By Azura · Updated June 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.
On open ground (the most common case)
- 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.
- Hardware cloth first — if you have gophers, voles, or moles, staple ½-inch hardware cloth across the bottom before the cardboard. This is the only “barrier” most gardeners need.
- Then fill — leave the bottom open to the soil for drainage and worm access, then add your soil mix.
For a deep bed — fill the bottom third cheaply
If your bed is 18 inches or deeper, you don’t need premium soil all the way down. Fill the bottom third with logs, branches, leaves, and woody debris — the hugelkultur method. It cuts your soil bill by about a third and holds moisture as it breaks down.
On a patio, deck, or concrete
There’s no soil below, so:
- Add a liner with drainage holes (landscape fabric) so soil doesn’t wash out but water can escape.
- Go deeper — 12–18 inches minimum, since all the root volume must fit inside the bed.
What NOT to put at the bottom
- ❌ Plastic sheeting — traps water, causes root rot, blocks roots and worms.
- ❌ Rocks or gravel “for drainage” — actually creates a soggy perched water table. Skip it.
- ❌ Treated-lumber scraps or glossy/colored cardboard — can leach chemicals.
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. That’s it.
Next: see how to fill a raised bed cheaply and how much soil you’ll need.
Frequently asked questions
What should I put at the bottom of a raised garden bed?
For a bed on soil, lay cardboard to smother weeds — it breaks down and lets roots reach the ground. Add hardware cloth first if you have gophers or voles. For a deep bed, fill the bottom third with logs and branches (hugelkultur) to save on soil. Leave the bottom open for drainage.
Should I line the bottom of a raised bed with plastic?
No. Plastic traps water, causes root rot, and blocks worms and roots from reaching the ground. Use cardboard or landscape fabric instead, or leave it open.
Do you need landscape fabric under a raised bed?
Usually not. Cardboard works better for smothering weeds because it breaks down. Use landscape fabric only if the bed sits on a hard surface like a patio and you want to keep soil from washing out.
Can I put rocks at the bottom of a raised bed?
No — the old advice to add rocks 'for drainage' actually creates a perched water table that keeps roots wetter. Skip the rocks and use a free-draining soil mix instead.