Raised bed gardens are a great way to deal with a large expanse of concrete without having to tear out the concrete pad. Not only do they add a welcome note of green, growing plants to what is otherwise a bare and uninviting space, but you can grow everything from annual vegetables to small trees in these versatile container gardens.
When installing raised garden beds on concrete, pay attention not only to the materials used to construct the sides, but also to what you use at the bottom of the beds. You will get years more out of a well-constructed raised garden bed that has good drainage and is filled with a good growing medium.
Table of Contents
- Can You Build a Raised Garden Bed on Concrete?
- The Importance of Good Drainage
- Explaining the Layers of a Raised Garden Bed on Concrete
- The Frame
- Interior Lining
- Filling Up the Raised Garden Bed
- Now Add Your Plants/Seeds!
- Other Types of Raised Garden Beds
- Final Thoughts
Can You Build a Raised Garden Bed on Concrete?
Yes, you can build and place raised garden bed frames directly on a concrete surface, but you could have some problems develop down the line if you don’t add in some features to facilitate good drainage and prevent soil from leaking out and making a mess on your concrete base.
The Importance of Good Drainage
Water always has to go somewhere, and even in a bed raised a couple of feet above the surface, you run the risk of the soil becoming waterlogged if there’s nowhere for it to drain out. And for most plants, waterlogged soil will lead to root rot, killing them dead. That’s why flowerpots need to have drainage holes in the bottom, and why you need a way to get excess water out of your raised beds. Otherwise, one good downpour could spell disaster for your raised garden beds on concrete.
Explaining the Layers of a Raised Garden Bed on Concrete
With the growing popularity of raised garden beds, you have many different alternatives for constructing the frame.
The cheapest way is by scavenging free pallets, which can be taken apart to use the wood for constructing the sides. If you can’t find pallets, lengths of rough-cut 1x4s from the lumberyard will also work. Do not buy treated lumber, as the chemicals may leach into the soil. You can treat the raised bed wood frame with a non-toxic wood treatment that will prevent rot and keep you safe from chemicals.
If you want to spend more money, look into rot-resistant cedar for a longer-lasting raised bed frame.
Cut the wood into lengths for whatever height you want, nail them to 2x4s at the top and bottom to create sturdy walls, and screw them together at the corners.
Of course, wood is not the only frame option but it is the most popular for good reason. You can build a raised garden bed frame with galvanized steel, plastic materials, cinder blocks, and DIY concrete.
Because the lumber will shrink over time, it’s a good idea to line the raised bed sides to prevent soil from leaking out. However, do not use an impermeable barrier such as a plastic tarp, because you need to ensure that water can drain away without taking the soil with it.
Interior Lining 1: Wire Basket
One way to line your raised bed is with small-gauge wire fabric. Cut it to fit and tack it to the sides and bottom of the frame, creating a wire basket to hold in the soil. Another advantage of using metal wire is that it will prevent small creatures such as mice or chipmucks from burrowing into your beds from below.
Interior Lining 2: Heavy-Duty Landscape Fabric
If rodents are less of a concern than leaking soil, simply staple heavy-duty landscape fabric around the frame and across the bottom. Use a quality product that is guaranteed for several years.
Filling Up the Raised Garden Bed
Filling a raised garden bed can seem to be a daunting and expensive undertaking, as the volume of soil required to top up even a small bed is substantial. The good news is that you don’t need to fill the whole box with top soil, or even soil at all! Layer your raised garden bed with a variety of materials that will over time break down and turn into a superior growing medium.
Fill 1: Gravel
When filling a raised garden bed on a concrete base, you should lay down a few inches of gravel right at the bottom. This will allow excess water to drain away. If your raised bed is directly on soil, use a layer of flattened cardboard boxes instead.
Fill 2: Cardboard, Branches, Stumps, and/or Untreated Lumber
Next, take care of some of your yard waste to add some bulk. Depending on the size of your raised bed, this could be anything from twigs and small branches to stumps. You can also add scraps of lumber as long as they haven’t been treated with chemicals.
This is also a good opportunity to get rid of your accumulated egg cartons, torn up cardboard, and other paper products.
While this layer of wood won’t break down in even a year, the slow process of decomposition will add valuable fertility and humus to your soil. This should be the single thickest layer in your raised bed, taking up between 40 to 50% of the total.
Fill 3: Plant Waste
Top the wood with a thin layer (10% of the total) of fresh plant waste. This will kick-start the decomposition of the wood layer, and add a quick dose of nitrogen for your growing plants, whose roots will reach down through the soil layers.
Fill 4: Compost
Getting closer to the top, use either your own well-rotted compost, or purchase bulk mushroom compost or bagged composted cow or sheep manure from a garden centre. Put down about as much as you did of plant waste.
Fill 5: Top Soil
Finally, fill the bed up to the top with good quality top soil.
Now Add Your Plants/Seeds!
One of the advantages of raised garden beds is that the soil never becomes compacted, since it is never walked on. That means that you can set plants closer to each other, as their roots will tend to grow deep down through the loose soil structure rather than shallowly spreading out.
Other Types of Raised Garden Beds
Rolling Raised Garden Beds
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One of the great things about using raised garden beds is their versatility. You can even put your beds on wheels to move them around! Of course, you will need more than wire netting or landscape fabric on the bottom, in that case. Add a wooden or metal bottom with plenty of drainage holes when building your box, and then line it with wire or fabric.
Raised Garden Bed Kits
Don’t feel like getting out your woodworking tools? Raised garden beds have become so popular in recent years that you can now buy galvanized metal or recycled plastic kits that are easy to assemble. You can find all sorts of options online or at big box stores.
However, many of those kits will have no bottom on them, as they are usually designed for use on the ground rather than concrete. You may have to modify them to work on a concrete base by adding one of the options discussed above.
See a few options on Amazon below:
- Reinforced PP materials – NACREEN Raised Garden Bed Planters (Photo above)
- Modern Galvanized Steel KING BIRD Raised Garden Bed (Rectangle)
- Land Guard Galvanized Raised Garden Bed Kit (Oval)
Just Use Pots!
You don’t have to build raised garden beds to grow a variety of plants on a concrete base. Instead, use a collection of pots, from small to large, to accommodate everything from herbs to flowers to any vegetables that you care to try. Even carrots can be grown in a deep flowerpot!
Plastic pots are inexpensive and lightweight and will last for several years with proper care. You can also use galvanized pails or pick up ceramic or terra cotta pots of varying sizes at yard sales or on sale. Try to buy them in the fall when retailers will be trying to get rid of their seasonal inventory.
Even where you don’t have a big backyard, you can still grow flowers and vegetables in raised beds on a concrete patio or balcony. In fact, because you can create the perfect growing medium in these containers, you’re almost guaranteed a long season of successful gardening!
Janice is a retired High School teacher who is spending her leisure years keeping busy with all sorts of projects. Aside from freelance writing, she’s an enthusiastic amateur chef, home wine maker, and tends a large raised-bed vegetable garden, while at the same time running a Bed & Breakfast.