Do Raised Garden Beds Need Drainage? A Complete Guide

Raised garden beds are a great way to grow vegetables, herbs, and flowers They elevate your plants above the ground, reducing bending for easier maintenance. Raised beds also improve drainage in clay or compacted soils. However, some questions come up when building raised beds – mainly, do they need additional drainage material underneath the soil?

The short answer is it depends. In this complete guide we will look at the pros and cons of adding drainage to raised garden beds and help you decide what’s best for your unique situation.

Why Add Drainage Materials to A Raised Bed?

There are two main reasons you may want to add drainage material like rocks or gravel to the bottom of your raised bed:

1. Improve drainage

Raised beds contain the soil and plants like a big pot or container. In heavy rains, they can become waterlogged if drainage is poor. Adding a layer of gravel, sand, pebbles or other coarse material helps excess moisture percolate out of the soil. This creates a healthier environment for plant roots.

2. Save money on fill materials

Bagged garden soil and compost can get expensive. Drainage materials like gravel cost a fraction of the price. Adding rocks, pebbles, etc to the bottom third or half of the bed saves you money vs filling the entire bed with soil.

Do All Raised Beds Need Drainage Materials?

The short answer is no. Many raised beds will perform just fine without additional drainage material.

Here are some instances where drainage materials may NOT be required:

• Raised beds that are knee-high or less: The roots of most vegetables only grow 12 to 18 inches deep. As long as you have at least this much soil, any extra water will drain out of the bottom of shallower raised beds without the need for gravel, etc.

• Raised beds with open bottoms: Beds that are built right on the ground can soak up extra water from the ground below. The roots of the plants can grow through the bottom, which lets the soil drain naturally.

• Beds located on fast-draining sites: Raised beds located on sandy or gravely sites tend to have sufficient drainage without adding more rock. Likewise, beds positioned on a slope or hill will drain faster.

• You amended the native soil: Sometimes poor drainage is due to compacted native soil under the bed. If you broke up and amended this soil prior to installing your raised bed, you may find sufficient drainage without gravel.

So when is additional drainage material more critical? Keep reading…

Situations Where Drainage Materials Are Recommended

Here are some instances where it’s wise to add drainage rock, gravel or other coarse materials to your raised garden bed:

• Beds with solid bottoms: Manufactured raised beds with solid plastic or wood bottoms should include drainage holes. Add gravel and/or landscape fabric to prevent the holes from clogging.

• Taller beds: Especially those over 2 feet tall. There’s greater risk of waterlogging without drainage materials with all that soil.

• Beds on very dense, compacted ground: Amending the native ground under the bed may still not suffice if the soil is heavily compacted. Gravel will improve drainage.

• Beds on cement, asphalt or other hardscape: Without drainage materials, water can puddle under and around the raised bed causing leaks, mud and other issues.

• Beds on clay soils: Clay is prone to compaction. Incorporating sand, pebbles or other coarse materials creates air space for better drainage.

• Beds used to grow plants that prefer “fast drainage”: For example – basil, parsley, sage, onions, carrots, peppers, and other plants prone to root rot.

How Much Drainage Material Do You Need?

A good rule of thumb is to make your drainage layer about 30-50% of the total depth of the raised bed.

Here’s a quick guide on how much drainage material to use:

• For a 6-12 inch deep raised bed, no drainage layer is required (fill entirely with soil).

• For a 12-18 inch deep raised bed, use 3-6 inches of drainage material.

• For an 18-24 inch deep raised bed, use 6-10 inches of drainage material.

• For a 24-30 inch deep raised bed, use 10-12 inches of drainage material.

This formula leaves sufficient room for plant root growth while improving drainage. You can also save money on fill materials.

Best Drainage Materials for Raised Beds

What type of drainage material works best? Here are some top options:

• Gravel or pea stone: Smaller gravel pieces provide greater surface area for moisture to percolate through. Larger stones tend to shift over time. Gravel blends are best.

• River rock or pebbles: Provides good drainage. Smaller sizes are easier to work with. Be sure rocks are washed clean before using.

• Broken bricks or crushed concrete: Recycles waste materials. Different sizes provide good drainage. Rinse well to remove dust.

• Coarse sand: Use a coarse builder’s sand, not fine play sand. Allows moisture to seep through easily. May compact over time.

• Landscape fabric: Adds a layer of separation between soil and drainage materials. Keeps soil from filtering down.

Avoid using organic materials like wood chips which will decompose and collapse over time.

Installing Drainage Materials in a Raised Bed

Once you’ve calculated how much drainage material you need, here are some tips for installation:

• Dig down 6-12 inches below where the raised bed will sit and loosen the soil if possible.

• Cover the bottom of the bed frame with landscape fabric if desired. This keeps soil and fines from migrating into the rocks below over time.

• Add your drainage material first. Use shovelfuls, not dumps, for best results. Level it out as you go.

• Gently tamp down the drainage layer every few inches with a hand tamper as you build up depth. This ensures good water flow.

• Add soil next. Some gardeners add another layer of landscape fabric between the rock and soil. Others mix the soil and rock layers for better root penetration.

• Top off with 2-3 inches of mulch like wood chips to prevent erosion and retain moisture.

• Consider adding a thin layer of sand on top of the landscape fabric before adding soil. This helps prevent soil compaction from above.

Adding drainage materials to raised garden beds improves drainage and reduces cost but isn’t mandatory in all cases. Consider factors like bed height, construction, location and soils when deciding whether to add gravel, rock or sand layers.

Aim for a drainage layer of 30-50% of the total raised bed depth. Gravel, crushed concrete and coarse sand make good drainage materials. Install the drainage layer first before adding soil and mulch on top.

With the proper construction considerations, your raised beds will provide ideal growing conditions for vegetables and flowers! Just be sure to assess your unique situation first before filling them up.