Start with a barrier, then build up drainage and soil

The bottom layer of your raised bed does three jobs: it stops weeds and grass from growing up into your soil, it lets water drain so roots don't rot, and it can break down over time to add nutrients. Most gardeners use a combination of materials rather than just one. The order matters—you want a weed barrier closest to the ground, then materials that drain well, then your growing soil on top.

What you choose depends on how long you want the bed to last, how wet your site gets, and whether you want the bottom materials to eventually become part of your soil. A bed that sits on bare ground needs different protection than one on a patio or deck.

Key Takeaways

  • Cardboard or landscape fabric goes directly on the ground to block weeds, and cardboard breaks down over one to two years while fabric lasts longer.
  • A layer of coarse material like wood chips, straw, or gravel on top of the barrier improves drainage and prevents soil from washing down into gaps.
  • Avoid treated wood chips and glossy cardboard, which can leach chemicals into your soil as they break down.
  • If your ground is already compacted or very wet, add a drainage layer of gravel or perforated pipe before the weed barrier.

Cardboard as a weed barrier

Cardboard is the cheapest and most common choice for blocking weeds. Lay it flat and overlapping—each piece should cover the one below it by at least six inches so weeds can't push through the seams. Wet the cardboard as you lay it so it stays in place and starts to conform to the ground. Avoid shiny or glossy cardboard (the kind used for some produce boxes), because the coating can contain chemicals. Plain brown boxes from appliances, groceries, or online orders work well.

Cardboard breaks down in one to two years depending on moisture and soil life. By then, it has usually done its job—the weeds underneath have died from lack of light, and the cardboard has turned into organic matter that feeds your soil. If you want a longer-lasting barrier, use landscape fabric instead, but know that fabric doesn't add anything to your soil as it breaks down.

Landscape fabric for durability

Landscape fabric (also called weed cloth) lasts five to ten years and blocks weeds just as well as cardboard. It's thinner and easier to cut around corners and posts. The downside is cost—a roll of good fabric runs more than a stack of free boxes—and it doesn't improve your soil when it eventually tears.

Use landscape fabric rated for gardens, not the thin black plastic sometimes sold as weed barrier. Plastic doesn't let water through and can trap moisture under your bed, which causes root rot. Fabric breathes. Overlap pieces by at least four inches and pin them down with landscape staples or rocks so wind doesn't lift the edges.

Drainage layer: wood chips, straw, or gravel

On top of your weed barrier, add a two- to four-inch layer of material that lets water move through but doesn't compact. Wood chips are popular because they're often free from tree services, they drain well, and they break down into organic matter that improves soil structure over time. Use chips from untreated wood only—avoid dyed mulch and chips from treated lumber, which can leach chemicals.

Straw (not hay, which contains seeds) also works and breaks down faster than wood chips, usually within one season. It's lighter and easier to move, but it can blow around and may mat down if your bed gets heavy rain. Gravel or pea gravel drains the fastest and lasts indefinitely, but it doesn't add organic matter and can be harder to dig through when you're planting. Some gardeners use a mix—gravel on the very bottom for drainage, then wood chips or straw on top for organic matter.

Special situations: wet ground and compacted soil

If your ground stays wet or you're building on clay that doesn't drain, add a drainage layer before the weed barrier. Lay down two to three inches of gravel or coarse sand, or use perforated drainage pipe laid along the bottom of the bed. This lets water move sideways and down instead of pooling under your roots. Then add your weed barrier and the rest of your layers on top.

If the ground is very compacted (like old lawn or a parking area), you can skip the weed barrier and go straight to wood chips or gravel. The compacted soil underneath won't support weed growth for the first season or two, and by then your raised bed soil will be deep enough that any weeds that do push through are straightforward to pull.

How deep to build before adding soil

After your weed barrier and drainage layer, you should have used up one to six inches of your bed's depth, depending on what you chose. For most vegetables and flowers, you need at least eight to ten inches of growing soil above that. If your bed is twelve inches deep, you might use four inches for cardboard plus wood chips, then fill the remaining eight inches with soil. If your bed is twenty-four inches deep, you can use more drainage material and still have plenty of room for roots.

Don't overthink this—the exact depth of each layer matters less than making sure water can drain and weeds can't push through. A straightforward setup of cardboard plus three inches of wood chips works for most gardens.

Materials to avoid

Don't use treated wood chips or dyed mulch in a vegetable bed. Treated wood contains copper compounds that can build up in soil over time, and dyes can leach into the ground. Avoid plastic sheeting as a weed barrier—it traps water and causes root rot. Don't use glossy cardboard or cardboard printed with heavy inks. Skip hay in favor of straw; hay contains seeds that will sprout in your bed.

If you're using wood chips from a tree service, ask whether the wood was treated or came from diseased trees. Free chips are great, but chips from trees treated with fungicide or from diseased wood can introduce problems into your garden.

Frequently Asked Questions

Can I use newspaper instead of cardboard?

Yes. Newspaper works the same way as cardboard and breaks down at about the same rate. Use multiple layers (at least ten sheets thick) and overlap them well. Avoid glossy inserts and colored inks if possible, though standard black-and-white newspaper is safe.

Do I need a weed barrier if I'm putting my raised bed on concrete?

No. Concrete already blocks weeds, so you can skip the cardboard or fabric and go straight to your drainage layer and soil. You may want to add landscape fabric anyway to keep soil from washing into cracks, but it's not required.

How often do I need to replace the bottom layers?

Cardboard lasts one to two years, then breaks down completely. Landscape fabric lasts five to ten years. Wood chips break down over three to five years depending on moisture and soil life. You don't have to replace them—as they break down, they become part of your soil. If you want to refresh the drainage layer, you can add more wood chips on top of the old ones.

What if I'm building a bed on top of an existing garden?

Use a weed barrier even if the ground underneath is already cultivated. Old roots, dormant weed seeds, and perennial weeds can still push up through your new soil. Cardboard or fabric stops them from reaching your new plants.

Can I use leaves or grass clippings at the bottom?

Leaves break down quickly and can mat down, blocking drainage. Grass clippings can mat even faster and may contain herbicide residue. They're better mixed into your soil or used as a top mulch after planting. Stick with wood chips or straw for the drainage layer.