Do Rocks Make Good Mulch?

You may have noticed an increasing amount of rocks used for city landscaping installations and even in your neighbor’s gardens. Rocks have been used as mulch for quite some time, and the phenomenon is not new. 

This might lead people to wonder if rocks really do make good mulch. They may wonder if they break down like other natural mulches, providing a small, slow-release of nutrients to the garden soil. Safety for the garden is a concern when you are considering their use.

The truth about rock mulch lies very much in the types of rocks that are used. There are so many different types; some are useful, while others should be left for different projects. The type of garden comes into play as well.  

Common Rock Uses

Rocks are an inorganic material, as opposed to organic mulches like straw, wood chips, and leaves. This means rocks don’t break down over time in any way that can benefit the soil, at least not immediately. 

While there are geological regions where rock contents are present in the soil, it takes a long time for them to decompose naturally. The rocks we’re talking about are mostly still whole or only slightly decomposed. In that vein, there are a few different ways to use them. 

Pathways and Borders

Use rocks to add interest to the garden via accents, edges, and pathways. A river rock border can provide color to the landscape and act as a channel for water to flow through.  

Add some rocks to the area along the foundation of your house to keep it clear of dirt that could damage your home over time. Gravel pathways can be set, and even gravel patios can be set somewhere in the landscape to level the ground and include a seating area. 

Architectural Interest

Large rocks are often employed to provide dimension and aesthetic appeal to a growing area. In my time as a landscaper, we often used smaller mossy rocks to create a boundary between lawns and driveways and build up retaining walls for planting beds. 

Employ large rocks to provide seating in a garden, or let them act as a space to build up the soil for adding topography to a flat area or tiers to a slope. You may notice these large rocks in municipal landscaping, as they’re pretty expensive and difficult to move around at home. 

Rocks as Mulch

With the somewhat recent popularity of xeriscaping and its ability to conserve water, people began to tear out their lawns in favor of gravel. This is probably the most popular use of rocks as mulch, and this is most likely how you’ve seen it in the garden. 

River or lava rocks are employed similarly. In some situations, they act as the top layer of a garden, and their main role is to suppress weeds while allowing water through to the soil layer. Whether or not this is the best use of rocks as mulch is up for debate, though.

Is Rock Mulch Okay for Gardens?

There are some ways to use rocks as mulch that are actually effective and beneficial to the garden. There are also other ways to use rocks that will cause problems down the line. Let’s start to tackle this topic by discussing the ups and downs of rock mulch. 

Benefits

If you have problems with annual and perennial weeds, rock mulch can stifle much of their growth. With rocks, locating growing weeds is easier, making bed maintenance simpler. When weeding, a color contrast between the green weeds and your rock of choice helps to simplify the task. 

Another highly beneficial part of rock mulch is its ability to drain water much more readily than wood chips, straw, or leaves. Just like grains of sand, rocks don’t have stable bonds that hold in water and nutrients, and some plants do not want to be in an area where water retention exists. 

Smaller rocks are also great for areas where you want to grow plants, but you know foot traffic will be heavy. By installing rocks rather than mulch, you prevent soil erosion below the mulch line. No matter the size of rock you use, when installed properly, you can count on it keeping up over time.  

Downsides

All of the problems with rock mulch originate from improper applications. If you’re using rocks larger than ½ inch in diameter, you may be hindering your plants’ growth. This is most common when the rock mulch layer is too thick and can have the opposite of the desired effect.

Types of Rock Mulch

Knowing the type of rocks you need will help you get the most out of a rock mulch. You should determine which is best for your garden based on their best uses. But sometimes, it’s better to avoid rock mulch and choose natural mulch instead. 

Use your best judgment, and remember that you can remove the rocks if you find they don’t work in your garden. In that case, they could be used in areas where plants aren’t growing. 

River Rocks

This kind of rock is generally one to two inches in diameter and has a smooth surface. This smoothness is attributed to exposure to running water for a long time. River rocks are often sourced from rivers, stream beds, and beaches. 

They’re multicolored and perfect for areas where you want to fill in spaces between larger landscaping features. Build ditches to direct water away from your site. After a long install of a dry creek bed in one of the sites, it’s very satisfying to see what these rocks provide.

However, these are not a good choice for rock mulch, as their size isn’t suited to growing plants. While you can install them with fully grown plants, those plants will have a hard time pushing out new growth among heavy rocks. Planting in a bed of river rocks is also very laborious.   

You will also find they’re harder to source than some of the other options here. They’re also the second most expensive of the choices on our list. 

Pea Gravel

The smaller rocks you find along beaches, streams, and river beds are what is known as pea gravel. These are usually no more than ⅜ of an inch in diameter. They tend to have a slightly wider range of colors than river rocks, too, with more tans and reds present. 

Pea gravel is one of those rock mulches that can work with growing plants in a garden setting. It’s a good choice for a waterwise garden in a hot area, with native plants that appreciate the heat and dryness. It’s not great for a veggie or a shade perennial garden, though. 

The greatest benefit of pea gravel is its price. It’s the cheapest on the list here and easy to find in hardware stores and garden centers. 

Lava Rocks and Pumice

Both of these rocks are porous, which makes them distinct from the other rocks we’ve touched on up to this point. That means they have the ability to hold water in a garden setting. They are lighter than the other rocks on this list, too. It’s, therefore, easier to install and adjust them.

Lava rocks are mined from ancient, inactive volcanoes in the western United States. They aren’t technically lava but hardened magma. Pumice, on the other hand, is sourced from the same areas but is made of the glassified froth of lava. 

If you’re working with plants that like heat, these are useful sources of mulch. However, both types of igneous rock can be difficult to plant within. It may also be difficult to remove fallen leaves from a garden with volcanic rock, and lawn equipment can be damaged by the hard material. 

Lava rocks are by far the most expensive option for rock mulch. This is probably due to their ability to hold water and the fact that they are harder to source. You can find them in hardware stores and garden centers easily, though. 

Decomposed Granite

This rock is derived from granite that has flaked away and takes on a gravely or almost sandy texture. It’s a very common igneous rock formed of the minerals feldspar, mica, and quartz. Granite is pinkish to peach-colored and comes from over 100 sources in the United States.

This is the rock mulch most often employed in xeriscaped gardens. Just like the others mentioned here, it gets hot and drains well. Therefore, desert plants and other hot weather plants that don’t want to be waterlogged benefit from it.

It has its benefits, but the small bits of decomposed granite can wash away in heavy rains if not installed in a successive and deliberate manner. It is easier to set up in the garden due to its size, but you must moisten each layer in the installation process. 

You can find decomposed granite in bulk or at a big box store with little issue. Another great thing about decomposed granite is that it’s the second least expensive type of rock mulch we have discussed.