Volcanic Rock for Garden: The Pros and Cons of Lava Rock as Mulch

Volcanic Rock For Garden: The Pros And Cons Of Lava Rock As Mulch

Mulching is an essential component in landscaping.

It ties together the various elements in the garden and gives a natural flow to the yard, prevents excess evaporation, improves soil drainage, prevents weeds, and so on. In this article, we discuss the pros and cons of using volcanic rocks for garden mulch.

What Exactly is Volcanic Rock?

Lava Rock or volcanic rock is quite a popular decorative material in yard landscaping. It belongs to the category of igneous rocks and is formed when erupted lava cools down.

It is porous in nature and also has air bubbles that pop as it dries. The air bubbles are the reason why lava rocks are really light and also why they have bubble-shaped holes throughout.

Lava rock is available in three colors; reddish-brown, black, and gray. The minerals present in the rock and the cooling process determine its color. The reddish-brown tint appears only if the iron in the lava begins to oxidize when it is cooling down.

Ingredients of Volcanic Rock

Volcanic rocks are primarily categorized into two main categories, the felsic rock, and the mafic rocks. The difference is dictated by their composition. Felsic rocks are rich in silicon and aluminum minerals, whereas mafic rocks comprise of iron and magnesium.

Because of their chemical composition, lava rocks classify as basalts.

They are not only rich in magnesium and iron but also calcium. They also constitute numerous minerals, including magnetite, pyroxene, plagioclase feldspar, hornblende, olivine, quartz, pyroxene, biotite mica, and amphibole.

Volcanic Rock as Mulch

Volcanic Rock As Mulch

Volcanic rock is a naturally occurring substance that can help you fulfill your landscaping needs. While it may not provide the benefits organic mulch does, it does eliminate a lot of the side effects brought by artificial mulch.

It might very well be the perfect option for those looking for something low maintenance yet beneficial. Here are some pros and cons of using volcanic rocks for garden mulch.

Volcanic Rock Pros

  • Lightweight – lava rock is quite lightweight because of the air pockets in it. Its lightweight makes it ideal as mulch because it does not sink into the soil, nor does it compress it. It lies on the surface, protecting it from the environment while allowing it to maintain its structural integrity.
  • Porous – Lava rocks porosity allows for better drainage. It ensures water does not pool between the rocks or under them, which prevents fungal growth keeping the yard healthy.
  • Protects the soil – When used as mulch, lava rocks also prevent soil erosion while allowing all the nutrients to seep through. It also slows down evaporation from the soil, which can drastically reduce watering needs for your yard.
  • Weed Suppressor – By adding a fabric weed barrier beneath the layer of lava rock mulch, you can effectively protect your grass from a weed invasion for a very long time.
  • Low maintenance – Unlike organic mulch, lava rock does not disintegrate or decompose. It lasts a very long time and requires almost no maintenance. Add it in your yard and forget about it. You don’t even have to worry about replacing it for a while.
  • Available in Different Colors – the reddish-brown lava rock can add a much-needed pop of color to the yard. Though it may not be for everyone, it is quite a popular option for many landscape designers, especially in desert regions. For those who don’t like red could always opt for the greys and the blacks to add earthier tones in the yard.

Volcanic Rock Cons

  • Too Light – While its lightweight has advantages, it also presents various disadvantages. In fact, it could potentially fly away in case of a storm or really strong winds.
  • Possibly Hazardous – When mowing your lawn, you will have to be extra careful. The machine could possibly pelt the rock backward, and it could hit the mower. In fact, if it hits the eye or exposed skin, it can cause deep cuts.
  • Can Sink into Soil if Stepped on – Imagine rocks embedded into the soil. If you have used lava rock as mulch, the probability of that happening is quite high. You cannot let the grass grow as tall as it wants to be, and you will have to step on it to mow it. That’s when lava rocks for garden mulch become a major pain.
  • Costly to Replace – Once you add it into your soil, taking it out to replace it with any other kind of mulch is very costly. It is expensive not only in terms of money but also in terms of time and effort.

How to Use Volcanic Rock when Mulching your Garden?

How To Use Volcanic Rock When Mulching Your Garden?

For those with a semi sloped garden, using lava rock as mulch can do wonders. It is one of the most effective ways of preventing soil erosion while maintaining optimal soil health and promoting natural growth, except for weeds.

While nothing stops weeds permanently, lava rocks can prevent the invasion for quite some time. To use it as mulch, you can follow the steps mentioned below.

  • Clean the area of all debris and make sure the soil is free from weeds. If there are any that you miss, they will multiply and grow despite the presence of the lava rock or weed barrier fabric.
  • Identify the size of the rocks you want. Landscape designers recommend using a mixture of different sizes to add more depth to the yard. You can opt for rocks ranging from 2 to 4 inches for mulch around trees and shrubs. For the area around plants and flower beds, use rocks 1 to 2 inches in size.
  • Lay out the mulch at a 2 feet distance from all plants. You can pile it close to trees and shrubs, but the jagged edges of volcanic rock can pierce through plants as they are more delicate.

Three steps are all it takes to add volcanic rocks as mulch for your garden. You can get creative and use different colors for different areas. You can even use a mix of red, grey, and black to undermine the brightness of the red that many find awkward. It also adds a natural, earthy look to the yard.

Different Types of Mulches

Different Types Of Mulches

There are various types of mulches that people use for landscaping and gardening needs. Using mulch does offer substantial benefits.

  • It prevents soil erosion by adding a protective layer.
  • Improves soil drainage to prevent waterlogging and fungal growth.
  • Prevents weed growth.
  • Maintains soil temperature.
  • Prevents rapid and excess evaporation.

Mulches can be divided into two main categories, organic and inorganic. Organic mulches consist of:

  • Shredded or chipped bark
  • Shredded leaves
  • Newspaper shaving
  • Pine needles
  • Straw

You can also choose from the different commercially available inorganic or synthetic mulches. They consist of:

  • Landscape fabric
  • Black plastic
  • Gravel/stone

Both organic and inorganic mulches offer numerous benefits. Your choice will largely depend upon your landscaping requirements, budget, and how much effort you are willing to put into maintenance. Here we list down the pros and cons of both.

Pros of Organic Mulch

  • Decomposes overtime – Organic mulch naturally decomposes over time. You don’t have to worry about taking it out and replacing it with a new one. It naturally disintegrates and merges into the soil.
  • Adds nutrients to the soil – The decomposition process results in additional nutrients making their way into the soil. It not only improves overall garden health but also promotes the growth of vegetation.
  • Easier to replace – Organic mulch is easily available at any gardening store. Since you don’t have to worry about getting rid of the remnants of the previous batch, all you have to do is add in some more to continue enjoying its benefits.
  • Improves soil structure – When organic mulch disintegrates and merges into the ground, it enhances the structural integrity of the soil. It allows for better nutrient penetration and optimal root development.

Cons of Organic Mulch

  • Not as low maintenance as inorganic mulches – You will have to replace them at least once a month for you to continue benefiting from it.
  • Organic mulch, when applied wrong, can turn into sour mulch and damage the plants instead of benefitting them. It can absorb nitrogen from the soil and stunt the growth of vegetation.
  • They have to be quite thick to be efficient. Two to three-inch-thick bark shavings don’t always make a pretty sight and can destroy the yard’s visual appeal.

Pros of Inorganic Mulch

  • Low maintenance – Once you add in inorganic mulch, you don’t have to worry about maintenance unless you want to revamp the look of the yard. Stones, gravel, and other inorganic mulches do not decompose and are long-lasting. They are ideal for those who do not want to pay much attention to landscaping needs.
  • Visually appealing – Where organic mulches can look dull and dreary, inorganic mulches can enhance the yard’s visual appeal. They offer a nice backdrop for plants and create a beautiful contrast letting the greenery stand out.
  • One time cost – Because inorganic mulch does not decompose, you don’t have to worry about the additional cost of regularly replenishing it. It is ideal for those looking for budget-friendly options for their gardens.

Cons of Inorganic Mulch

  • Harmful for the soil– Plastic and rubber mulch, in particular, is not the best option for the yard. They can destroy the soil’s structure and can even lead to the formation of brown patches in the future.
  • Higher installation cost – Though one-time cost, inorganic mulches cost more than organic ones do.
  • Hinders soil drainage – Gravel and stone are not as harmful to soil drainage, plastic, and rubber shavings are. They can lead to waterlogging and can even become embedded in the soil, which is harmful to plant growth.
  • Does not add nutrients – Organic mulch decomposes and improves the quality of the soil, whereas inorganic mulch does not. It may, in fact, do more harm than good as plastic and rubber can release harmful chemicals over time, especially when exposed to harsh sunlight.

Organic Vs. Inorganic

Organic mulch is definitely better for your gardening needs, especially if you are passionate about the plantation and want to include flowers and shrubs in your yard.

However, it is also very demanding in terms of maintenance and upkeep. You will have to replace it regularly, which can be off-putting for those looking for a low-maintenance option.

Here, inorganic mulch can be a better option. It is a one-time cost and does not require additional maintenance. Some of the artificially produced mulches can be harmful to the yard and the environment.

You can, however, avoid the harmful side effects by opting for stone, lava rock, or gravel. They offer the perfect low maintenance and environment-friendly mulch option. They also add beautiful textures to the yard, which makes them even more appealing.

Volcanic rock is quite popular as garden decoration and is now gaining popularity for mulching as well. We have already talked about how well it works as mulch, let us take a look at how we can use it for landscaping.

Volcanic Rock for Garden Decoration

Volcanic rocks come in different sizes. You can opt for bigger slabs and incorporate them into your landscape design.

You can use a combination of slabs and smaller rocks to create a beautiful walkway that is incredibly easy to maintain. You don’t have to worry about mopping it as you can simply wash it with a hose, and the porosity of the rocks provides optimal drainage.

Your herb garden makes for another amazing area for lava rocks. Herb growing and picking can become tiresome with loose soil. You have to maintain it regularly to ensure the soil does not clog together, preventing nutrients from seeping in.

Adding a layer of lava rock can prevent the soil from clumping together. It provides for ideal drainage without while adding a protective cover so as not to damage the integrity of the soil.

It also ensures you can go pick your herbs without having to worry about soil sticking to your feet.

However, it is not suitable for use in kitchen gardens that grow a variety of vegetables and fruits. Such gardens require regular soil aeration and fertilization. They are also replanted quite frequently, and using lava rocks as garden mulch in these areas is not ideal.

The fact that volcanic rock is a naturally occurring substance is what makes it’s an excellent choice.

Synthetic materials can hinder the growth of natural vegetation, but lava rock does not. While it doesn’t necessarily increase the nutrients in the soil, it also doesn’t take away from them, making it the ideal option for many.

Black lava rocks, in particular, add a wonderful contrast to the yard.

Adding that to the landscape can not only add texture but also enhance the colors of the plants and flowers in the garden. The difference in texture and color makes for the perfect backdrop for green foliage.

How to Use Volcanic Rock for Landscaping

The steps for installing lava rock into the landscape are as simple as those of using it as mulch.

  • First, you should clear out the land of all debris and level it. If the turf is uneven, it not only takes away from the beauty of the rocks but also their effectiveness as low maintenance landscape decoration. You will have to fix and rearrange them every few days or so.
  • Draw a line and clearly demarcate the area where you want to install the lava rock. You can even use a string or a hose for a clearer definition and then add the lava rock. Make sure to add in an even layer as it looks more visually appealing.
  • Create an edge for the area that holds the rocks. They should be added to an area 4 inches below the rest of the landscape for it to present a uniform look after completion. If you can’t create a 4-inch depression, you can add in 4-inch tall cement, tiled or brick edge to keep the rocks in place and for ease of maintenance.

Lava rocks make interesting and beautiful additions to the yard. You can even create a small rockery exhibit in a corner using different stones. Many add it to waterfall features they put into their yards, and others use it to designate spaces.

It has various diverse uses and can help transform your yard into a functional and beautiful space.


Volcanic rocks for garden mulch are an ideal option for those hoping for a low maintenance option for their yard.

It does not require regular top-ups as it does not disintegrate. It is a naturally occurring substance, so it does not release harmful chemicals into the soil, nor does it damage its structural integrity.

Volcanic rock offers dual functionality, as it is also ideal for garden landscaping. You can choose from three different colored rocks to create a beautiful yard that is also weed-free and vegetation friendly.

With volcanic rocks for garden decoration and mulch, you can create beautiful landscape designs without having to worry about soil erosion.

About The Author

Gina Harper grew up dreaming about farms and growing her own food. She began an urban garden to feed herself and turned it into an incredible hobby. Gina is here to teach you everything from raised beds to container gardening, how to keep plants alive and well in a smoggy city, and the works. It’s time that we carve our own piece of green earth and reap what we sow—she’s here to help you with that.