Nothing, as long as your bed sits on open ground. A frame under 12 inches (30 cm) deep should have no bottom at all, so roots can reach down into the dirt below. Bare soil is the best raised bed bottom layer you can give a shallow box. A few cases do call for a layer down there. Every one of them turns on what your frame sits on, and none of them turns on the soil you shovel in.
"The whole side yard is old concrete," my neighbor told me over the fence. "Can I set the boxes down and fill them?" Not without something in between. On concrete or asphalt, the University of Delaware Cooperative Extension calls for a barrier fabric between the bed soil and the ground. The same goes for soil that comes back dirty from a lab, whatever you plan to grow in the box on top of it. Geotextile barrier fabric is the material named there. We built her four boxes in late fall. She first planted them the following spring.
The layer under your bed does one of two opposite jobs. Leave the bottom open and roots run past the frame into the ground below, so a shallow box behaves like a much deeper one. Put fabric down and you cut that contact on purpose, because whatever is under there should not reach the roots of your food crops. You cannot have both, so work out which of the two you need before you buy a thing. Depth pushes the answer one way. The shallower your frame, the more of the root run has to happen in the ground under it, and fabric there costs you that room for good.
A raised bed over pavement is the clearest case for a barrier. Nothing roots into asphalt, and your soil will wash out through the seams of the frame unless fabric holds the soil back while water still passes. Ground that tests high for lead is the second case, and you only care because you plan to eat what grows there. A metal trough has the same problem in a different shell. Oregon State University Extension records troughs used as beds with drainage holes added, and that is the step most people skip.
Cardboard turns up in every online thread on this question. I could not find one extension source in this research that has looked at cardboard as a bottom for a raised bed, which is not evidence that it fails. It means nobody running trials has weighed in, so you should not count on cardboard doing the job people promise. Handle your weeds on top of the soil where you can watch the result, because a layer you buried in April tells you nothing in July.
Test your soil before you build a thing. Send a sample of your existing garden soil to a testing lab, and ask for a lead test if the bed will grow food. That result decides whether the ground itself rules out an open bottom, and skipping the test leaves you guessing about the only thing you cannot see.
One more cost comes with burying wood. If branches or chips go under your soil, budget 1/4 pound (0.11 kg) of actual nitrogen per 100 square feet (9.3 sq m) for every 1 inch (2.5 cm) of that woody material. Rotting wood ties up nitrogen while it breaks down, and your first crop pays the bill in pale leaves and slow growth. Skip the buried wood if you would rather not do that math.
Read the full article: Raised Garden Bed Specs Backed by Research
