If you’re planning on using raised container beds in your garden, you’d be right to wonder what to put on the bottom of it because the entire structure depends on good drainage. It’s all well and good using well-draining soil but all that moisture needs to go somewhere.
The versatility of raised garden beds means they open up gardening to everyone, regardless of whether you have a garden or not. With the right set up, raised garden beds can be used almost anywhere.
A few example locations are balconies, roof terraces and on solid concrete areas, provided there’s sufficient drainage and the beds aren’t running parallel with the house without a damp proof membrane between the garden bed and the structure.
Water needs to flow, but regardless of whether you have a garden or not, raised garden beds can be a terrific solution for any outdoor space, provided care is taken in the initial setup.
What Should You Put Under a Raised Garden Bed?
What you use to put under raised garden beds will be determined by the type of groundwork you’re working with.
Building a Raised Garden Bed on Grass
Grass can’t simply be turned top-side down and then used a base layer for a raised garden bed. There’s going to be grass and weeds in the soil so it needs to be prepared so you can use it for planting.
To prepare your grass for planting, there’s a couple of ways you can go. You can:
1 – Sod the Lawn
This method is the most labor intensive, if you’re doing it manually with a spade and edger. There are powered sod cutters available for hire, but those are really only needed for larger areas that would otherwise be back-breaking.
It’s worth bearing in mind that the amount of work involved depends on the existing state of your lawn. If it’s over-grown with weeds and what-not, that will need to be seen to first.
Before you start digging a lawn up, it’s easier when it’s mowed as far down as you can get it. A trimmer can take it right down to soil level, but the lowest setting on any mower will be sufficient.
With the weeds under control and out of the way, it’s still not the ideal time to get digging. To make it easier on your back, lightly soak the soil and let it dry overnight and dig it the next day. It’s easier to turn your grass over when it’s moist, but not soaking wet because water adds to the weight and sod is heavy enough.
When you’re ready to get digging, go to a depth of about 6-inches to make sure you get to the roots of grass and stubborn weeds. Cut the grass/sod into manageable sections for lifting. If you’re doing this yourself, that’ll be small squares or rectangles rather than rolling it into strips to lift it up. The sod you remove can be used for compost.
For the base of a raised garden bed on a removed piece of lawn, you can cover the area with a 3-inch thick layer of newspaper. Today’s papers use non-toxic ink so it doesn’t matter if it’s colored pages or black only.
To prevent the papers from blowing away while you’re layering them on the ground, lightly spray them with water, then add some organic mulching material like grass clippings, leaves or other organic matter over the newspaper.
Both newspaper and cardboard are biodegradable. The purpose they serve is to block out light from any grass or weed seeds still in the soil, preventing photosynthesis so anything in the soil beneath the newspaper or cardboard dies. Add your new soil above it, and you’ll be good to get planting.
2 – Smother the Lawn to Kill the Grass
A much easier way to prepare a lawn for a raised garden bed is to use what’s called soil solarization. The purpose is to use the heat of the sun, and trap that under the tarp so it reaches high enough temperatures to kill everything.
For it to work effectively, the soil, or grass area needs to be watered to a depth of at least six inches, and the plastic tarp needs to be secured in place to avoid heat from escaping. The water trapped in the soil turns to steam and evaporates.
While there are a variety of landscaping fabrics suited to solarization, the Texas A & M Agrilife Extension found that 1- 4 ml thick poly plastic sheeting such as that used by decorators to be more effective than black or white plastic as those don’t allow sufficient heat through. Other types of fabrics used for solarization can take longer.
An advantage of soil solarization using clear plastic is the soil gets nutrients from the process so you can build your bed right on top of it, fill it with some aggregates like stones or gravel to help with drainage, add some top soil and start planting.
Building a Raised Garden Bed on Solid Ground
If you’re putting a raised garden bed on any solid surface such as concrete, block paving, or a patio, it needs to be able to drain moisture, therefore it needs to be elevated off of the ground.
If you’re building your own raised garden bed using timber, there’s two things you need to do. The first is to raise it off the ground to allow moisture to drain and the second is to have an inner base because otherwise, the soil would stain the surface, and the water runoff would cause damp problems.
That would eventually lead to root rot in the plants you’re growing if the water wasn’t able to escape. So, water needs to be able to drain and the surface that the raised bed is on needs to be protected.
A permeable geotextile membrane also needs to be added to allow for drainage while keeping the soil in place. If you’re using other landscaping fabrics that aren’t designed specifically for drainage, you’ll likely need to pierce some small holes in the bottom to allow moisture to escape.
The only other thing you need to add is a few inches of crushed rocks or other aggregates to the base, then you can add your topsoil.
The only thing to remember with raised garden beds is that they need to be able to drain moisture without causing water damage to surfaces. In an existing garden, the soil may be enough. If the soil is clay-like rather than sandy soil, crushed rocks and aggregates can help to improve drainage.
When the raised garden bed is on solid ground that can’t drain, it needs to be elevated more, and a geotextile membrane needs to be added along with aggregates to allow moisture to drain. This will prevent waterlogging in your raised garden bed, which would lead to root rot in any plants and weaken the garden bed’s structure.