What Should You Put at the Bottom of a Raised Garden Bed?

For most open-bottom raised garden beds, you do not need to install a solid bottom or fill the base with rocks. The bed can usually sit directly over the existing soil so excess water drains away and plant roots can grow deeper when necessary.

However, the right preparation depends on where the bed is installed and what problems you need to solve.

Cardboard may help suppress grass temporarily. Hardware cloth can block burrowing animals. Logs, branches, leaves, and other organic materials can reduce the amount of soil needed in a deep bed. A raised planter with a solid bottom requires drainage holes and a different filling method altogether.

Before adding anything, determine what type of raised bed you have, how deep it is, and whether weeds, drainage, contamination, or digging animals are concerns.

Quick Answer: What Goes on the Bottom?

Avoid using solid plastic, trash bags, carpet, pressure-treated scraps, painted wood, or a deep layer of gravel.

First, Identify Your Type of Raised Bed

The phrase “raised garden bed” can describe several different structures. The correct bottom layer depends on the design.

Open-bottom raised bed

An open-bottom bed is a frame placed directly over soil or grass. It has no built-in floor.

This design allows:

• Water to drain into the ground
• Earthworms and other soil organisms to enter
• Deep roots to grow below the bed
• Existing soil to interact with the new soil

Most open-bottom beds do not require a liner.

Raised bed placed over concrete

A bed installed over a patio, driveway, or other hard surface cannot drain into the ground. Water must escape through gaps or drainage openings around the base.

These beds need adequate soil depth and a clear route for excess water. They may also release water and soil stains onto the surrounding surface.

Elevated planter

An elevated planter stands on legs and has a built-in bottom. It behaves more like a large container than an in-ground garden bed.

It requires:

• Drainage holes
• Lightweight growing media
• A permeable liner if soil can escape
• More frequent watering than many ground-level beds

Do not use ordinary garden soil alone in an elevated planter because it may become compacted, heavy, and poorly drained.

Do Raised Garden Beds Need a Bottom?

Open-bottom raised beds usually do not need a constructed bottom.

Direct contact with the ground improves drainage and gives deep-rooted vegetables additional growing space. A solid barrier can prevent this natural connection and create drainage problems.

A bottom barrier may be useful when:

• Gophers, voles, or other animals dig into beds
• The bed is installed over aggressive turf
• The underlying soil is contaminated
• Soil would otherwise fall through an elevated planter
• The bed sits on a balcony, deck, or hard surface
• Tree roots repeatedly invade the growing area

The material should address a specific problem. Adding layers without a purpose can make the bed more expensive and difficult to maintain.

Calculate How Much Soil You Need

Before choosing filler materials, calculate the approximate volume of the bed.

For a rectangular raised bed:

Volume = length × width × soil depth

If the measurements are in feet, the answer will be in cubic feet.

For example, a bed measuring 8 feet long, 4 feet wide, and 1.5 feet deep requires:

8 × 4 × 1.5 = 48 cubic feet

Bulk soil is often sold by the cubic yard. One cubic yard equals 27 cubic feet, so:

48 ÷ 27 = approximately 1.78 cubic yards

Do not calculate volume using the full height if you plan to leave space at the top. Leaving approximately 1–2 inches below the rim can make watering and mulching easier.

Irregular or rounded beds may require a different formula, but an approximate calculation is usually sufficient for ordering soil.

Best Materials for the Bottom of a Raised Garden Bed

1. Existing Native Soil

If the bed sits over reasonably healthy, uncontaminated soil, the simplest option is to use no bottom layer.

Loosen compacted soil with a garden fork before installing or filling the bed. This helps water move between the imported soil and the native ground and gives roots an easier path downward.

You do not need to remove every stone or sift the soil. Focus on breaking up severe compaction and removing large perennial weed roots.

This approach works especially well for shallower beds because vegetables with deep root systems can continue growing below the frame.

2. Plain Cardboard

Cardboard can serve as a temporary weed-smothering layer when a raised bed is installed over lawn or annual weeds.

Use plain brown corrugated cardboard with:

• All plastic tape removed
• Shipping labels removed
• No glossy coatings
• Minimal colored printing
• No wax or plastic lining

Overlap the pieces so grass cannot immediately grow through the seams. Wet the cardboard thoroughly before adding soil to help it conform to the ground and begin decomposing.

Cardboard is not permanent. Soil organisms, moisture, and plant roots will gradually break it down. This is beneficial when you want temporary grass suppression without permanently separating the bed from the soil.

Cardboard will not reliably stop:

• Gophers
• Voles
• Tree roots
• Bindweed
• Bermuda grass
• Japanese knotweed
• Other persistent perennial weeds

Remove difficult roots and rhizomes before building the bed rather than expecting cardboard to solve the problem by itself.

3. Galvanized Hardware Cloth

If burrowing animals are a problem, attach galvanized hardware cloth to the bottom of the raised bed before filling it.

Hardware cloth is stronger than lightweight chicken wire and has smaller openings. The appropriate mesh size depends on the animal you need to exclude.

Install it so that:

• The entire bottom is covered
• Adjacent pieces overlap
• Edges are attached securely to the frame
• No large openings remain around corners
• Sharp cut edges are folded or covered

The mesh allows water, roots, and many soil organisms to pass through while reducing access for larger burrowing pests.

Choose corrosion-resistant material suitable for long-term outdoor use. Plastic netting is generally easier for animals to damage and can break into fragments as it ages.

4. Compostable Organic Materials

Deep raised beds can be expensive to fill entirely with purchased soil. Logs, sticks, leaves, straw, and other plant materials can occupy some of the lower space.

Possible bottom materials include:

• Partially decomposed logs
• Small branches and twigs
• Untreated wood
• Autumn leaves
• Straw
• Finished or partially finished compost
• Chemical-free plant debris
• Old potting mix
• A limited amount of untreated wood chips

Cover bulky materials with enough growing mix to support the crops you intend to plant. Most vegetable roots should have access to a substantial layer of stable soil and compost rather than being planted directly into coarse woody debris.

The organic matter will decompose and settle, so expect to add compost or soil to the top of the bed in future seasons.

5. A Permeable Planter Liner

A liner can be useful in an elevated planter or a bed with wide gaps that allow soil to escape.

Choose a material that:

• Allows water to pass through
• Is strong enough to hold moist soil
• Does not block the drainage holes
• Is suitable for outdoor growing
• Can be replaced if it deteriorates

A liner should retain soil without turning the planter into a watertight container.

For wooden beds, lining the vertical sides may reduce direct contact between moist soil and the wood. Do not seal the entire base unless the structure includes a separate drainage system.

6. A Root Barrier

A specialized root barrier may be necessary when invasive tree or bamboo roots repeatedly enter raised beds.

Unlike cardboard, a root barrier is designed to remain intact for a longer period. However, it also separates the raised-bed soil from the ground and may limit drainage.

If using one:

• Make sure excess water has somewhere to escape.
• Provide adequate soil depth above the barrier.
• Inspect the bed periodically for roots entering from above or around the sides.
• Follow the manufacturer’s installation directions.

Whenever possible, move the bed away from aggressive roots instead of relying only on a bottom barrier.

Using the Hugelkultur Method in Raised Beds

Hugelkultur is a gardening method that incorporates woody material and other organic matter beneath the growing layer.

In a raised bed, a simplified version may include:

1. Coarse logs or branches at the bottom
2. Smaller sticks and woody debris
3. Leaves, straw or plant material
4. Compost or aged organic matter
5. A substantial upper layer of planting mix

This can reduce the volume of purchased soil required for a deep bed. As the organic material decomposes, it contributes organic matter and may retain moisture.

However, raised-bed hugelkultur has limitations.

Expect settling

Wood and plant material lose volume as they decompose. The bed surface may sink unevenly, particularly during the first few years.

Fill gaps carefully during construction and plan to top up the bed with compost or soil.

Use enough growing soil

Shallow-rooted crops may tolerate a thinner growing layer, but tomatoes, peppers, carrots, parsnips, and other larger or deeper-rooted plants need more stable soil above the woody material.

Do not fill most of a moderately shallow bed with branches and leave only a few inches of soil on top.

Avoid contaminated wood

Do not use:

• Painted wood
• Stained or sealed wood
• Plywood
• Particleboard
• Railroad ties
• Construction scraps of uncertain origin
• Chemically treated lumber
• Diseased plant material

Use clean, untreated woody material.

Consider the wood species

Highly decay-resistant wood will take longer to break down. Some woods may also contain compounds that affect sensitive plants, especially when used fresh and in large quantities.

Black walnut is a common concern because certain parts of the tree may contain juglone. When the wood source is uncertain, use well-aged, partly decomposed material or leave it out.

Watch for temporary nitrogen competition

Microorganisms use nitrogen as they decompose carbon-rich wood. This effect is strongest close to the wood-soil interface.

Keeping woody materials in the lowest part of a deep bed and providing an adequate compost-rich planting layer helps reduce problems. Fresh wood chips mixed throughout the root zone are more likely to interfere with plant growth than intact logs buried well below it.

How Deep Should the Topsoil Layer Be?

The required soil depth depends on the crops being grown and whether roots can continue into the ground below.

General guidelines include:

These are general ranges rather than strict rules. Compact varieties may need less space, while large plants may benefit from more.

If the bed has a root barrier, solid base, concrete surface, or dense layer of woody filler, count only the usable growing medium when evaluating root depth.

What Not to Put at the Bottom

Plastic Sheets or Trash Bags

Solid plastic blocks drainage. Water can accumulate above it, depriving roots of oxygen and increasing the risk of root disease.

Plastic may also tear and become difficult to remove after being buried under a large volume of soil.

If you need a liner, choose a permeable material and keep all drainage openings clear.

A Thick Layer of Gravel or Rocks

Adding rocks to the bottom does not automatically improve drainage.

Water movement depends on soil texture, pore size, and the ability of water to exit the bed. When fine-textured soil sits above a coarse gravel layer, water may remain in the soil above the interface until it becomes sufficiently saturated to move downward.

Gravel also:

• Reduces available root depth
• Adds considerable weight
• Becomes mixed with soil
• Makes future excavation difficult
• Increases the amount of material that must eventually be removed

Improve drainage by using suitable soil, avoiding compaction, keeping drainage holes open, and providing a clear exit for water.

Landscape Fabric Without a Specific Need

Landscape fabric may temporarily reduce weeds, but it can also restrict root growth and separate the raised-bed soil from the ground.

Over time, soil and organic matter accumulate on top of the fabric, allowing new weeds to germinate anyway. Roots may also grow into the fabric, making it difficult to remove.

For open-bottom beds over ordinary lawn, biodegradable cardboard is often a better temporary option. Reserve durable barriers for situations involving severe weeds, contaminated soil, or persistent root intrusion.

Carpet

Old carpet may contain synthetic fibers, adhesives, dyes, stain treatments, and other materials that are not intended for garden soil.

It also creates a difficult-to-remove barrier as it deteriorates. Do not use carpet beneath a food-growing bed.

Pressure-Treated or Painted Wood Scraps

Do not bury wood of unknown origin in a vegetable bed. Paint, preservatives, adhesives, and surface coatings may contain substances you do not want in the soil.

Choose untreated natural wood from a known source.

Large Amounts of Fresh Wood Chips

Wood chips can be useful as surface mulch, but mixing large quantities of fresh chips into the planting layer may temporarily reduce nitrogen availability around the decomposing material.

If using chips as deep-bed filler:

• Keep them below the main root zone.
• Combine them with nitrogen-rich compostable materials.
• Add a generous layer of finished growing mix above.
• Expect the bed to settle over time.

For weed control, wood chips are generally more effective as a 2–3-inch surface mulch than as a bottom liner.

How to Prepare a Raised Bed Over Grass

For ordinary lawn without severe perennial weeds:

1. Mow or cut the grass very short.
2. Mark the raised bed’s position.
3. Loosen heavily compacted soil if necessary.
4. Remove persistent weed roots.
5. Install hardware cloth if burrowing animals are present.
6. Lay overlapping plain cardboard over the grass.
7. Wet the cardboard thoroughly.
8. Place the raised-bed frame.
9. Add filler materials if it is a deep bed.
10. Add the growing mix.
11. Water the bed to settle the soil.
12. Top up low areas before planting.

If the lawn contains Bermuda grass, bindweed, nutsedge, or another persistent weed, remove as much of its root or rhizome system as possible first. A thin cardboard layer alone may not stop it.

How to Prepare a Raised Bed Over Concrete

A bed over concrete needs special drainage planning.

Make sure:

• The frame allows water to escape.
• Drainage routes are not sealed.
• Water will not collect against a building.
• Runoff will not create a slipping hazard.
• The surface can support the bed’s saturated weight.
• Soil cannot wash out through large openings.

Use a suitable raised-bed mix containing stable organic matter and ingredients that preserve air space. Avoid filling the bed with dense native clay soil.

A shallow bed over concrete has limited root depth. Select compact crops or use a deeper structure for large vegetables.

What Soil Should Go Above the Bottom Layer?

The upper growing zone should provide drainage, water retention, nutrients, and physical support.

A practical raised-bed blend may contain:

• Quality screened topsoil or mineral soil
• Finished compost
• A structured soil conditioner or other stable organic component

Exact ratios depend on the materials available. Avoid filling the entire bed with compost because compost continues decomposing and may hold excessive water or contain nutrient concentrations unsuitable for some plants.

Also avoid using only lightweight potting mix in a large open-bottom bed unless it is specifically formulated for that purpose. It may dry quickly, settle significantly, and become expensive.

Purchase soil and compost from reputable suppliers, and ask about the source, screening process, and any available testing.

Do Metal Raised Beds Need a Liner?

A properly manufactured metal raised bed usually does not require a bottom liner when installed over soil.

A liner may be considered when:

• Soil escapes through gaps
• The bed sits on concrete or decking
• A root barrier is required
• Burrowing animals are present
• The manufacturer specifically recommends one

Do not add solid plastic merely because the bed is metal. The growing area still needs drainage.

If you are concerned about the sides becoming hot in strong sunlight, adequate watering, soil volume, mulch, and plant coverage are usually more relevant than installing an impermeable liner.

Frequently Asked Questions

Should I put cardboard at the bottom of a raised bed?

Cardboard can temporarily suppress ordinary lawn and annual weeds. Use plain cardboard, remove tape and labels, overlap the pieces, and wet them before adding soil. It will decompose and will not permanently stop aggressive weeds or burrowing animals.

Should I put rocks in the bottom?

No. A gravel or rock layer does not reliably improve drainage and reduces usable root space. Use a well-structured soil mix and make sure water can leave the bed.

Do I need to remove grass first?

Removing sod provides the most thorough preparation, but it is not always necessary. Short grass can be covered with overlapping cardboard beneath a sufficiently deep bed. Persistent perennial grasses should be removed more carefully.

How do I keep gophers out?

Attach galvanized hardware cloth securely across the open bottom before filling the bed. Select a mesh size suitable for the animals in your area.

Can I fill the bottom with logs?

Yes, particularly in a deep raised bed. Use untreated, disease-free wood and cover it with enough soil for the intended crops. Expect decomposition and settling over time.

Can I use leaves in the bottom?

Leaves can be used as part of an organic filler layer. Avoid placing an extremely thick, compressed layer of whole leaves beneath a shallow amount of soil, as it may settle unevenly.

Is landscape fabric necessary?

Usually not. It may be useful for a specific severe weed or root problem, but it can restrict root access and complicate future maintenance.

Can I put straw in the bottom?

Clean straw can be combined with other organic materials in a deep bed. Make sure it has not been treated with persistent herbicides and provide a substantial growing layer above it.

What should go under an elevated planter?

Use a permeable liner only if needed to hold soil inside. Do not cover or block the planter’s drainage holes. Fill it with a lightweight, well-draining growing medium.

How much space should I leave at the top?

Leaving approximately 1–2 inches below the rim provides room for watering and mulch without soil washing over the sides.

Final Thoughts

For an open-bottom raised bed installed over healthy soil, the best bottom is often no added layer at all. Loosen the ground, remove serious weeds, and allow the raised-bed soil to connect naturally with the soil below.

Use cardboard when you need temporary grass suppression and hardware cloth when burrowing animals are the problem. In very deep beds, clean logs, branches, leaves, and other compostable materials can reduce soil costs, but they will decompose and settle.

Avoid solid plastic, carpet, contaminated wood, and unnecessary gravel. Every bottom material should solve a real problem without blocking drainage or reducing the healthy root zone.