Are you dreaming of homegrown veggies, herbs, and flowers – but the only space you have for a garden bed is on concrete, a patio, or other hard surface? Or perhaps you’re like us and have a good-sized yard, but want to maximize growing space by adding raised garden beds on your hardscape areas as well. Maybe your patio is simply the sunniest spot you have. No matter the reason, I fully support your effort to grow – wherever you can!
Read along to learn tips and best practices for building a raised garden bed on concrete or other hard impervious surfaces. In this example, I’ll show you how we prepped a new wood raised garden bed to go on top of our asphalt driveway. We’ll discuss important concerns such as drainage, soil retention, and soil quality.
Finally, we’ll also cover alternate options for putting planter boxes on hard surfaces – such as self-contained pots and beds.If you’re not up for building your own, check out this awesome selection of elevated raised garden bed kits from Gardener’s Supply.
Quick Summary of Tips
- Choose a sunny, level location for the raised bed.
- Keep in mind that installing a raised garden bed on top of a deck may cause staining or water damage, unless drainage is controlled. Discoloration on concrete should be easy to remove with a pressure washer if needed.
- Provide adequate drainage, bed depth, and high-quality soil for the plants to grow best.
- I don’t recommend placing the soil itself right on the concrete. Yet I don’t necessarily recommend adding a solid bottom on the bed either. Instead, we create a wire and fabric “basket” or bottom on our raised bed to both contain the soil but also promote drainage.
- If our example doesn’t suit your situation, read the “alternative options” for more ideas to add growing space to your patio, deck, or other hardscape area!
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Getting Started: Evaluate Your Space
Before you go slapping together a raised bed and plopping it down on your patio, take a moment to think about your space. As with any garden space, an ideal location should receive full sun (or as much as possible). It should also be fairly level. Even more, building a raised garden bed on top of concrete or other hard impervious surfaces brings about a whole new set of considerations. What is the hard surface like? Can it handle the moisture and weight of a heavy raised garden bed?
For example, if you’re hoping to add raised garden beds on top of a nice wood deck, the style of bed I’m going to show you in this article may not be the best choice – which sits right on the ground and drains from the bottom. Instead, you’d likely want to protect your deck by using an elevated raised bed kit, or one that has a solid bottom and contained drainage system. In that case, See the “Alternative Options” section at the end of this article.
On the other hand, the steps we used to modify our newest driveway garden bed will work well on top of concrete, asphalt, pavers, or similar surfaces. If and when you ever decide to move the garden bed, it may have some discoloration below. Yet a pressure washer should be able to remove it!
Drainage & Water
All garden beds need good drainage, including raised garden beds installed on hard surfaces! Soggy soil and drowning roots lead to unhappy, unhealthy plants. Therefore, it is not a good idea to add a secure solid bottom on your bed. That is, unless there are plenty of drainage holes, an internal drainage system, or it otherwise has the ability to freely drain excess moisture from the soil. On the other hand, I don’t necessarily suggest putting soil directly on top of concrete. (You’ll see what I mean in our driveway bed example and “containing the soil” section below.)
There is a common misconception that putting a raised garden bed on concrete or other hard surfaces will prevent it from draining well. The opposite is actually true, as long as the bed is open on the bottom! A well-built raised garden bed on concrete will actually drain faster than one sitting snugly down within the soil of your yard. Therefore, you may even find the need to water a garden bed on concrete more often than others, like most container gardening. Especially since the surrounding concrete may slightly raise the soil temperature, increasing evaporation and drying as well.
But where does all that water drain to?
Well, excess runoff from your raised bed will run to wherever rain water usually collects on your hard surface. For example, if you have a drain system, a slight slope to direct water flow off the edges, or a depressed area where water usually collects – that is where the excess raised bed water will go too. However, that doesn’t necessarily mean a ton of water is going to be flowing out of your beds all the time!
With a combination of tailored watering practices (not too much!), quality soil (one with both adequate drainage but also good moisture retention), and the right bed height (taller is better!), your garden bed should be able to retain most of the water you provide it. When you water your new raised garden bed, provide enough water to keep the soil consistently damp. On the other hand, avoid watering so much that water comes pouring out from the bottom. You’ll find your groove.
Recommended Bed Depth
Most common vegetable plants need a minimum of 12 inches of soil to grow big and healthy. The more space for roots, soil, and nutrients – the better! In fact, many plants prefer 18 to 24 inches or deeper, including tomatoes, carrots, peppers, eggplant, and even kale. With a traditional in-ground garden or raised beds open to the soil below, roots can grow deep and uninhibited. In contrast, putting a raised garden bed on concrete is essentially like creating a large pot or container. The roots (and general plant health) are restricted to what space you provide it.
All that said, I suggest a minimum depth of 12 inches (preferably 18”) for any raised bed that will be put on a hard surface. In addition to increasing root space, providing ample depth and soil will reduce water runoff and keep the soil moist longer. The bed we built in the example below is 18 inches, or three 2×6” boards tall. Stacking four 2×6” boards to make a 22 to 24-inch deep bed is also great, but will take a lot more soil to fill.
When it comes to filling a garden bed on concrete, invest in high-quality soil and compost to help compensate for the shallower root space. We filled the new driveway bed with the same soil and compost mixture we fill all of our raised beds with. You can learn about building the ideal organic soil in detail in this article. In a nutshell, we use a combination of high-quality bagged soils, bagged compost, and homemade compost. When filling many large beds at once, we also have bulk soil and compost delivered.
The ideal raised bed soil is rich in nutrients and organic matter. It should allow for adequate drainage, but also have good moisture retention properties. Do not use “potting soil” only. It is quite fluffy and will drain and dry out too fast. I suggest using a combination of some potting soil, some general planting or raised bed mix, and plenty of compost. Compost is key! Compost and worm castings will help greatly with moisture retention. You don’t want all the water and nutrients to run right out of the bottom of the bed! Mulching the soil surface also increases moisture retention and reduces your need to water as frequently.
Curious about compost? Learn composting basics (and 6 different ways to compost at home) in this article. Or check out our detailed vermicomposting 101 article to learn all about composting with worms!
Containing the Soil
It isn’t the best idea to add soil straight into a garden bed directly on top of concrete. Why? For one, some of the soil will seep and wash out from the bottom of the bed. That will make a huge mess around your bed surface. It will also slowly reduce the amount of soil in the bed entirely. Also, concrete is alkaline while most garden soil is neutral to mildly acidic. I have heard it may slightly increase the pH of your soil over time when in direct contact with concrete.
Consequently, the best way we have found to contain the soil within a raised garden bed on top of concrete is to create a sturdy “basket” for the soil within the wood frame of the bed. That is, in lieu of adding a solid bottom to the bed. We’ll also talk more about solid-bottom raised beds in the Alternative Options section near the end of this article.
Now, let me show you what I mean…
Building a Wood Raised Garden Bed to Install on Concrete
Step 1: Build or Obtain a Raised Garden Bed Frame
First, you need to build (or otherwise obtain) a wood raised garden bed frame for your space. If you need any pointers here, please check out our detailed tutorial. There is even a step-by-step video included! For our new driveway raised bed, we simply used our standard bed design – but modified the bottom and installation. The dimensions in this example are 6 feet long, 20 inches wide and 18 inches tall. We needed to keep it quite narrow so I wouldn’t hit it with my car going in and out of the driveway!
Note that the following steps will work best as-described below with a wood bed frame that is about 2 or 3 feet wide. For beds wider than 3 feet, I suggest you consider adding a couple wood cross-beams along the bottom of the bed. They will help support the weight of the soil on top of the fencing and landscape fabric we’re going to add next. Sorry, I don’t have a photo but I did my best to draw what I mean! See below. Use a durable wood such as cedar or redwood 2×4” or 2×6” boards. Install them within the frame (flush or just up from the ground level) NOT attached to the outer bottom of the bed frame. That would inhibit your bed from sitting flat on the ground. Make sense?
Step 2: Line the Bottom with Hardware Cloth
Next, we are going to line the inside bottom of the raised bed frame with hardware cloth wire fencing material. The hardware cloth serves as a sturdy and durable bottom for the bed, which landscape fabric will lay on top of next. Without hardware cloth, landscape fabric alone would easily rip under the weight of the soil. It would also be easy to accidentally tear the fabric open on rough ground, such as when you’re setting the bed in place or if you ever move it.
For this step, I highly suggest using hardware cloth over something like chicken wire. Galvanized hardware cloth won’t rust, is extremely strong and long-lasting. In contrast, chicken wire sags, rusts, and breaks down over time. The gauge and hole size of the hardware cloth doesn’t matter all that much. We used this 2-foot wide 1/2” opening hardware cloth, though wider rolls are available too.
Truth be told, we put hardware cloth under all of our garden beds, including those in the soil. Gophers are a real problem here, and it stops them from getting in the beds and eating everything! It is also the material we use for predator-proofing our chicken coop and run.
Create a wire basket bottom
Using aviation snips or metal snips, cut the hardware cloth in a manner that gives you a few extra inches of slack hanging over every side of the bed. With the bed sitting right-side-up, place the hardware cloth on top of the bed. Bend up each side to follow the shape of the bed, and then press it down into the bed. Try to keep an even amount of slack on all sides. Then carefully form the hardware cloth wire bottom to fit the inside of the bed. The corners get a bit awkward. Just do your best to meld and bend the wire to be as flush as possible with the wood. Wear good poke-proof gloves! I’ve also found it helpful to stand inside the bed and step on the wire to press it into place.
Why don’t we just attach it to the bottom side of the bed frame, you ask? I mean, you certainly could. Yet in my experience, wire attached to the bottom edge pops loose more easily from the weight of the soil pushing down on it compared to wire attached on the inside walls of the bed. You know, physics. Also, a totally flat wire bottom doesn’t create the same type of “basket” we’re aiming for, and may not drain quite as well.
Attach the hardware cloth
Now, go around the perimeter of the hardware cloth and secure it to the lower inside walls of the bed every 6 inches or so. We used wide-head cabinet screws, positioned in a way that caught and pinched the hardware cloth in place. You could also use galvanized U-nails or poultry staples. Long staples may work too, but could also pop loose more easily.
Step 3: Add Durable Landscape Fabric
Once the bottom wire basket is in place, add a layer (or two) of commercial-duty landscape fabric on top of it. This is what will keep the soil inside your bed, but also allow for excellent drainage! I emphasize choosing commercial-duty or high-quality landscape fabric because there is a lot of cheap crap out there. And I don’t just mean “cheap” as inexpensive; I mean poorly made. The common thin stretchy black plastic-like stuff will rip into shreds over time and make a huge gross mess. Two I suggest are this one or this option.
Add the landscape fabric to the inside bottom of the bed in a similar manner as the hardware cloth. We had a roll of 4-foot wide landscape fabric, so we folded it in half to create a dual layer in the just-under 2 foot wide bed. It is important to leave the fabric very loose rather than taut, to provide give as it rests against the hardware cloth for support. The role of the fabric is to prevent soil from leaking from the bed, but it should not be responsible for bearing the weight of the soil. That is the hardware cloth’s job. Because there shouldn’t be much weight pulling down on the fabric, we simply used a staple gun to secure the cloth in place around the inner lower perimeter of the bed.
Step 4: Place & Fill Your Bed!
Ta-da! Your new raised bed is all prepped and ready to be installed in its new hardscape home. Set the bed in place, as level as possible. Make sure you like the location before filling it with soil! Now, add soil to the bed, taking care to keep the fabric flush against the inside of the bed as you go. Speaking of taking care… remember to be gentle and dig only lightly in this bed, now and in the future.
We are huge fans of no-till gardening here anyways, so we don’t intend on digging way down into the soil any time soon anyways. But if you do need to do any digging, use caution to not accidentally rip open your fabric bottom. It isn’t the end of the world if it gets a little tear. To learn more about ongoing soil care for your raised bed, check out this article about how we amend and fertilize our raised beds before planting (or between seasons) using a no-till method.
If this style and example doesn’t work in your situation for whatever reason, there are still a TON of other options to garden on a patio, deck, balcony, driveway, or other hardscapes!
Raised Bed Kits
For instance, there are several quality elevated raised bed kits out there. Some have drainage holes that you could add drip pans underneath, effectively protecting the surface below. Others have a fully solid bottom and an internal drainage collection system, and you can direct runoff with a hose or valve. Check out the awesome selection of elevated raised bed kits from Gardener’s Supply here. Vegepods and Earthbox additional examples of a fully-contained systems, and would be great on a deck or balcony.
Other DIY options
Kits aside, there are also many other DIY raised bed variations from what I shared here today. In fact, we have built our standard raised bed design and added a wood bottom on a few occasions! See the photos of our tree box below. The bottom is essentially solid, made of the same redwood 2×6” boards used for the rest of the box. We drilled several large half-inch drainage holes in the bottom, before lining the interior with landscape fabric and adding soil. We chose this style because we wanted to elevate the small beds on heavy-duty furniture dollies with wheels, making them mobile. Otherwise, we personally avoid adding wood bottoms since they do inevitably inhibit some drainage and are prone to rotting over time.
You could use the same design and idea as our solid-bottom rolling boxes, but prop the bed up on bricks instead. Then, a drip tray can slide under and in line with the drainage holes. However, keep in mind that some water will still leak from the seams in the boards. Yet it will be more concentrated from the large drainage holes.
I am leery to suggest adding legs to a raised bed, unless you design it very carefully. For example, using short 4×4” posts supported with metal braces. Do NOT underestimate the extreme weight of lumber and soil, especially once it is wet! The dollies we use for our rolling boxes are rated up to 1500 pounds.Plan to make beds that you wish to elevate on legs or dollies smaller than ones you’d put on the ground (either in depth, or width and length).
Containers & Pots
Finally, there is the whole wide world of container gardening out there. There is nothing wrong with growing things in pots. We heavily rely on fabric pots or grow bags to supplement our raised bed gardening space. Truthfully, we prefer the mobility and added benefits (such as “air pruning”) of grow bags over raised beds for many of our crops, including to grow potatoes. Heck, they even make raised beds out of grow bag materials! We also use half wine barrels as planting containers, with drainage holes added to the bottom. Essentially, any container with drainage can be used to grow food!
And that is how you build a raised garden bed to put on concrete.
Or perhaps I should say… how we like to do it! Clearly, there are many different ways you can garden on top of hard surfaces. With a little creativity, food can be grown in more places than most people imagine! I hope this article gave you plenty of new ideas for your space. Please let me know if you have any questions. Happy planning, building, and growing.
Don’t miss these awesome related articles:
- How to Fill a Raised Garden Bed: Build the Perfect Organic Soil
- Companion Planting 101, with Companion Planting Chart
- Seed Starting 101: How to Sow Seeds Indoors