What to Put at the Bottom of a Raised Garden Bed

By Azura · Updated July 2026 · Raised Garden Hub is reader-supported. When you buy through links on our site, we may earn an affiliate commission at no extra cost to you.

For a raised bed sitting on soil, put a layer of cardboard at the bottom to smother weeds — nothing else is required. It breaks down over time and lets roots and worms reach the ground below. Here’s exactly what to use for each situation, and what to avoid.

Why the bottom layer matters

The bottom of your raised bed does three jobs: it blocks weeds from growing up into your soil, it controls drainage, and — if you use it wisely — it saves you money on fill. Getting it right takes about ten minutes and costs almost nothing.

If your bed sits directly on ground, weeds and grass will grow up through plain soil from below. A physical barrier, even a temporary one, gives your plants a head start before it decomposes. On a hard surface like a patio or deck, the concern is the opposite: you need to keep soil inside while still letting excess water escape.

On open ground (the most common case)

For most gardeners, this is the setup: a wooden or metal bed frame sitting directly on lawn or bare soil.

• Cardboard — the best all-rounder. Lay 1–2 layers of plain brown cardboard (remove tape and glossy print) to smother grass and weeds. It rots away in a few months and lets roots grow down into the native soil below. Overlap the edges by at least 6 inches so weeds cannot sneak through gaps.
• Newspaper — works the same way as cardboard, just thinner. Use 10–12 sheets stacked together and wet them down before adding soil so they don’t blow around. Fully break down in a season.
• Hardware cloth first — if you have gophers, voles, or moles, staple ½-inch hardware cloth across the bottom of the frame before the cardboard. This is the only “barrier” most gardeners need long-term because it does not break down. It keeps burrowing animals out while still allowing drainage.
• Then fill — leave the bottom open to the soil for drainage and worm access, then add your soil mix.

Rule of thumb: Cardboard + hardware cloth (if you have pests) is everything you need on the bottom. Do not overthink it.

For deep beds — save money with bulk organic fill

If your bed is 18 inches or deeper, you do not need premium soil all the way down. Roots need the best mix near the top; the lower section can be cheaper organic material that improves over time.

Fill the bottom third (or even half for very deep beds) with logs, branches, leaves, straw, and woody debris — this is the hugelkultur method. As the wood breaks down it holds moisture, feeds soil microbes, and slowly enriches the mix above. It can cut your soil bill significantly for a tall bed.

Other good bulk fillers for the bottom layer:

• Untreated wood chips or bark
• Straw or old hay (not fresh grass clippings, which mat and smell)
• Shredded leaves
• Spent garden compost

Then top it with 10–12 inches of quality growing mix. See how to fill a raised garden bed for the full layering strategy — and work out exactly how much mix the top layer needs right here:

On a patio, deck, or concrete

When there is no soil below, drainage becomes your main concern — water must be able to escape, but you also do not want soil washing out onto your deck or patio.

• Use landscape fabric or a porous liner with drainage holes so soil stays inside but water escapes freely.
• Go deeper — 12–18 inches minimum, since all the root volume must fit inside the bed. Check how deep a raised bed should be for specific guidance by crop type.
• Elevate slightly if possible so water does not pool under the frame. Pot feet or lumber scraps work fine.

For a 4×8 raised bed on a patio you will likely need significantly more soil than you expect, since there is no native soil below to supplement.

What NOT to put at the bottom

Some common advice will actually hurt your garden. Avoid these:

• Plastic sheeting — traps water, causes root rot, blocks roots and worms from reaching the ground below. Even “perforated” plastic degrades and clogs over time.
• Rocks or gravel “for drainage” — this is a persistent myth. A layer of gravel beneath soil actually creates a perched water table, keeping the soil above it wetter than it would be without any gravel. Skip it entirely.
• Landscape fabric as a long-term weed barrier — it works for the first season or two, but it does not break down. Eventually weeds root into it, it tears, and it becomes a nuisance to remove. Cardboard is better because it disappears.
• Treated lumber scraps or glossy/colored cardboard — can leach chemicals into your soil. Use plain brown cardboard only.
• Fresh grass clippings in a thick layer — they mat, heat up, and can go anaerobic. Thin layers mixed with other material are fine, but a solid mat of clippings blocks drainage.

For more on keeping unwanted plants out of your bed from above, see how to keep weeds out of raised beds.

The simple rule

On soil: cardboard (plus hardware cloth if you have burrowing pests), then fill. On a hard surface: a draining liner and a deeper bed. For deep beds anywhere: bulk organic fill on the bottom third.

That covers every situation most gardeners encounter. When you are ready to choose the frame itself, see our best raised garden beds guide for top picks across wood, metal, and kit options.

Next: see how to fill a raised bed and use the soil calculator to estimate how much you need.