So, you have just built your raised garden bed and now you are ready to fill it and start growing. But what should you put on the bottom? Your raised bed can be part of your garden for years to come, so it is important to start off on the right foot.
The ideal bottom layer should suppress weeds, which help with drainage, improve your soil, keep rodents from burrowing in, and protect your soil from potential contaminants.
Some great materials to put at the bottom of your raised garden bed are cardboard, newspaper, straw, woody material, leaves, grass clippings, rocks, burlap, wool, and hardware cloth.
Each material has its unique benefits for your raised bed and they can be combined to get your garden off to a great start.
Let’s look at the pros and cons of each material so you can decide what will work best to line the bottom of your raised garden beds with.
Should I Line The Bottom Of My Raised Bed?
Of course, you can simply put your raised bed right on the ground to fill it and start growing, and while this is the cheapest and fastest way to get going it might not be the the best option. Wether or not you put something on the bottom of your raised bed depends on your situation, and you should ask yourself questions like:
Benefits Of Lining Your Raised Garden Bed
Building a raised garden bed is a long-term investment that takes a lot of work, so you want to make sure the project is a success. Lining the bottom of the raised beds has several advantages that might be worth the extra time and effort.
Here are some of the benefits of lining your raised bed:
Should You Line My Raised Garden With Landscape Plastic?
There are several reasons that landscape fabric should NOT be used under raised beds.
1: Landscape fabric is not biodegradable
Landscape fabric is thought to be a great option because does not decompose. However, the reason it does not decompose is that it is made of plastic. There is a lot of concern about using plastic in the garden, especially to grow vegetables. If in doubt, it is better to err on the side of caution.
2: It is impermeable to beneficial insects
Earthworms and other beneficial soil-dwelling creatures do not easily pass through landscape fabric. Not only can they become trapped underneath, but they will not be able to travel upwards and your raised bed will not reap the benefits of their assistance.
3: It does not work when buried
While landscape fabric can be very beneficial as a ground cover, it does not when it is buried underground. Any soil on top of the fabric will sprout weeds, and you will end up with a bunch of weeds growing on top of the fabric.
Also, once weeds start growing through the fabric, they are almost impossible to pull out and you will have to remove all the fabric and start over.
If you want to use landscape fabric with your raised beds, consider covering the top of the soil to prevent weeds rather than the bottom.
10 Great Materials To Put Under A Raised Garden Bed
Before your start filling your raised bed with soil, take special consideration about what you put at the bottom. Here are 10 great materials to use to line the bottom of your raised garden bed:
Helpful Tip: If you are using cardboard, newspaper, straw, or other organic matter, have it extended outside the bin by at least 6 inches. This will keep weeds from growing under the edge of the bed and into your soil.
Cardboard is the ultimate mulching material for anywhere in the garden, including under a raised bed. It chokes out weeds, keeps moisture in the soil, encourages earthworms, and adds organic matter as it decomposes. Carboard will take about 8 to 10 months to decompose, at which time most of the weeds underneath will be dead.
Cardboard also successfully chokes out tough weeds such as quack grass when a thick enough layer is applied and it is topped with a layer of organic matter such as straw.
Cardboard is free and easy to come by. Try asking your local grocery store, and they will often gladly give you more than you can use.
To use cardboard under your raised bed, remove and staples and tape from the cardboard. Lay down at least two layers of cardboard over the bottom of your raised bed (don’t forget to extend it outside the box), and make sure the edges are overlapped by a few inches so the weeds can’t slip in between.
No matter what other material you put on the bottom of your raised bed, it can always be paired with a bottom layer of cardboard.
Newspaper has similar benefits as carboard and makes a great bottom layer for your raised bed. It will smother weeds, is excellent at holding moisture, earthworms love it, and it decomposes into nice compost.
While it will break down a little faster than cardboard, it will still last for most of the season.
One caution of newspaper is some of the ink can contain undesirable chemicals.
Thankfully, most newspaper and printing services are switching to soy-based ink which is safe even for the vegetable garden. Check with your local printing or recycling facility to be sure.
To use newspaper on the bottom of your raised bed, lay down at least 10 sheets with the edge overlapping.
As with cardboard, newspaper can be combined with any other material to form a good bottom to your raised bed.
Straw is a great way to keep moisture in your raised beds while adding organic matter at the same time. While straw will smother weeds on its own, it is better when applied on top of cardboard or newspaper.
Straw adds carbon-rich matter to your raised bed, and the humus that forms as the straw decomposes under the soil will do wonders in the long run.
For best results, add 10cm to 15 cm (4-6 inches) of straw on the bottom of your raised bed.
Be aware that the straw will shrink as it decomposes, so you might have to add a bit more soil to the top of your bed next year.
There are a few considerations when purchasing straw for use in your garden. First, make sure you know your source as a lot of straw comes infested with weeds seeds.
We have noticed in the last few years that anywhere we have used straw from certain farms will sprout thousands of Canadian thistles in subsequent years.
Second, try and source organic straw as this will be free from the harmful fertilizers and pesticides that are used on conventional fields (and no, organic straw does NOT have more weed seeds than conventional farming).
4: Wood, Woodchips, And Other Woody Material
If you really want to choke out the sod below your raised bed, consider lining it with wood planks or old boards.
This creates a more solid weed barrier that will still decompose over time and feed the soil. Avoid using pressure-treated lumber, or glued material such as plywood or OSB as these can leach chemicals into the soil.
Decomposing lumber makes an excellent home for beneficial soil-dwelling bacteria.
You can also put a layer of wood chips on top of cardboard or newspaper. A layer of wood chips that is a few inches thick is excellent at choking out weeds and will retain lots of moisture.
However, too many woodchips can limit nitrogen in the soil and make the soil more acidic, so monitor your soil if you decide to use wood chips.
Adding a layer of wood material, such as branches, twigs, and small logs will also benefit your raised beds. While this will not suppress weeds, the decomposing wood will benefit the soil similar to hügelkultur practices.
Leaf mould (or decomposing leaves) will really benefit your soil by creating lots of beneficial humus at the bottom of your raised bed. The thick mat of leaves works great at smother weeds that might try to sneak through.
Add 5 to 10 cm (2-4 inches) of leaves to the bottom of your bed (preferably on top of cardboard or newspaper).
You can use leaves from most trees, but avoid using leaves from black walnut and eucalyptus trees as these will inhibit plant growth.
The leaf mat will shrink as it decomposes so you might need to add soil in the following years.
6: Grass Clippings
Grass clippings will form a thick mat at the bottom of your raised bed that will decompose into wonderful humus while choking weeds at the same time.
Apply a layer that is about 5 to 10 cm (2-4 inches) of grass clippings to the bottom of your raised bed.
Make sure that the grass did not go to seed before cutting or you will be fighting grass in your raised bed for years.
Also, a lot of grass that is mechanically clipped can have an oily-gassy smell from the mower, and you might prefer to avoid adding potential toxins to your garden.
Rocks can be beneficial to your raised bed in certain circumstances but they should be used with caution. When used properly, rocks can improve drainage but they can also cause the ground to become saturated.
If you have very heavy clay soil beneath your raised bed, a layer of rocks in the bottom of the bed can help. The water can settle in the rocks until it filters through the clay keeping the soil in the bed from becoming waterlogged.
However, too many rocks, or if the layer of rocks is too dense, can actually trap water on top of the rocks (similar to a river bed) and the soil will not drain and become saturated.
Carpet can be used at the bottom of your raised bed, but be careful which kind of carpet you use. Most carpets are made from plastic and will never decompose, potentially leach chemicals, inhibit drainage, and interfere with the roots of your plants.
However, natural carpets made from organic materials (such as hemp, jute, or cotton) can be an excellent bottom layer. These carpets will successfully choke weeds and still be permeable for water and deep roots to pass through. They will generally take a few years to decompose.
You can staple the carpet to the sides of the raised bed to form a solid base, or stick the carpet out past the edges of the bed to keep weeds from slipping in the sides.
There is not much information about using raw sheep’s wool as a bottom layer of your raised bed, but some gardeners have been using wool in their raised beds for years.
There are many benefits to using sheep wool as a mulch and a layer that is 15cm (6 inches) thick will successfully smother weeds.
It is also natural, contributes to healthy soil, and retains moisture while still allowing good drainage. Wool works great on top of cardboard to keep the weeds down.
10: Hardware cloth
If burrowing critters are a plague in your garden, then hardware cloth is the product for you. Hardware cloth is a strong wire mesh used in construction.
It will corrode and break over time, but it will give you at least 10 years of protection from hungry critters digging under your raised beds.
Lay hardware cloth over the bottom of your raised bed and staple it to the sides.
Hardware cloth comes in a variety of different sizes and thicknesses, so check your local hardware store for availability.
Building raised garden beds is not an easy task, so it is important to get it right the first time. I hope this article has given you some ideas on how to line the bottom of your raised bed so you will have a successful and plentiful harvest for years to come.