What to Put at the Bottom of a Raised Garden Bed – 11 Materials to Use

Raised beds have a number of advantages over traditional garden beds. Not only do they help to improve the soil quality and drainage, they are much more accessible than beds at ground level, allowing those with limited mobility to garden with greater ease. 

Once your raised bed has been made and installed, it’s time to think about filling it. This post will take you through the different materials that can be put on the bottom of your raised bed. 

Do Raised Beds Need Lining? 

Whether you should line your raised bed or not depends on a few factors. 

If there’s annual weeds or sod underneath your bed, it’ll be better off with a thick lining. This will stop weeds growing up from below. 

Some raised bed liners can lead to poor drainage. When selecting a liner to use, you should ensure it’s water-permeable. Plastic liners that don’t allow water to soak through can lead to overly-wet soil, which will eventually cause root rot. 

You should also think about what your bed is made from. Pressure-treated wood beds require a lining to prevent chemicals from seeping into the soil. Galvanised steel beds should also be lined, as exposure to acidic soil can shorten their lifespan. 

Benefits of a Raised Bed Lining

Adding a lining or a layer of organic material to the bottom of your raised bed has a number of benefits. 

Weed Prevention

The primary reason most gardeners choose to line their raised beds is to prevent the growth of weeds.

A thick layer of material at the bottom of your beds will stifle weeds before they have a chance to grow. By the time the bottom of your raised bed has decomposed (which it will do if it’s made from biodegradable materials), the weeds will have been killed off. 

Better Drainage

Raised beds need more water than in-ground beds as there’s less soil. This means that in warm weather, your beds will dry out faster. 

Lining the bottom of the bed is the easiest way to help it retain moisture, saving you time and labour! 

Better Quality Soil

Using a biodegradable material at the bottom of your raised bed can improve your soil quality over time. 

This can also prevent your soil from becoming contaminated. Any toxins that have gotten into your garden soil won’t be able to get into your  raised bed if you add a thick lining to the bottom. 

Raised beds made from pressure treated wood should be lined at the sides. This is because some pressure-treated wood is toxic. These toxins can leach into the soil and harm your plants. If the wood was purchased before 2003, it’s very likely to contain toxins. 

If your beds are made from pressure treated wood, you should consider lining the edges with a heavy duty plastic liner, in order to prevent the wood making contact with the soil.

Protection Against Rodents

Rodents are a nightmare for gardeners, and those that burrow can access your precious veggies from underneath. A thick layer of material at the bottom of your bed will prevent pesky rodents from ruining your crops. 

Better Insulation

Raised beds are much more sensitive to temperature fluctuations than in-ground beds. This is because, being elevated above the ground, they are more exposed. The soil will get hotter and colder faster than it would if it was in the ground. 

Using a liner can help limit the changes in temperature. The liner will help the soil to retain heat from the sun, preventing a sudden drop in temperature if it frosts overnight. It’ll also stop the soil drying out so much on a hot day.

The Best Materials for the Bottom of Raised Beds

There’s a range of materials that can be used for the bottom of raised beds, with some more cost-effective than others.

1. Newspaper

Affordable and readily available, you can make the most of old newspapers by using them to line your beds! It’s usually possible to pick up old newspapers for free.

Spread a few layers of newspaper to the bottom of your bed, add soil and get planting. Newspapers will break down over time, releasing carbon which can improve your soil quality. Contrary to belief, newspaper ink is soy-based, so it won’t release toxins into the soil. 

Newspaper is one of the best and most popular lining materials, but if you’d rather a lining that doesn’t break down over time, this isn’t the one for you. 

2. Corrugated Cardboard

Corrugated cardboard is another good choice if you’re looking for a material that will eventually break down over time to improve your soil quality. 

Cardboard takes a lot longer to break down than newspaper as it’s thicker and more durable. You should find it around 6 months – longer if you add it in thick layers. 

Cardboard can also be picked up for free, but it’s important not to use your standard recycling. While cardboard boxes are fine, don’t use cereal boxes with glossy or colourful ink, as the ink can release toxins into the soil. 

3. Landscape Fabric

A slightly more expensive option is landscape fabric, although this will last a lot longer and will help your raised bed to drain better. 

Unlike cardboard and newspaper, landscape fabric doesn’t decompose, so your beds will be protected from the bottom for a long period of time. It’s a very durable material, and won’t need replacing for around 10 years. 

Additionally, it provides very good drainage, allowing excess water to simply pass through. 

However, some gardeners are opting to move away from landscape fabric as a lining. This is because it contains some plastic, which prevents it from decomposing. Those who don’t wish to use plastic in their garden should opt for a biodegradable material instead. 

4. Plastic Lining

There are many gardeners who see no issues with using plastic in their beds, and there are certainly a few benefits. 

If the soil in your raised beds is very dry, using plastic can help to retain moisture by slowing drainage. Alternatively, line the bed with plastic but add drainage holes. This will allow excess water to drain through. 

Plastic liners don’t allow good drainage, so are best used at the sides of raised beds, with a water permeable material at the bottom. 

5. Burlap

For those who want a long lasting material that will eventually break down, burlap is a good choice. This offers good drainage and won’t decompose for many years, but it’s more eco-friendly than landscape fabric which contains plastic. 

It’s also cheaper than landscape fabric. 

The only downside is that the edges fray fairly easily. Frayed edges will lead it to break down faster, and can also provide a gap for weeds or rodents to get through. 

6. Grass Clippings

Another material that’s absolutely free is grass clippings. Collect the debris after mowing the lawn, then add a layer of grass clippings to the bottom of your raised bed. The weight of the added soil will cause the clippings to compact and form a thick layer which can prevent weeds from growing.

Over time, the clippings will decompose to help form a nutrient rich soil.

7. Natural Carpet

If you’ve got an old carpet you don’t know what to do with, this can be added to the bottom of a raised bed. Carpets made from hemp or cotton will decompose over time, and they can also choke weeds. 

Cheaper carpets can contain plastic and can release toxins into the soil, so these shouldn’t be used as a lining, 

8. Straw

If you’re wanting to keep a lot of moisture in your raised beds, straw is one of the best materials. This will smother weeds, while also adding organic carbon-rich matter to your soil.

While straw works well on its own, it’s even more effective at stifling weed growth when combined with other materials like cardboard or newspaper. 

It’s worth bearing in mind that some bales of straw will contain weed seeds. Ensure you purchase good quality straw from a reliable seller if this is the material you wish to use. 

9. Leaves

If you’re on a budget, leaves are a great option as the lining for your beds. They decompose in around a year, and when they do, will add a layer of organic matter to your soil. 

Provided you’ve got a tree or two near your home, leaves are also free. Simply collect them up in the autumn, then fill the bottom of your raised beds before adding the soil. The more you add, the slower the decomposition. 

You could add other pieces of garden debris such as tree branches or small twigs.

10. Stones

Stones aren’t the most popular choice, but they do offer a couple of benefits. They won’t decompose, so provide a permanent barrier against weeds. 

They’re also very easy to source, and can usually be picked up for free.

The drawback is that they can reduce the drainage, meaning your soil may become waterlogged. It’s also hard to lay stones without at least some gaps, meaning weeds can still grow up into your beds. If you’re going to use stones, fill in the gaps with another material. 

11. Hardware Fabric

If it’s pests you’re worried about as opposed to weeds, hardware fabric is the best option. This won’t prevent weeds from growing, but it can prevent your produce from being eaten by rodents. 

This also adds an extra layer of protection when combined with another material, such as newspaper. While rodents can very easily chew through newspaper, they’ll have a much harder time getting through when it’s combined with a layer of hardware fabric. 

This fabric can usually be picked up at hardware stores.

FAQs

How deep should a raised garden bed be?

The optimum depth of your raised garden bed depends a lot on what you are going to be growing. If your plants or vegetables have deep roots, they’re going to need more space. 

For best results, your raised bed should be at least 12” deep. 

The plants that need the most space (between 18” and 24”) include broccoli, Brussels sprouts, cabbage, corn, cauliflower and tomatoes,

Beans, cucumbers, kale, peas, peppers, squash and short vine tomatoes require a depth of between 15” and 18”.

Carrots, celery, cabbage, garlic, chard, leeks, lettuce, oregano, potatoes and strawberries require a 10″ to 15″ deep bed.

If you’ve got less space, plant basil, chives, coriander, onions, mint, spinach, thyme and oregano, as these require a shallower depth of between 6” and 10”. 

Do raised beds need foundations? 

Raised beds can usually be built on any surface, with no need for foundations. They can be installed directly on soil, grass, paving stones or concrete, as long as the surface is level. 

If you’ve got grass underneath that you’d like to remove, you could cover the area with a layer of cardboard, then cover the cardboard with a layer of soil. Do this in the autumn, and the grass will break down over the winter ready for planting in the spring. 

Do you need to line the sides of raised beds?

Many people also choose to line the sides of their raised beds, but this isn’t always necessary.

If your raised bed is made from galvanised metal, lining the sides is a good idea. Acidic soil can break down the zinc coating, shortening the lifespan of your bed. A thick lining will prevent the growing medium from making direct contact with the galvanised metal.