Cardboard can be a useful carbon material in some composting and vermicomposting systems, but it should be balanced with suitable moisture and other organic inputs.

Does Cardboard Add Nutrients?

Cardboard is primarily a carbon-rich material. As it decomposes, it contributes a small amount of organic matter, but it is not a complete soil amendment.

It should not replace:

• Finished compost
• Mineral soil
• Balanced fertiliser
• A soil test
• Appropriate nutrient management

Microorganisms need nitrogen while decomposing carbon-rich materials. Because the cardboard remains beneath the primary root zone rather than being mixed throughout it, significant nitrogen problems are less likely. Still, plants should grow in a suitable compost-and-soil blend above the layer.

Will Cardboard Stop Weeds?

Cardboard can temporarily suppress grass and many annual weeds by depriving them of light.

It is less reliable against:

• Bermuda grass
• Bindweed
• Nutsedge
• Quackgrass
• Horsetail
• Bamboo
• Japanese knotweed
• Tree roots
• Established brambles
• Weeds with deep storage roots

These plants may penetrate the cardboard, grow around its edges, or return after it decomposes.

Cardboard also cannot prevent new weed seeds from landing and germinating in the soil above it. Surface mulch and routine weeding remain necessary.

Will Cardboard Keep Gophers and Voles Out?

No. Wet cardboard is not a durable animal barrier.

Gophers, voles, rats, and other digging animals can chew or tunnel through it. If burrowing pests are a known problem, secure galvanized hardware cloth across the open bottom before adding soil.

Use cardboard for grass suppression and metal mesh for animal exclusion. They solve different problems.

Ensure the mesh:

• Covers the entire base
• Has an appropriate opening size
• Overlaps at seams
• Is secured to the frame
• Has no exposed sharp edges

Chicken wire may corrode or have openings too large for smaller animals.

Cardboard vs. Landscape Fabric

Landscape fabric is not automatically better. In an open-bottom vegetable bed, it can separate the imported soil from the ground and become entangled with roots.

Use it only when a longer-lasting barrier is genuinely required and drainage will remain adequate.

Cardboard vs. Newspaper

Both can be used for temporary sheet mulching.

Cardboard is:

• Thicker
• Easier to cover large areas with
• Less likely to shift
• Slower to break down
• More effective with fewer sheets

Newspaper is:

• Thinner
• Faster to decompose
• Useful for small gaps
• More likely to blow away during installation
• More difficult to apply evenly

Avoid glossy advertising inserts and coated paper. Wet newspaper as it is installed so it stays in place.

Common Mistakes

Leaving Plastic Tape Attached

Tape does not decompose like cardboard and becomes difficult to remove after the bed is filled.

Using Glossy or Waxed Boxes

Coatings can slow decomposition and may introduce unwanted materials.

Failing to Overlap Seams

Grass quickly finds gaps between separate sheets.

Expecting Cardboard to Stop Every Weed

Persistent rhizomes and roots can grow around or through a temporary layer.

Making the Layer Too Thick

A deep stack may restrict early water movement and root access without providing additional long-term control.

Using Cardboard as a Gopher Barrier

Burrowing animals can penetrate it. Use appropriate metal mesh instead.

Blocking Drainage Holes

Never place cardboard over the drainage openings of a solid-bottom or elevated planter.

Planting in Too Little Soil

Young plants need enough growing medium above the cardboard before their roots can reach the soil underneath.

Frequently Asked Questions

Should every raised bed have cardboard underneath?

No. A bed installed over clean, healthy soil usually does not need it. Cardboard is most useful for temporary grass and weed suppression.

Can I plant immediately after installing cardboard?

Yes, provided there is enough suitable soil above it for the selected crops. You do not need to wait for the cardboard to decompose.

Should cardboard go under or inside the frame?

Cover the entire interior footprint and overlap the seams. Extending it slightly under the edges can help reduce grass entering around the perimeter.

Can I use printed shipping boxes?

Plain corrugated boxes with limited ordinary printing are generally preferable. Remove tape, labels, plastic, staples, and heavily coated sections.

Will plant roots grow through cardboard?

As wet cardboard softens and decomposes, roots can grow through it. The timing depends on the thickness, moisture, temperature, and plant.

Can cardboard cause drainage problems?

One properly wetted layer beneath an open-bottom bed generally breaks down over time. An excessively thick, compressed stack or cardboard covering container drainage holes can interfere with water movement.

Does cardboard make soil acidic?

Ordinary cardboard is unlikely to create a meaningful, lasting change in the pH of a large raised bed. Test the soil rather than using cardboard to adjust acidity.

Can I use cardboard around existing plants?

Yes, cardboard can be used for sheet mulching around established plants, but leave space around trunks, crowns, and stems. Do not pile wet cardboard against plant tissue.

Does cardboard attract termites?

Cardboard contains cellulose and may be consumed by termites where they are present. Avoid placing large amounts against wooden buildings, decks, or structural timber, especially in termite-prone regions.

Can cardboard be used on garden paths?

Yes. Lay overlapping cardboard over short weeds and cover it with a substantial layer of wood chips or another suitable path material. Expect to renew the path as both layers decompose.

Final Thoughts

Cardboard can be a useful, inexpensive material when installing an open-bottom raised bed over ordinary lawn. It temporarily blocks light, suppresses shallow vegetation, and gradually decomposes without requiring removal.

Its benefits should not be exaggerated. Cardboard does not permanently stop aggressive weeds, protect against gophers, improve badly drained soil, or replace compost and fertile growing mix.

Use clean, plain corrugated cardboard, remove all tape and plastic, overlap the sheets, wet them thoroughly, and add enough soil for the intended plants. When the ground is already clean and healthy, leaving the raised bed open to the soil below may be the better choice.