Start with drainage, then build up

The bottom layer of a raised bed determines whether your plants get water or rot. Put drainage material first — anything that lets water pass through but doesn't compact. The most common choice is landscape fabric laid directly on soil, which stops weeds from growing up into your bed while letting water drain down. On top of that, add 2 to 4 inches of coarse material: gravel, wood chips, or broken terracotta pieces all work. This layer keeps soil from washing down and clogging the drainage layer below.

After drainage comes the bulk of your bed. Most gardeners use a mix of compost, topsoil, and aged bark or wood chips in roughly equal parts. This combination holds moisture without staying soggy, feeds plants as it breaks down, and stays loose enough for roots to push through. If you're filling a deep bed (12 inches or more), you can put cheaper filler — shredded leaves, straw, or partially decomposed wood — in the middle layers, then save the good compost for the top 4 to 6 inches where roots actually feed.

Key Takeaways

  • Landscape fabric or cardboard on bare soil stops weeds from growing up into your bed and is the first layer you should lay down.
  • A 2 to 4 inch layer of gravel or wood chips on top of fabric prevents soil from washing down and blocking drainage.
  • Fill the rest of the bed with a mix of compost, topsoil, and aged bark — roughly equal parts — so water drains but soil holds moisture.
  • Deep beds can use cheaper filler material in the middle layers and save finished compost for the top 6 inches where plant roots feed.
  • Avoid fresh wood chips, peat moss, and clay-heavy soil, which compact, hold too much water, or rob nitrogen as they break down.

Landscape fabric or cardboard: the weed barrier

Landscape fabric is the standard choice because it lasts several years and lets water through while blocking light. Lay it directly on the ground before you add anything else. Overlap the edges by at least 6 inches so weeds can't creep in at the seams. Cardboard works too and costs less, but it breaks down in one to three years depending on moisture and sun — you'll need to replace it or accept that weeds will eventually push through from below.

Do not use plastic sheeting or thick rubber. These stop water from draining into the ground below, which means water pools under your bed and creates a swamp instead of a garden. If your raised bed sits on concrete or asphalt, plastic is fine because there's nowhere for water to drain anyway — but on soil, it will cause root rot and fungal problems.

Gravel and wood chips: the drainage layer

Once fabric is down, spread 2 to 4 inches of coarse material. Gravel (pea gravel or larger) is inert and lasts forever, but it's heavy and expensive to fill a large bed. Wood chips are lighter and cheaper, and they break down slowly into organic matter that improves soil structure. Avoid fresh wood chips from tree trimming services — these pull nitrogen from the soil as they decompose and can heat up as they break down, cooking roots. Aged chips (sitting in a pile for at least six months) are safe.

Broken terracotta, slate, or other rubble works if you have it on hand. The goal is anything chunky enough that soil particles can't filter down and clog the space. This layer keeps your good soil from washing down into the gravel and keeps the gravel from mixing up into your planting mix.

Compost, topsoil, and aged bark: the main fill

The bulk of your bed should be a mix, not a single material. Compost alone is too rich and expensive for a large volume, and it compacts over time. Topsoil alone is often clay-heavy and doesn't drain well. Aged bark or wood chips alone don't hold enough moisture or nutrients. A three-way mix in roughly equal parts gives you the best balance: compost feeds plants and improves structure, topsoil holds moisture and provides minerals, and aged bark keeps everything loose and drains excess water.

If you're filling a bed deeper than 8 inches, you can save money by using cheaper material in the middle. Shredded leaves, straw, grass clippings (from untreated lawns), or partially rotted wood chips can fill the center layers. Just make sure the top 4 to 6 inches is your good mix, because that's where roots spend most of their time and where they pull nutrients.

What not to put at the bottom

Fresh wood chips will tie up nitrogen as they decompose, starving your plants for the first year or two. Peat moss holds water so well that it creates soggy conditions, especially in the bottom layers where drainage is already slow. Clay-heavy soil compacts and blocks water movement. Sawdust from treated wood can leach chemicals. Newspaper is fine as a weed barrier (use it instead of fabric if you prefer), but don't layer it thick — one or two sheets, overlapped, is enough.

Avoid putting rocks or stones throughout the bed. They don't break down, they make digging and replanting harder, and they can trap water in pockets. Save rocks for the drainage layer only, where they're meant to be.

Depth and settling: how much to fill

Most vegetables and perennials need 8 to 12 inches of good soil to root properly. Shallow-rooted plants like lettuce and herbs can work in 6 inches. Fill your bed to the top when you're done — soil will settle 10 to 20 percent in the first year as organic matter breaks down and water compacts it. Plan to add an inch or two of compost each spring to keep the level up and refresh nutrients.

If you're building a very deep bed (18 inches or more), the math changes. You can use cheaper filler for the bottom 8 to 10 inches, then layer your good mix on top. This saves money without sacrificing plant health, because roots rarely go deeper than 12 inches anyway.

Frequently Asked Questions

Can I put cardboard instead of landscape fabric?

Yes. Cardboard is cheaper and biodegradable, but it breaks down in one to three years depending on moisture. Landscape fabric lasts longer and is easier to work with if you need to add or remove soil later. Either one stops weeds from growing up from below.

Do I need to add a drainage layer if my bed is on concrete?

No. Concrete blocks water from draining into the ground anyway, so a drainage layer won't help. You can skip the gravel and go straight to your soil mix. Just make sure your bed has drainage holes or a slight slope so water doesn't pool on top of the concrete.

What if I don't have landscape fabric?

Cardboard works. Newspaper (several layers, overlapped) works. Even untreated wood scraps laid flat will slow weeds for a season or two. The goal is just to block light and give you time before weeds push through. If you skip a barrier entirely, weeds will grow up from the soil below within weeks.

Can I reuse the soil from my old raised bed?

Yes, but refresh it first. Old soil compacts and loses nutrients. Mix in 2 to 3 inches of fresh compost, stir it in, and let it sit for a week before planting. If the old soil had disease problems (like powdery mildew or root rot), replace the top 6 inches with fresh mix to reduce the risk of the problem returning.

How often do I need to replace the bottom layers?

Landscape fabric lasts three to five years before it starts breaking down. Wood chips in the drainage layer last five to ten years. The soil mix above breaks down and settles every year, so you'll add fresh compost annually anyway. You don't need to dig out and replace everything — just top-dress with an inch of compost each spring.