You should put nothing in the bottom of a raised garden bed according to UMN Extension research. The best raised bed base layer sits right on top of loosened native soil with no barrier between them. This lets roots grow deep and water drain freely in both directions.
I know this advice goes against almost every gardening blog and YouTube video out there. When I built my first beds, I followed the popular tip of laying cardboard under raised bed frames to kill weeds. It seemed like a smart shortcut at the time and every guide I read backed it up. Two months later my plants looked stunted and the soil stayed soggy after every watering session. I pulled up the cardboard and found a slimy, airless layer that smelled awful.
UMN Extension explains why barriers cause these problems. Any material you place at the bottom of a bed blocks roots from reaching deeper soil layers. Plant roots in a raised bed can grow 12 inches (30.5 centimeters) or more below the frame if you let them. Cardboard, landscape fabric, and plastic tarps all create a wall that traps water above them. Roots that try to push through get suffocated. Remove any existing cardboard or tarps before you set your bed frame in place.
Many gardeners also ask about using a raised bed liner to prevent weeds from poking up through the soil. In most cases you don't need one. A bed filled with 6-8 inches (15.2-20.3 centimeters) of quality soil mix blocks light from reaching weed seeds below. The weeds that do sprout from the native soil underneath run out of energy before they reach the surface. Any stray weed that does break through is easy to pull from loose raised bed soil.
There is one exception to the no-barrier rule. OSU Extension warns that bermudagrass and other fast-spreading grasses can invade a raised bed in just 1 month. These grasses send runners deep underground. You need a root barrier buried at least 1 foot (30.5 centimeters) deep around the outside edges of your bed to stop them. A bottom layer won't help here since the grass creeps in from the sides, not from below.
Here is what you should do instead of adding bottom layers. Loosen the native soil below your bed frame with a garden fork to a depth of 4-6 inches (10.2-15.2 centimeters). This creates a smooth transition zone between your raised bed soil and the ground beneath it. Roots will grow right through this zone and tap into the deeper moisture and nutrients below your frame.
If moles, voles, or gophers are active in your area, lay a sheet of half-inch hardware cloth at the bottom of the bed. The mesh gaps are wide enough to let roots and water pass through. But they stop burrowing animals from eating your carrots and beets from below. Staple the hardware cloth to the inside of your frame so it stays flat and doesn't bunch up when you add soil.
One more tip that makes a big difference. Place your bed on level ground so water spreads evenly across the entire soil surface. A tilted bed sends all the water to one end and leaves the other side dry. Use a basic level across the frame before you fill it with soil.
Skip the cardboard, forget the landscape fabric, and save your money on fancy liners. Your plants will grow stronger and your drainage will work better when you let the bottom of your bed connect straight to the earth below it. The ground under your frame is not your enemy. It is the foundation that makes everything above it thrive.
Read the full article: Raised Beds: A Complete Growing Guide
