Start with a base layer of coarse material

The bottom of your raised bed should not be solid soil. Instead, lay down 2 to 4 inches of coarse material that lets water drain through but keeps soil from washing away. Broken terracotta, gravel, wood chips, or straw all work. This layer prevents the bed from becoming waterlogged and gives roots room to breathe.

If your raised bed sits directly on grass or bare ground, you can skip this step—the earth below will drain naturally. But if the bed sits on concrete, a patio, or compacted soil, the drainage layer is essential. Without it, water pools at the bottom and roots rot.

Key Takeaways

  • A raised bed needs a drainage layer at the bottom (2 to 4 inches of gravel or wood chips) unless it sits on unpaved ground.
  • The middle layer should be partially decomposed organic material like compost, aged manure, or shredded leaves—this feeds plants as it breaks down.
  • The top 6 to 8 inches should be finished compost or a soil mix, where seeds and roots actually grow.
  • You can fill a bed all at once with a purchased soil mix, or build it in layers using materials you have on hand.
  • Avoid fresh wood chips, peat moss, and uncomposted manure, which either lock up nitrogen or introduce pathogens.

Add a middle layer of partially broken-down organic matter

Above the drainage layer, add 4 to 6 inches of material that is partway to becoming soil. Aged compost, aged manure (at least 6 months old), shredded leaves, or a mix of these all work. This layer feeds the plants above it as microorganisms break it down over the season. It also holds moisture and improves soil structure.

Do not use fresh manure—it burns plant roots and can carry harmful bacteria. Do not use peat moss; it breaks down slowly and offers little nutrition. Shredded leaves from last fall are free and excellent. If you have access to aged horse or chicken manure from a stable or farm, that is ideal.

Top with finished compost or a purchased soil mix

The top 6 to 8 inches is where seeds germinate and roots grow, so this layer must be loose, dark, and rich. Use finished compost (fully broken down, crumbly, dark brown) or a bagged soil mix designed for raised beds. Finished compost is better if you have it, because it is alive with beneficial microbes. If you buy a mix, look for one labeled "raised bed soil" or "garden soil"—not potting soil, which is too light and drains too fast.

If you are filling a 12-inch-deep bed, you might use 4 inches of drainage material, 4 inches of aged compost, and 4 inches of finished compost. A 10-inch bed could be 2 inches drainage, 4 inches aged material, and 4 inches finished compost. The exact proportions matter less than having all three layers present.

Fill all at once with a single purchased mix

If you do not have materials on hand, you can buy a raised bed soil mix and fill the entire bed with it. This is faster and simpler than layering. The mix is usually a blend of compost, peat or coconut coir, and perlite or vermiculite. It drains well and holds moisture at the same time.

Calculate how much you need by multiplying the bed's length × width × depth in feet, then dividing by 27 to get cubic yards. A 4-by-8-foot bed that is 12 inches deep needs about 1.2 cubic yards. Bags are usually sold by cubic feet (typically 2 cubic feet per bag), so a 4-by-8-by-1-foot bed needs about 32 bags. Buying in bulk from a landscape supplier is cheaper than buying bags at a garden center.

Avoid materials that cause problems

Fresh wood chips tie up nitrogen as they decompose, starving plants. Use them only if they have aged for at least a year. Uncomposted leaves mat down and block water. Shred them first and let them sit for a few months, or add them to a compost pile before using them in the bed.

Peat moss is slow to break down and offers little food value. It also dries out completely when it gets dry, and then water runs right through it. Fresh manure (less than 6 months old) can burn roots and carry E. coli and salmonella. Treated wood in the bed frame itself is fine (modern treated wood is safe), but do not put treated wood chips inside the bed.

Adjust your mix based on what you are growing

Most vegetables and flowers do well in the standard three-layer setup. But some plants need tweaks. Blueberries and rhododendrons need acidic soil; add sulfur to the top layer or use a mix made for acid-loving plants. Succulents and cacti need extra drainage; add perlite or coarse sand to the top layer so water does not sit around the roots.

Root crops like carrots and parsnips need loose, deep soil with no rocks or clumps. If your finished compost is chunky, sift it or break it up before planting. Shallow-rooted plants like lettuce and spinach do fine with just 6 inches of finished compost on top of the drainage and aged material layers.

Refresh the bed each season

After the first year, the layers compress and settle. Before planting in spring, add 1 to 2 inches of finished compost to the top and mix it into the upper few inches with a garden fork. This replaces nutrients the plants used and keeps the soil loose. Every three to four years, the bed will need a more thorough refresh—remove the top 4 inches, replace it with fresh compost, and work in some aged manure or leaf mold.

If you are growing heavy feeders like tomatoes or squash, add extra compost or aged manure before planting. If you are growing nitrogen-fixing plants like beans or peas, you can use less compost because they add nitrogen to the soil as they grow.

Frequently Asked Questions

Can I use garden soil from my yard instead of buying a mix?

Garden soil from your yard works for the middle layer but not the top. Use it as part of the aged material layer (mixed with compost), but buy or make finished compost for the top 6 inches where seeds go. Yard soil often compacts, drains poorly, and may contain weed seeds or disease.

What if I do not have a drainage layer and the bed is already full?

If water pools at the bottom after rain, drill or punch holes in the bed frame near the base to let water escape. If the frame is solid wood, you may need to remove some soil, add a drainage layer, and refill. If the bed sits on grass, you can usually skip drainage altogether because water soaks into the ground below.

How much does it cost to fill a raised bed?

A 4-by-8-foot bed 12 inches deep costs roughly $40 to $80 in bagged soil mix, depending on your region and whether you buy at a garden center or landscape supplier. Buying in bulk from a supplier is cheaper. If you use materials from your yard or compost pile, the cost is zero.

Can I reuse soil from last year?

Yes. Remove the top 2 inches (which may have compacted or contain disease spores), then refresh the bed with 2 inches of fresh compost mixed into the top layer. The rest of the soil can stay. This saves money and is better for soil life than replacing everything.

Should I add fertilizer to the bed when I fill it?

No. Finished compost and aged manure contain all the nutrients most plants need for the first season. Adding extra fertilizer can burn roots or create excess nitrogen that makes leafy plants grow at the expense of fruit. Wait until mid-season to add fertilizer if plants show signs of hunger (pale leaves, slow growth).