What to Put at the Bottom of Raised Garden Beds for Optimal Growth

Raised garden beds provide many benefits for growing vegetables, flowers and other plants. The contained structure keeps soil loose and nutrient-rich while elevating plants to reduce pressure on your back. But before you start filling your new raised beds with fertile soil it’s important to consider what to place at the base below the main soil layer.

Using the proper materials on the bottom of raised beds will enhance drainage prevent weeds and even add organic matter to your growing medium. Read on to learn about various options for base layers that will get your raised bed off to a great start!

Why Add a Base Layer?

Filling up raised beds entirely with soil or compost can get quite expensive. A base layer allows you to use less of the main planting medium by creating fill at the bottom. But base layers aren’t just for saving money on soil. Other important benefits include:

• Drainage – Gravel, sand, or wood chips facilitate drainage underneath the soil layer. This prevents waterlogging.

• Weed Prevention – Landscape fabric, cardboard, or leaf litter blocks weeds from sprouting up from below into the clean soil.

• Added Nutrients: As leaves, sticks, straw, and other organic matter break down, they feed plants.

• Pest Deterrence: Burrowing animals are less likely to enter beds if hardware cloth is layered at the base.

• Leveling – A thin sand or gravel layer can help level an uneven garden area prior to setting up the raised bed.

Take advantage of these perks by choosing the right base material for your unique needs and raised bed style.

Recommended Base Layer Options

Here are some of the top options to use at the bottom of raised beds:

Weed Barrier Fabrics

Landscape fabric, burlap, or cardboard create an impenetrable barrier preventing weeds from underneath taking root in garden soil. These breathable layers don’t impede drainage. Weed barrier is ideal for any style raised bed.

Gravel or Pebbles

A 3-4 inch layer of small gravel rocks or pebbles improves drainage and deters burrowing pests. Opt for river rocks or pea gravel. Larger stones may poke through landscape fabric over time.

Sand

Washed play sand or horticultural sand maintains excellent drainage under the soil layer. Level and smooth sand before installing the raised bed frame.

Wood Chips

Use a thin layer of coarse wood chips, taking care to avoid wood treated with chemicals. The decomposing wood chips add organic material.

Leaf Litter or Straw

Dry leaves or straw provide organic matter and nutrition as they break down. Compress leaves and wet them down to prevent excessive settling.

Sticks and Logs

The “hugelkultur” technique uses sticks, logs, and woody debris for a nutrient-rich base layer as they slowly decay. Use aged wood, not fresh green wood.

Plastic Bottles or Bags

While not ideal, recycled plastic debris can act as filler at the base. Avoid glossy printed plastic. Remove labels, rinse thoroughly, and puncture drainage holes before using plastic bottles or bags.

Hardware Cloth

For root crops like carrots and radishes, a layer of 1⁄4 inch galvanized hardware cloth keeps out burrowing pests. Use corrosion-resistant metal.

How to Install a Base Layer in a Raised Bed

Adding a base layer is simple with the following steps:

1. Clear and level the raised bed site of all vegetation, rocks and debris.

2. Optionally, spray the site with white vinegar to kill weeds and grass before covering.

3. Cut landscape fabric or cardboard to fit the dimensions of the planned raised bed.

4. Cover the entire area under and around the planned raised bed with the weed barrier.

5. If using gravel, sand, wood chips or other loose materials, contain them in your selected shape using a temporary enclosure.

6. Spread your chosen loose base material 3-5 inches deep, keeping it contained.

7. Set raised bed frames in place over the base layer, pressing into the soil underneath to secure.

8. Use a shovel to fill frames with nutrient-rich potting mix to within 2 inches of the top of the frame.

9. Water thoroughly until water drains freely from the base. Allow it to dry before planting.

The bed is now ready for seeds and transplants! Having a proper base established gives plants the ideal growing environment for their roots.

Considerations for Choosing a Base Layer

As you select a base layer material, keep these considerations in mind:

• Intended Crops – Root crops may penetrate plastic or be obstructed by gravel. Leafy greens do fine over landscape fabric.

• Climate – Improve drainage with sand or gravel in wet climates. Wood chips retain moisture well in arid regions.

• Weighted Materials – Heavy gravel and sand stabilize frames against frost heaving. Light leaves require reinforcement.

• Decomposition Rate – Organic weed barriers like leaves or cardboard break down faster than permanent options.

• Pest Pressure – Use hardware cloth at the base to deter destructive chipmunks, voles and armadillos.

• Eco-Friendliness – Opt for recycled materials or locally sourced organic matter when possible. Avoid non-biodegradable plastics.

Choose materials suitable for your specific garden conditions and the types of plants you intend to grow. Blending two or more base layers can provide added benefits.

Alternatives to Full Base Layers

While using a full base layer is recommended, you can still gain some advantages with the following simplified options:

• Line just the bottom of beds with cardboard or a double layer of newspaper before adding soil.

• Place 4-6 inch diameter boards or timbers under the corners and along edges of a raised bed to deter burrowing pests.

• Incorporate a 2-4 inch bottom layer of compost, leaf mulch or wood chips directly into soil to add organic materials.

• Mix gravel or pea stones into the soil layer rather than using a solid bottom layer.

• Use gravel or bricks under the edges of raised beds only for leveling and stabilizing the structure.

Even these abbreviated approaches will improve drainage, nutrition and pest control to some degree. But a full base layer provides maximum benefits.

Common Questions About Raised Bed Base Layers

Can I just put compost in the bottom of raised beds?

Yes, several inches of compost or composted manure at the base provides organic matter. Just be aware it will decompose faster than wood chips and may compress over time. Top off beds with additional soil as needed.

Do I need weed barrier with a raised bed?

Not 100% necessary, but highly recommended. Weed seeds in the underlying soil can still germinate and take root in raised bed soil. An impermeable base layer prevents this.

Do gravel or rocks at the bottom help with drainage?

Yes, gravel provides a reservoir for excess water to drain into, preventing soggy soil. Use a coarse gravel or pea gravel size. Larger rocks take up needed root space.

Can I use grass clippings or leaves at the bottom?

Wet grass clippings will become compacted, smelly and anaerobic. Use only dry leaves, compressing them to decompose slowly. Cover leaves with landscape fabric to prevent sinking.

Is landscape fabric safe to use in vegetable gardens?

Yes, modern landscape fabrics are inert once installed. Avoid glossy, coated fabrics. Stick with natural fiber or black polypropylene materials.

How long does cardboard last as a weed barrier?

Expect cardboard to decompose within 4-6 months. It works fine under raised beds but avoid using it in exposed areas or pathways. Opt for longer-lasting commercial weed barrier.

Get the Most From Your Raised Beds

A well-designed base layer sets your raised garden up for success. Weed prevention, moisture control, and added nutrition help plants thrive in the ideal raised bed environment. Observe how your plants respond over the first growing season and make any needed adjustments to ensure your base layer is fulfilling its purpose. With healthy soil from bottom to top, your raised beds will produce bountiful flowers and vegetables year after year.