How to Fill Raised Garden Beds Properly (2024)

You built some great raised garden beds and now you need to fill them but soil is heavy to move and expensive. You look around for a better alternative and find a hundred suggestions online. Which is the best option?

I started using raised beds back in 1974 and have tried a lot of variations over the years. In this post I’ll combine the science with my experience and tell you what works and what doesn’t, and give you the best option for filling raised beds.

Height Matters

If your raised bed is less than 12″ (30 cm) high, use only soil to fill it. Plants need all that space for their roots and adding anything else in a short bed will lead to poor growth.

If the bed is more than 12″ tall you can consider some of the other options listed below to save money, but the best option is not to put other junk in the bed. Fill it with only soil.

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Think in Terms of Layers

The fill for raised beds can be divided into three layers; a bottom layer, a middle layer and a top layer. To better understand the filling process I have divided this post into three sections.

What to Put in the Bottom of a Raised Bed?

You can place material under the raised bed or place it inside the bed as the very first layer. This is usually a very thin layer that provides a function other than growing plants.

Weed Barrier or Landscape Fabric

Weed barrier, which is also called landscape fabric, is commonly placed at the bottom to keep out weeds but it doesn’t work.

If the bed is placed over lawn grass, 6″ (15 cm) of soil is enough to kill the grass and most weeds in the lawn, so weed barrier is not needed. Some very noxious weeds such a bindweed and Canada thistle are not stopped by the soil, but they are not stopped by weed barrier either. Most weeds in a raised bed come from the air above the bed, not from below.

Weed barrier can also inhibit the flow of water, making the lower levels of soil too moist. Don’t use landscape fabric anywhere in the garden.

Cardboard or Newspaper

People have heard about lasagna garden, also called sheet mulching, and are convinced that cardboard is good for soil and earthworms – that is a myth. Some people add it to kill lawn grass and weeds but as explained above, 6″ of soil will do that. You don’t need paper.

There is a concern that cardboard is not safe for soil, but that is a myth.

If you are trying to get rid of some cardboard or newspaper, feel free to put it in the bottom of the bed. It will slowly rot and disappear. Understand that it adds no value to a raised bed.

Wire Mesh (Hardware Cloth)

Wire mesh or hardware cloth can be added to the bottom of raised beds to keep mice, rats and chipmunks from burrowing from under the bed. This will work if you use a mesh that is small enough (1/4″, 0.6 cm). The problem is that all of these guys are good climbers and they can easily climb up the side of most raised beds and get at the food that way.

Adding wire mesh might be a good idea if you have groundhogs because they don’t climb as much.

If you do add wire mesh to the bottom it needs to be attached tightly so there are no gaps between the bed and the wire.

Stones for Drainage

Adding a layer of stones in the bottom for drainage seems to make a lot of sense, but it does not work. Raised beds are just big containers and gravel in pots has been shown not to work. It creates a perched water table at the bottom of the container and reduces drainage.

Never add stones for drainage.

The Middle Layer

Taller beds require a lot of soil to fill them. You can calculate how much they need with our handy Soil Calculator. This soil can be very expensive and therefore people start looking for things to put in the raised bed to reduce the amount of soil they need to buy.

Adding a middle layer of inexpensive material will save you some money but it also reduces the amount of soil available for plant roots. Rather than adding fill material, it is much better to make shorter beds.

Many of the materials suggested as filler are organic. The problem with these is that they eventually decompose and shrink in size. Then you have to buy soil to top off the bed. It is better to just buy the soil you need and get the job done. But if you want filler material, consider the following.

Logs and Branches

Woody material like logs and branches take up significant space and decompose slowly. The bigger the logs, the slower they decompose. They do absorb moisture which may help keep plants watered.

This idea of adding wood stems from a gardening technique called hugelkultur, which is fully described in . There is almost no scientific testing of this gardening technique and even less evidence that it adds value to raised beds. Besides, most people don’t have a lot of logs laying around.

Decomposition of wood by microbes requires nitrogen and microbes get that nitrogen from the soil around the wood, thereby reducing the amount of nitrogen available for plants. You can compensate for this by adding more nitrogen fertilizer but it is real hard to determine how much to add.

Large logs decompose more slowly but even they will eventually rot and as they do, the soil level in the bed will drop. At some point you have to add more soil, so you might as well do it right from the start.

Dry Leaves and Straw

Dry fall leaves and/or straw can also be used. These materials are also high in carbon and will take nitrogen from the soil as they decompose. They decompose much faster than wood.

Adding such material does not make any sense unless you are just trying to get rid of some plant refuge to clean up the garden.

Kitchen Scraps

Kitchen scraps have a carbon to nitrogen ratio (i.e. a good brown to green ratio) that is ideal for microbes and such material will not rob nitrogen from soil. It is good material to add to a raised bed, but who has enough to make much of a difference to a tall bed? It won’t really change the amount of soil you need unless you have access to a lot material.

Rocks

This is different than the small stones for drainage discussed above. Rocks are larger stones that are distributed throughout the soil layer so they take up space, but form a single layer. Soil needs to be place between the rocks to allow water to drain properly. It is also a good idea to keep them away from the top 1 foot (30 cm) of soil so they don’t get in the way of digging.

This method works quite well and they don’t decompose like most of the other suggests. One issue with this method is that if you don’t already have them, they might cost just as much as soil. However, you can probably find them for free at construction sites.

Top (Main) Layer of the Raised Bed

This layer provides the soil material for growing plants. If a raised bed is less than 2 feet (60 cm) tall, it should be filled entirely with this material – skip the middle layer. You want enough soil to provide a good root system.

If the raised bed is taller than 2 feet (60 cm), you can fill the extra space with other material, as long as you end up with 2 feet of soil.

There are many options for this layer. Some are soil based and some are completely soilless. You can also mix up an unlimited number of combinations. In this section I will describe the ingredients and in the following section I’ll give you my recommendations.

To understand these options better have a look at, Topsoil, Compost, Triple Mix – What’s the Difference?

Real Garden Soil

This could be taken from your garden or purchased. In most cases you will have to purchase it and it will cost a lot less if you by it in bulk rather than in bags. If you don’t have enough of your own soil consider the following.

Top Soil – this is soil that is removed from new building sites and consists of the top few inches of soil. This is a great option, but can be either high in clay or in sand, depending on your local soil type. In extreme cases, triple mix might be better.

Triple Mix – consists of 1/3 top soil, 1/3 compost and 1/3 peat moss. It is a great mix for growing plants, it’s more nutritious than top soil and is a better option when top soil has too much clay or sand in it. The problem with this soil is that since it is 2/3 organic matter, which decomposes over time, the level of soil in the bed will drop each year for 3-5 years, requiring you to top it off every few years.

Myth: You Can’t Grow Food in Real Garden Soil

I see this myth quite a bit and I think it is one reason so many people are building raised beds. They think their garden soil can’t be used. Consider this; before your house was built, the land was either a field or wood lot that grew all kinds of plants or it was a productive farm. You can certainly grow food in your soil. It is what I have used my whole life.

Vermiculite and Perlite

These are common soil additives found in potting soil. They hold moisture and improve drainage. They are fully described in Perlite vs Vermiculite – Which Soil Additive is Better?

The problem with these materials are that they are man-made, not organic and won’t decompose. They add no nutritional value to the garden and are expensive. Once you add them to soil, they can’t be removed. I would not add them to outdoor gardens.

Compost and Manure

Compost and manure are great additives for any outdoor soil. They are organic and slowly decompose to release nutrients for plants. They also add carbon to the soil which results in better soil structure.

The fact that these materials decompose can be seen as a negative because it causes the soil level to drop.

These sound like a great option to add to raised beds and they are. However, some people go to extremes and add huge amounts. Too much of a good thing results in raised beds that have a high phosphate level and become toxic to plants. Add no more than 20% in the first year and in following years add it as a 2″ (5 cm) layer of mulch on top of the soil.

Soilless Mix

Some people go with a completely soilless mix which is mostly peat moss. You could also use coir and other material, but peat moss is common, has a price advantage and is just as sustainable as the other options.

A common soilless mix is Mel’s mix, named after Mel Bartholomew who promoted the idea of square foot gardening. His mix consists of 1/3 vermiculite, 1/3 peat moss and 1/3 compost.

Soilless mixes dry out quickly, can be difficult to wet once completely dry and shrink quickly as the organic matter decomposes. They are expensive to buy but grow good plants.

Best Soil Option for Raised Beds

Now that we have had a look at the options, let’s put this all together and decide on the best one that meets these criteria.

• Be reasonably priced
• Hold moisture well
• Limited drop in soil level
• Grows good plants with high yields

The best mix for raised beds is 75% top soil + 25% compost. Use only this mixture to fill the complete raised bed. Don’t add a middle layer or a bottom layer.

The preferred top soil is clay based. If your local top soil is mostly sandy, either add some clay soil, or increase the compost to 30%.

This mixture is less expensive than other options and it hold moisture well. It will settle a bit the first year and you can top it up a bit, but after that you can use if for many years without adding more soil. Add a 1-2″ (3 cm) compost mulch layer each year.

Mistake #1: Using Two Different Soil Layers

Some sites recommend two layers of soil. Garden soil in the bottom because it is less expensive and better soil above. Never do this. Water may not drain properly from the top layer into the second one, creating a perched water table and wet plant roots.

Mistake #2: Using Raised Beds that are Too High

Too many people are building tall raised beds with walls (wood, bricks etc). Raised beds can be made with or without a wall. Ones without a wall are just areas that have a higher soil level than the pathways. They are easy to build and maintain, and they grow just as much great food as tall beds with walls.

Raised beds with walls are more aesthetically pleasing – I get that. If you want this type of bed, build it no more than 8″ tall and fill them with the soil described above. Except for cases where you have a disability, there is no good reason to build tall beds.

You might also be interested in: Best Building Material for Raised Garden Beds

Mistake #3: Lining Raised Beds

Don’t line your raised beds with plastic or weed barrier. I see this recommendation all the time and it will shorten the life of wooden raised beds. It might seem as if the plastic keeps water away from the wood which in turn reduces wood rot, but the opposite happens. The plastic actually keeps the wood wetter and it rots faster. Don’t put anything between the wood and the soil.

Mistake #4: Building a Bath Tub

The raised bed is essentially a container without a bottom. Water needs to drain out of it to work and this only happen if it can easily move between the soil in the bed and into the ground under the bed. If the soils are very different, water won’t easily move between these two layers. To overcome this create a gradual transition between the two layers.

Loosen the soil below the bed and mix in some of the soil that will be used to fill the bed. Then put more bed soil in the spot and mix it around again. This will create a transition where the soil slowly changes from one type to another.

FAQ for Filling Raised Beds