Growing flowers and vegetables in raised garden beds have become very popular. They can provide easier access to plants – gardeners don’t have to stoop and bend. They’re also wonderful for gardeners who use wheelchairs.
Raised beds are great to use when the soil in your yard is poor, contaminated, or even nonexistent. You can place them on a deck, patio, concrete slab, or gravel.
Whether your raised bed is made of wood, brick, metal, stone, or reclaimed material, you may wonder if you should install a liner before adding soil. Sometimes a liner is necessary, but it’s not a lot of the time.
A raised bed is a container. It’s like a giant flower pot or a planter, but usually without a bottom. Depending on the materials used to build it and the location of the raised bed, a liner performs two services – it can keep things in, and it can keep things out.
Things you’ll want to keep in include soil, nutrients, and plant roots. Things to keep out are critters, toxins, and weeds.
Reasons to Use a Liner in a Raised Bed
A liner keeps in the soil, keeps in the nutrients, and keeps the roots in the bed. Roots can grow into the wood of the raised bed or the soil under it.
A liner can keep out any toxins that might be in the ground or the material used to build the bed.
A liner can help keep out burrowing pests like voles, moles, gophers, groundhogs, and prairie dogs.
A liner can suppress weeds that might grow up from the ground under the raised bed, but that’s only necessary if your bed is lower than 6 inches. In a deeper box, the weeds will be smothered by all the soil that’s been put on top of them.
If the soil underneath is contaminated, a liner can prevent plant roots from drawing the toxins up. If you have used treated lumber to build your boxes, or you have used reclaimed lumber and are not sure what’s in it, a liner can help keep out toxins from the lumber as well.
A liner can help moderate soil temperatures and can extend your growing season by a week or two.
When your raised bed is elevated or on a patio or deck, a liner can prevent soil from washing out from the bottom.
Without a liner to stop them, plant roots can grow into the wood used to build the box, shortening the wood’s life.
If your bed is tall and you don’t want to fill the entire thing with soil, a liner can be put in to make it shallower.
Types of Liners
If you decide to use a liner, several different materials can be used for different situations.
Hardware cloth or other metal mesh keeps out gophers, moles, voles, prairie dogs, and other animals that can burrow under and into the bed. Use stainless steel because it will not rust. Other meshes that can rust might have been treated with a preservative that you probably don’t want in your vegetables.
Attach the mesh to the bottom of your raised box during construction using a heavy-duty staple gun.
Hardware cloth can also be used to build a false bottom on a very tall raised bed. Then line it with landscape fabric to hold the soil in.
Newspaper and Cardboard
Newspaper and cardboard can be placed on the bottom of a bed. These materials decompose quickly and will only last one season. These materials help suppress weeds.
Landscape fabric (sometimes called a weed mat) and canvas or burlap are suitable for raised beds that are above ground on legs or on hard surfaces. When placed on the bottom, these materials hold in the soil while allowing water to drain. These fabrics can also be used on the sides of wooden raised beds.
Burlap will decompose faster than canvas, which decomposes faster than landscape fabric.
Plastic can prevent toxins in the wood from reaching your soil, but you shouldn’t build your boxes with toxic materials. Plastic can prevent water from draining freely and can inhibit oxygen. It’s can cause moisture to get trapped between the wood and the plastic, causing rot.
Plastic is excellent to use if you are gardening hydroponically or aquaponically. But plastic will keep water from draining and keep your soil too wet. An overly wet, poorly draining soil will drown the roots of your tomatoes and peppers, which will lead to death. Even if you punch a few holes in the bottom of the plastic, drainage can be impeded.
You can use plastic on the sides of your raised bed if you are concerned about toxins in the wood migrating into the soil and your harvest. A better option is to use raw linseed oil to seal the wood.
How to Install a Fabric or Plastic Liner
Remember that you don’t necessarily need a liner! But if you have any of the concerns listed above, then a liner might be right for you and your garden.
What you will need:
- The fabric or plastic (be sure it is food grade if you’re growing vegetables)
- Measuring tape
- Several big binder clamps, C-clamps, or spring clothespins to hold the fabric until you can secure it with staples
- Heavy duty staple gun and staples
- Wire cutters, if installing metal mesh
- Scissors or utility knife, if installing fabric or plastic
- Helpful to have: Another set of hands, Straight edge
Measure: Calculate the area of all the sides and the bottom (Area = length x width) and then add these numbers together. Add an extra 6 inches per side.
Cut the material to the size you need. It’s not necessary to line the bed with a single piece. Using smaller, overlapping pieces can make the job easier.
Fold the fabric in thirds or halves lengthwise depending on the size of the bed, and center it in the middle of the bottom of the bed. Unfold the liner and pull each long side up. Ensure the fabric is even and that you have enough to cover the sides. Make sure the fabric is flat and smooth on the bottom and snug to the inside bottom corners.
Place a clamp or two in the middle on each of the longer sides to hold the fabric in place, smoothing it as you go.
Pull up the short sides and clamp those.
Once you are satisfied with the position of the material, start stapling beginning in the centers of the long sides.
Move to the shorter sides, staple the centers, then smooth, pleat, fold the liner into the corners, and secure with staples.
Excess material can be trimmed off with scissors or a utility knife, about 2 inches from the top edge. Some gardeners like to fold the edges under for a neater look, but plants will soon cover any exposed liner.
After that, you can fill your container with rich garden soil and start planting!