Reviewed by Specialty Soils Updated August 21, 2026
The answer depends on the problem you are solving. Do not add a bottom layer merely because every raised bed supposedly needs one.
Weed suppression
Cardboard may serve as a temporary biodegradable barrier over vegetation, but it is not permanent drainage infrastructure.
Drainage
Do not automatically add gravel. Drainage depends on the entire bed, underlying soil, grade, and where water can go.
Rodents
Physical mesh may be appropriate where burrowing animals are a real concern.
Existing soil
If the soil below is usable, root interaction can be beneficial. Avoid an unnecessary impermeable barrier beneath an open bed.
Plan the complete raised-bed project
Separate new fill, existing-bed improvement and quantity before choosing a product.
Sources & references
General horticultural guidance is supported by the following authoritative resources. Specialty Soils product statements and labeled company observations are first-party information.