Cardboard, Landscape Fabric, or Nothing Under a Raised Bed?

In most cases, put a raised garden bed directly on prepared soil—not on gravel, plastic, or permanent landscape fabric. Clear the vegetation, loosen compacted ground, level the site, and let the bed’s growing soil connect with the soil below. A temporary cardboard layer can help smother ordinary lawn grass, while hardware cloth can help exclude burrowing pests when the local pest problem justifies it. Landscape fabric is not a default weed barrier over healthy garden soil; on hardscape, a permeable liner may be useful for containing soil at gaps, depending on the bed and surface design.^[1]

Quick decision tree

• Grass or ordinary lawn weeds: Remove or cut the vegetation, loosen the soil, and use no permanent barrier. Add cardboard only if you need a temporary smothering layer while building the bed.
• Compacted soil: Use nothing underneath. Loosen the ground with a garden fork or spade so water and roots can move downward.
• Persistent weeds such as bindweed, quackgrass, or blackberry: Remove the major roots before building. Cardboard alone may not control these plants.
• Patio, asphalt, or another hard surface: A permeable liner is optional and may help retain soil where there are gaps or drainage openings. Make sure the bed and surface design provide a safe route for excess water, and make the bed deep enough for the crops you plan to grow.
• Burrowing rodents are common: Consider galvanized hardware cloth beneath the bed. Mesh size and installation should match the pest you are trying to exclude; evidence cited here specifically addresses mice, not every type of burrowing animal.
• Standing water or a marshy site: Do not try to solve the problem with a layer of gravel. Improve the site’s drainage or move the bed.

What should go under a raised bed on grass?

For a typical backyard lawn, the best base is prepared soil. Mow the grass as short as possible, remove obvious sod and large root masses, then loosen the top several inches of soil with a fork or spade. This improves the planting site and gives roots a path into the native soil beneath the bed.^[2]

You have two reasonable ways to handle the remaining grass:

1. Remove it: This is the most dependable option for a shallow bed or an area with vigorous turf. Slice beneath the sod and take out the upper root layer.
2. Smother it: Lay overlapping sheets of plain cardboard or heavy paper over ordinary grass, wet them thoroughly, and cover them with enough growing material to keep them in place. Oregon State University Extension recommends generous overlaps because gaps can allow weeds through.^[3]

Cardboard is not a permanent foundation. It breaks down. While a layer is still intact, it may temporarily reduce or slow water and root movement through the covered soil, particularly if it is thick, poorly wetted, or used with compacted fill. Use it as a temporary smothering layer rather than as a substitute for site preparation, and avoid relying on it to control aggressive perennial weeds.^[3]

When “nothing” is the right answer

A bare-soil base is usually best when the native soil is reasonably clean, drains adequately, and does not contain a serious weed or pest problem. A direct connection between the raised-bed soil and the ground allows roots to explore beyond the frame and avoids creating a restrictive layer beneath the bed. The University of Minnesota Extension notes that most raised beds do not need a barrier between the bed soil and the soil underneath.^[1]

Do not add a layer of gravel simply because the bed is raised. Gravel can reduce usable rooting depth and does not automatically improve drainage. Drainage is better addressed by loosening compacted soil, choosing a suitable location, and filling the bed with a well-structured growing mix.^[4]

Should you use landscape fabric?

Usually not under a raised bed placed on soil. Landscape fabric is permeable when new, but its openings can collect fine sediment and become less permeable over time. Weed seeds can also land on top of the fabric, allowing weeds to grow through or above it. That makes it a poor default solution for a bed where you want long-term root growth and easy soil management.^[5]

Landscape fabric can make sense in narrower situations:

• On a hardscape: A permeable liner may help keep growing media from escaping through gaps or drainage openings. It is optional, not a universal requirement. The bed, liner, and hard surface must work together so excess water can drain safely; do not use solid plastic across the bottom unless the design includes an appropriate drainage system.^[1]
• As a separate walkway treatment: Fabric may be used temporarily beneath mulch or in paths, but it still requires maintenance and is not a permanent guarantee against weeds.^[5]

When hardware cloth belongs underneath

Hardware cloth is useful when animals are tunneling upward into the bed. Install it across the entire bottom before adding soil, and fasten or overlap the edges so pests cannot simply enter around the perimeter. The University of Minnesota Extension recommends hardware cloth at the bottom of a raised bed to reduce tunneling by mice and chipmunks.^[1]

Choose welded, galvanized mesh rather than chicken wire. Smaller openings generally provide better exclusion for small rodents; the National Park Service rodent-exclusion manual identifies galvanized hardware cloth with openings of one-quarter inch or smaller for keeping mice out. The manual is hosted by Utah State University Extension, but it is a National Park Service publication.^[6]

A hardware-cloth bottom is not a complete pest-control plan: animals may still climb into the bed, enter through an unprotected gap, or damage plants from above. For gophers, voles, or other species, confirm the appropriate mesh and installation approach with local extension or wildlife-control guidance.

What belongs under a raised bed on a patio or asphalt?

A bed on a hard surface is different from a bed on soil. Roots cannot continue downward, the growing mix may dry faster, and the base must allow excess water to escape. Use a permeable liner only if needed to contain soil, and do not seal the bottom with an impermeable sheet. Beds on hard surfaces also need enough depth for the crops: the University of Maryland Extension lists at least 8 inches for some shallow-rooted crops and roughly 12 to 24 inches for crops such as peppers, tomatoes, and squash.^[2]

Check the surface after heavy rain before filling the bed. If water pools beneath it, a liner will not correct the underlying problem. Move the bed, improve the slope, or use a design that drains safely away from structures.

Site-preparation checklist

1. Choose a level location with adequate sunlight and no persistent standing water.
2. Mark the bed’s footprint and remove large weeds, sod, and woody roots.
3. Loosen compacted soil with a garden fork or spade rather than creating a hard boundary beneath the bed.
4. Install hardware cloth only if burrowing pests justify it.
5. Use cardboard only as a temporary smothering layer for ordinary grass or as part of a sheet-mulched bed.
6. Skip gravel beneath ordinary soil-based beds.
7. Use permeable landscape fabric or another suitable liner only when the bed sits on hardscape or specifically needs soil containment at gaps. If you suspect contaminated soil, test it and seek site-specific guidance rather than adding a generic liner.
8. Fill the bed with a quality growing mix and expect some settling as organic matter decomposes; top it up as needed.^[8]

Bottom line

For grass over ordinary soil, prepare the ground and build the bed directly on it. Use cardboard when smothering simple turf is more practical than removing it, hardware cloth when a documented burrowing-pest problem warrants it, and permeable landscape fabric mainly when a hardscape bed needs help containing soil. The fewer unnecessary layers you add, the easier it is to maintain drainage and healthy root growth.

Sources

1. Raised bed gardens — University of Minnesota Extension.
2. Growing Vegetables in Raised Beds — University of Maryland Extension.
3. Sheet mulching and lasagna composting with cardboard — Oregon State University Extension Service.
4. Gardening in Raised Bed and Containers in the Northern Great Plains — South Dakota State University Extension.
5. The disadvantages of landscape fabric — University of Illinois Extension.
6. Rodent-Exclusion Manual — National Park Service; hosted by Utah State University Extension.
7. Growing Vegetables in Raised Beds — University of Maryland Extension.
8. How to use compost in gardens and landscapes — Oregon State University Extension Service.