Should You Use Concrete in Your Garden?

Should You Use Concrete in Your Garden?

By Michael Jenkins

There are many possible approaches to creating garden beds, landscaping edging, and garden containers—if you can think of it or imagine it, someone’s probably tried it. And while we at Gardzen encourage DIY thinking in gardening, it’s important to know how different materials will function in your garden. There’s a great deal of conversation about the use of concrete and concrete products in garden spaces—how does concrete affect your soil? What about concrete and drainage? Are concrete planters a good idea? As with everything, there are pros and cons to using concrete in your garden, so let’s dig in and learn a bit more about this material and what it can do in your garden space.

What is Concrete, Anyway?

Concrete is an omnipresent material in our lives—but what is it, really? While concrete hasn’t been around as long as gardening, it turns out that concrete is older than we might think. Concrete is a mix of materials: it is typically made of an aggregate like sand, a cement binder like calcium aluminate or lime (not the fruit, a mix of calcium oxides and hydroxides), and water. The resulting slurry is easily pourable, meaning that it can be molded into useful shapes. As it dries and cures—curing is a chemical process when the materials bond—it becomes solid and strong, meaning that it can be used as a long-lasting and affordable building material.

It has its roots in ancient times, as the Sumerians and Egyptians started experimented with aggregate materials as binders for masonry. The ancient Romans developed the first known “true” concrete, using a formula of sand, lime (again, not the fruit!), and water. They used this remarkable material to create structures like the Colosseum, which stand to this day.  Concrete really came into its own during the 19th century, with the creating of Portland Cement—a strong binding material based on calcium aluminate—and steel reinforcement. Today, concrete is the most common building material in the world—every year we use twice as much concrete globally than we do steel, wool, plastics, and aluminum combined.

Affordable, widely-used, and long-lasting—concrete is a miracle material, but is it good to use in gardening applications? Turns out, that’s a surprisingly complicated question; let’s continue!

Concrete, Cement, and Soil Chemistry

As we allude to above, the creation of concrete is a chemical process—the various materials involved are mixed into a slurry and then cure and harden. The curing process can take years—doing research for this article we learned that the Hoover Dam’s concrete is still curing and it was completed in 1936! The chemistry of curing concrete is more than we can cover here, but we can talk about how it affects soil around it.

In brief: concrete curing isn’t harmful to your plants or your soil necessarily, but it does affect the chemistry of the soil around it. The curing process can alter soil pH, increasing its alkalinity in ways that may not be good for some plants. This process is more likely to be visible in smaller concrete planters with a more concentrated concrete-to-soil ratio. Using a liner of some sort—or just keeping an eye on your soil and measuring the pH every now and then—can help mitigate these affects should they appear.

We’ve seen some sources online suggest that cement can leach harmful heavy metals into garden beds and thus contaminate veggies. However, we can’t find a reputable, well supported source that makes this claim—many extension websites suggest concrete blocks as a garden edging material. There are larger concerns about using treated lumber in the garden—which should be avoided! So if you are using concrete blocks for edging or to make a raised bed, nothing we’ve read suggest that you have anything to worry about. If you know differently or have a source we missed, please get in touch and let us know!

The Actual Disadvantages of Concrete

While concrete is safe (with caveats) to use in the garden, that doesn’t mean it is without any drawbacks. There are a few disadvantages to concrete that we should touch on briefly. First and foremost: concrete doesn’t drain well. The use of concrete blocks—which by definition have gaps between them that water can move through—mitigates this somewhat, but concrete does hold water in soil fairly well. Remember, they used it to build the aforementioned Hoover Dam! If you need better drainage in a container or raised bed, or if you’re concerned about healthy air circulation around the roots, there are other materials that may be better choices for your raised beds or containers. This is less of an issue in landscape edging, so concrete is likely fine for that application if its installed correctly with drainage in mind. Flat concrete surfaces like patios or driveways are impermeable to water and may cause problems with drainage and runoff, so be mindful of that and if you’re considering installing such a surface look at more permeable alternatives that will help rain absorb into the ground.

Concrete also gets hot, and like its issues with drainage that may or may not be an issue depending on implementation and the scale of use. Small concrete containers and planters may overheat the soil in direct summer sun, so be mindful of where you locate them. Concrete surfaces definitely overheat, as anyone who’s ever walked on hot pavement knows. Putting your plant containers directly on such surfaces can cause damage to your plants, so be mindful of that issue as well.

Concrete: A Useful Garden Material

So, in conclusion—should you use concrete in your garden? The answer is yes if it’s the right material for the job. Like all gardening materials, it has both advantages and disadvantages, but it’s safe, useful when employed correctly, and while it may not be right for every task it certainly is useful. We’re grateful to have this ancient material around for all kinds of things in and out of the garden!

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