There are a lot of great articles out there regarding raised beds, but one point, which is often not talked about is the bottom Layer. There are a lot of important things you have to consider when preparing a raised bed. Choosing the Right Material for the base should also be one of them.
What should you put at the bottom of a raised bed? There are a couple of possibilities here, each having its own advantages and disadvantages. The go-to bottom layer is cardboard or newspaper. Cloth, Weed Fabric or wire Mashs are also good candidates. Plastic foil, on the other hand, should be handled with caution, as it can lead to some serious drainage issues when not done properly.
But let’s first dive into the question: why do we need a bottom Layer at all?
The importance of the bottom Layer
The Bottom Layer plays a couple of important roles. It doesn’t have to be regarded as a necessity, building a raised bed on normal soil without a bottom Layer will not be the end of the world. However, I do find it to be advantageous to include such a layer to suit the conditions of my garden.
First and foremost, if you build a raised bed on a surface which should not be damaged (for example a terrace) you should definitely and without any question, include a protective layer at the bottom of your raised bed. Later in this article, we’ll take a look at different materials helpful in this situation and other factors to consider.
Most raised beds are built on normal soil and therefore protecting the surface below the raised bed is not an issue. However, building a good foundation within your raised bed can help you to minimize the growth of weeds. Compostable Materials used as a bottom Layer will also function as fertilizer (just be careful not to use any toxic materials, which might leech into the soil). Additionally, you can also protect your raised bed from animals that might want to pay your veggies a visit by digging through the ground.
One thing which should always be considered when building a new gardening bed is drainage. Depending on the material used to create the bottom layer you can help your raised bed to drain better. This is especially important if your normal topsoil is very clay-based. Raised beds are a great way to work around atrocious soil, but water drainage might still be affected.
These are just some advantages of including a bottom layer. Now let’s take a quick look at some arguments on the other side of the coin.
Disadvantages of a bottom Layer
I am going to keep this relatively short, as I find there are not a lot of counterarguments to be had here.
Cost: One disadvantage of a bottom Layer is the initial cost or investment you have to make. Granted this cost is normally manageable but if you didn’t want to invest anything in your garden and you do not have access to any materials, ignoring the bottom layer can be cheaper. Normally I’d advise to just use the cheapest materials possible in this situation, (Newspaper, Cardboard) which in most cases will be free anyway.
Drainage: This is a big one. When using the wrong materials as a foundation you can inhibit the water von draining. This issue is not directly caused by there being a bottom layer, but just by the materials being there. This can easily be avoided and should not be considered a disadvantage. Leaching of toxic chemicals falls into the same category.
Temporary Effect: Cardboard and Newspaper will break down very quickly at the bottom of your raised bed. After one or two years pretty much nothing will be left. Cheap materials won’t last forever, but they still provide a benefit for the time they do.
Blocking Roots: Here’s another very important thing to consider: What do you plan on growing in your raised bed? Good Soil will encourage your plants to grow thick roots but once they reach your original surface soil growth will be slowed down (as this soil is much harder). This is mostly an issue about choosing the wrong depth for a raised bed – nonetheless, if space is already an issue, reducing it further, is probably not the best option.
Materials for the Bottom Layer
Cardboard/Newspaper/Cloth: Out of all the Materials you can use as a bottom layer Cardboard is by far my favorite. It’s very cheap easy to install and helps to block weeds, as well as breaks down over time. It’s easy to use and can last up to 3 years depending on the thickness of the cardboard. Newspapers provide similar advantages but have to be used in bigger quantities. To get Cardboard just ask in your local supermarket for used transporting bins, they mostly give them out for free anyway. The layer should be about 1,5cm to 3cm (0,6-1,2in). Another plus: Cardboard does not hinder drainage. On the other hand, it doesn’t block any pests from digging into your raised bed. This also means worms are going to love eating (composting) this layer. Cloth works in a very similar fashion.
Stones/Gravel: Adding Stones to the bottom layer is mostly advantageous if you have issues with drainage. The Airpocktes within the raised bed will help the water to drain quickly and prevent water from collecting at the bottom of your bed. There are not a lot of additional benefits to using stones, except maybe some cost-cutting (you won’t need as much soil to fill the whole bed).
Wire Mesh: Another situational bottom layer is a Wire Mesh (with relatively small holes). This is only useful when facing a garden invested by a large number of (digging-)pests like moles. While not preventing all animals from taking a bite out of your veggies it might protect a carrot or another from being snacked on. The Mesh will not prevent water from draining, but besides keeping some pests away has no other benefits.
Wood: Wood is an interesting one. The idea is similar to using Cardboard: introducing something to block weeds from growing, which will break down over time. Composting of wood is a gradual process and will take a lot of time. By combining smaller and larger pieces your raised bed will get a constant supply of nutrients for a long time. This type of raised bed is usually called Hügelkultur. These beds are typically a lot higher, will take more time to set up and will last for a long time. I would advise against using small pieces of wood in a normal raised bed, as cardboard has the same benefits.
Plastic: I normally don’t like Plastic in a raised bed. Plastic can be used to protect the wood of the framing of the bed. Otherwise, a firm plastic layer should not be used. It’s important to remember Plastic will not allow water to drain, so this is a good way to turn a raised bed into a raised bog. If you really have to protect the surface below your raised bed I would tend to use Landscape Fabric.
Landscape Fabric: This Material can be used to protect any surface below your raised bed. It still allows water to drain and will stop the soil from washing away. This is the only time I would advise using landscape fabric. So far I really dislike using this material when building a raised bad on normal soil. Especially over time, this fabric can become a pain, so it’s better to stay away from it. It pretty much offers no advantages over cardboard in the short run, is pretty expensive and will grow into a carpet of roots within a couple of years (removing this stuff is quite an exercise).
How to get rid of Weeds in a raised bed? Adding a protective layer at the bottom of your raised bed can help to prevent weeds from growing. Additionally covering your topsoil with newspaper before adding Mulch can further help to reduce weeds.
Other tricks to improve raised beds? Secure your raised bed! Over time the soil will put a lot of pressure on the wooden frame, which can cause it to shift. To prevent that from happening, check if your bed is draining properly (doesn’t hurt to do that sometimes). Also, add some supporting stakes to your frame. This does not require a lot of work but might save you a headache later on!
Can you install a raised bed on concrete? Yes, you certainly can. Keep in mind to protect the surface of the concrete by using some landscape fabric. Make sure you only plant crops that will not develop deep roots (Roots can penetrate Landscape Fabric). When installing the fabric remember to stretch it all the way to the top of your wooden frame. Gaps in the fabric will allow Soil to be washed away.