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Lava Rock as Mulch

17 years ago

I have a semi-sloped garden and am using Lava Rock as a mulch. I find it helps with erosion issues as well. Are there any negatives to using Lava Rock as a mulch? The only reason I ask, is everyone thinks I'm crazy when I let them know what I'm using. Also, is there a difference in the black and red, other than their color? Thanks for any help!

Comments (31)

  • kjggames
    17 years ago


    I would say lava rock around your plants might not be a great idea. Those suckers are jagged, and could tear up your plants around their base. Not to mention when you need to do some weeding, it could be a bit...uncomfortable. If you can keep them away from tender plantings, and lay down a layer of plastic underneath it however, it should look nice.

    As for color, black tends to absorb heat, and the only thing i've seen about red is the color may help tomatoes ripen quicker (plastic is usually used for these applications).

    Enjoy your garden,

  • sylviatexas1
    17 years ago

    You're in Texas like I am:
    Our gardens need insulation more from heat than from cold, & rock holds so much heat it'll cook your plants.

    I've always had good results with shredded wood mulches.
    Cedar & hardwood are my favorites.
    They're dense enough that they don't float away, & they look nice.

    Cypress is another heavy mulch, & it'll mat down somewhat, which might be just what you need on your slope.

    I've *heard* that shredded pine & pine bark will float away.
    I never have had that experience, but then my lot is as flat as a pancake!

    Weeding in rocky material is tedious & difficult at best, & porous lava rock meakes a perfect home for tenacious weedy roots!

    If you re-do the bed, removing rock mulch is a chore.

    Wood mulch (if it hasn't been in the bed long enough to have broken down & become part of the garden soil) is easy to push aside or shovel out.

    Lava sand is a good material to mix with garden soil to help retain moisture.

    Lava rock, seems like, would provide the same benefit, so, should you decide to use a different type of mulch, you might remove the bigger pieces & mix the rest into your soil.

    I never have used plastic:

    I still have bad dreams about the teeniest pieces of plastic in the universe, which a previous gardener had installed, & which I removed when I worked the beds.

    Removed piece by tiny, brittle piece...

    Best luck!


  • ocdgardener
    Original Author
    17 years ago

    Sylvia, Thanks for the advise. I'm so depressed! I need to hear what's what though!

    I was inspired when I put in a rock bed for my brother in Santa Fe. I loved it so much I decided I wanted to do the same for my little slope. I thought Lava Rock actually kept the plants cool in the summer and warm in the winter. I guess it does the opposite! ---The saddest thing is that it looks so purrrrty and the squirrels are no longer digging in the beds. The beds are already well mulched with cedar mulch thats added about twice a year. I kind of shoved that mulch aside though to put in the rocks. Oh well - guess I better start de-rocking! Boo Hooooo WaAAAAAAA!

    Oh - and I found the rock to be less cutting on my hands than cedar mulch. I always prick myself with that stuff and get splinters too.

  • sylviatexas1
    17 years ago

    Well, if you really love the look, & if your garden is not western or southern exposure, you might take a wait-&-see attitude: if the plants get too hot, you can change it, & if they thrive, you'll have the look you like.

    Best luck!

  • rusty_blackhaw
    17 years ago

    Some of these negatives have been mentioned before, but

    1) Weeds love to grow in lava rock, and it is painful to weed in,

    2) It does nothing to improve the soil,

    3) It looks unnatural and ugly, at least to me.

    To really bake your soil and turn it into a nonporous greasy pancake on the surface, place landscape fabric under the lava rock. ;)

  • garasaki
    17 years ago

    Mulch has 3 main purposes (in my mind):

    Hold moisture
    Regulate temperature
    Break down into organic matter (this, in my opinion, is the most important)

    I don't think lava rock would do a satisfactory job of any of that.

  • ocdgardener
    Original Author
    17 years ago

    I'm realizing now why I posted a message. I want to hear all the negatives so I can say yeah but.... :)

    From what I understand about Lava Rocks is: Lava rock breaks down into Lava Sand. From all the sand in the bags I know it happens! Lava Sand is recommended as a soil amendment to HOLD MOISTURE. Now, the only thing I'm not sure about is temperature. I did notice though oddly enough last night when I got home from work the plants in the same bed mulched with cedar were wilting, the plants surrounded by lava were not. It makes me want to find out why - because you would think, as some have pointed out, that the rocks would make things hotter.

    Here's the positives I've found thus far.

    On a slope where my bed is, compost, and every thing else added at the base of the plants washes down to the bottom and is replaced with bare soil. I have to add mulch over and over because it all slides away. The Lava Rocks are not only holding the compost in place but are keeping me from having bare soil. I know it does because we had a heavy rain this morning and I looked and everything was in place! --Not the usual bare soil I see after a rain!

    I think I should explain how I'm using the rock. I'm taking one rock at a time and pressing it into the soil. When you think of lava rocks as a mulch I think most people tend to imagine it piled up. I agree with Eric in that sense it's not very attractive. The way I'm placing the rocks looks very natural, very much like a rock garden.

    So, I'm still not convinced, I should remove the rocks, but keep trying! :)

  • ocdgardener
    Original Author
    17 years ago

    oh and btw you can't really see that much of the rock anyway - because the plants cover them. Example: This is what the yard looks like in the spring.


  • ocdgardener
    Original Author
    17 years ago

    Update on the lava rock! I bought a soil thermoter and tested the temps. Although not lower, the temps were pretty close to the same by a degree. Also, I have calendulas blooming in the bed in the HEAT of summer. I've never had that before! So --- I'm thinking L.R. as a mulch in my sloped bed is going to work. I don't plan on using it however in the other beds - I'll stick to the cedar mulch for those.

    Just wanted to give a report!

  • Kimmsr
    17 years ago

    Au Contrare, Lava Rock will aid in retaining soil moisture, suppress weed growth, and aid in keeping soil temperatures cooler if the layer is thick enough. But Lava Rock does not add anything of value to the soil. I use black lava rock (4 to 6 inch depth over landscape fabric) between the house and the flower beds and find that pulling weeds is quite easy but getting leaves and other plant debris is not easy. However I would not use Lava Rock as a mulch in any planting bed since the mulch I do use contributes to the overall health of the soil by allowing the soil bacteria to digest and incorporate that organic matter into the soil. Most planting beds (flowers and veggies) get shredded leaves because these are readily and abundantly available around here.

  • ocdgardener
    Original Author
    17 years ago


    Weeds are not a problem in my beds. I think I spend less than 2 percent of my gardening weeding. Not sure why, but I haven't had weeds since the first year I yanked them all out one by one by hand. I think because my plants shade the ground might have something to do with it.

    Since the lava rock breaks down in to lava sand - which is good for the soil doesn't it add something? I am placing the needed soil amendments around the base of each plant. The lava rock is helping HOLD it in place. So, although I'm not amending the soil as much, by adding it everywhere, I am placing it where it is most needed.

    btw- I have NEVER EVER used the "black plastic" stuff, and don't plan on ever using it - even though I'm replacing my grass paths with mulch in my backyard. I just can't bring myself to put plastic anywhere in my landscape. I use newspapers for that!

    Thanks for the input.

  • mrwsm_yahoo_com
    14 years ago

    Lava Rock has low thermal mass. It does not retain heat. Lava Rock keeps the soil soft like mulch, so if there is a weed, no problem, it pulls really easy. I prefer red lava rock, although I have both. Never put plastic underneath it! It keeps most weeds out as is. Nothing is weed proof, but it does a nice job. If you are tired of buying mulch every spring, use Red Lava Rock. Buy once. Lava rock is light weight and easy to work with. It weighs very close to mulch. Red Lava Rock is not actually red, it is closer to a medium brown and appears like an everlasting mulch from a distance. Lava rock is not sharp and does not hurt plants. Where do these people get their info from? Anyway, I'm just giving you guys my personal experience with it. I would never use regular rocks, too heavy and cause heat stress to the plants. Lava Rock you can use anywhere.

  • justaguy2
    14 years ago

    this post is 3 years old, why resurrect it?

    14 years ago

    Well it's better to continue existing threads rather than to keep starting new ones of the same topic. I think?

  • PoorOwner
    14 years ago

    I hope people would think twice about lava rock as a mulch in an area you don't want weeds to grow, not just thinking about the appearance the day you put it on but think about what would happen over the next 10-20 years.

    Leaves and debris will settle between rocks and compost to soil. Weeds WILL grow. If the area is subject to traffic, the rocks will eventually go into the soil, in our case it was clay soil.

    The previous owner put lava rock all over the place, in some area against the foundation, nothing is planted. After I removed the lavarock and what's left of broken down plastic in attempt to seperate the soil and lavarock, the ground is rock hard.. from what I have seen it only made the ground harder.

    Also try to repair or dig a fence post around these embedded lavarocks was complete nightmare. Or even try to plant anything. It was not acceptable for me as a mulch or ground cover.. do not ever put lava rocks near a tree, as leaves fall on it and you cannot easily rake them out. In some areas around my place, the rocks were inside the soil and weeds and grass grow over it and you won't be able to tell until you look close the lavarock is there.

    I used a shovel to skim off the lavarock embedded soil, and also shop vac to get the loose rocks. Week after week as garbage disposal allows. Finally I just paid a contractor to have the area redone with new concrete slabs, the lavarock probably mixed as a base/fill material for a retaining wall along with much more gravel. I was so happy to have it gone. I underestimated the threat of these lavarocks when I purchased the home. I have not looked up the definition of a "mulch", I would think a mulch needs to break down into organic matter. Lavarocks does not.

    I have included a link of what I had to deal with if anyone is interested, it's an extreme case, in a bad neglected area. Although I have seen lavarocks used in commercial landscape, but still I wouldn't ever touch this stuff ever again.

    Here is a link that might be useful: My lavarock nightmare

  • david52 Zone 6
    14 years ago

    For a really good time, put lava rock down in your driveway. It gets crushed by the cars, and then the fine grit sticks to your shoes, and works something like 80 grit sand paper on polished wood floors.

  • horsefeathers2008
    14 years ago

    The best source of Lava Rock that is both a soil conditioner and flow ability of water and keeps the ground cool, and as it breaks up over time adds micro nutrients to the soil is they can handle small orders up to extremely large orders.

    Remember not all Lava Rock is the same. Many types are dense and heavy this type is has its uses but not in the soil, the best type in the soil is commonly called "foamed odsidian"This type of Lava Rock is found in the high desert region of Utah. It was formed under the great extinct lake called Lake Bountiful. This material is very light and airy with loads of micro elements that are easily digressable to tender plants. Check out for more answers to your soil re-mineralization questions.

    Here is a link that might be useful: the best source for Lava Rock that I have found

  • peter_6
    14 years ago

    I assume lava rock comes from volanoes. Most vocanic rock is highly paramagnetic. Paramagnetic rock increases soil conductivity and, consequently, mineral uptake by plants. But I suppose you would need to pulverize it and incorporate it into the soil, which doesn't at all serve your purpose. Ah well! Regards, Peter.

  • rdak
    14 years ago

    I think it is better than no mulch and maybe it does incorporate some beneficial "sand" while slowly breaking down, but, I prefer organic woodchip types of mulches.

    I've tried both and plantings do better, where I live, with wood mulch.

    (Now I do use compost and some of my neighbors' grass clippings under the wood chips, along with a sprinkling of grains and alfalfa pellets now and then. So that is a huge factor in my "improved" opinion of woodchip mulches over lava rock mulches.)

    Overall though, lava rock probably isn't all that bad IMHO. I mean, some of the members posting here have good results with lava rock mulch. So, it's probably ok.

  • maryt-2009
    14 years ago

    I just finished racking old leaves and undesirables to prepare for the season. One area of the yard (next to the garage) still has a good portion of Lava Rock and I don't want to spend the time taking it out. Is it OK to add red mulch to the Lava Rock? Eventually it will be all mulch. If I add the mulch, I would have 60% mulch and 40% LR. Any thoughts?

  • idaho_gardener
    14 years ago

    Lava rock is a good source of trace nutrients. I don't know why people were slamming lava rock in the previous posts.

  • Kimmsr
    14 years ago

    Lava Rock is not a substitute for organic matter in any soil, no matter what the people that are selling it try to tell you. Since the soil you have is composed of minerals (the sand, silt, and clay particles) why would you need to spend money to buy more mineral to add to your already mostly mineral soil? Just add organic matter.

  • maifleur01
    14 years ago

    You would add more minerals to the soil the same way and reasons you add the other minerals that many suggest, limestone and dolomite.

    Also the terms silt and clay are particle sizes not types of particles. Do some checking and enlarge your information bank.

  • stacyp9
    14 years ago

    I've spent three years removing the lava that previous owners put down as a mulch all around the house. It's awful stuff. Leaves get and sticks get stuck in it, you can't dig in it. It cooks plants like, well lava. Unless you are looking for a mulch that you will never dig in, I would avoid it at all costs.

  • bryanvolvette
    13 years ago

    I think all of you are crazy. I have used lava rock for 8 years. There have been no plants getting burnt, it does not radiate heat or hold in heat (un-like wood based mulch), and it doesn't cause any kind of bugs. I have a wide variety of plants and none of them have been harmed by the rock. Sure you cant get in the rock on your knees but who would want to get in regular wood/dirt mulch. Its made from garbage. I also carries bacteria that is harmful to humans and animals.
    The best thing about lava rock is the cost. It's cheap and you don't have to re-apply it twice a year like most mulch. I haven't had to add any in about 5 years, its a time saver

  • gjcore
    13 years ago

    I'm just guessing here and I'm probably not going to use mass amounts of lava rock as mulch. Maybe though using some lava rock would be a good deterrent to snails and slugs placed in a circle around some plants.

  • rickjones
    12 years ago

    I don't know what happened to my original post, but here is a short repeat. LAVA ROCKS ARE WONDERFUL PLANTERS, ESPECIALLY IN THE EXTREME HEAT OF THE MOHAVE DESERT. Yup. I build and use cedar planters and window boxes all over my home, they are great, but do not compare with the flowers grown in the lava rocks that I hollow out and plant a variety of species in. Plants in these rocks see temps to 120 degress in our summers and do just fine! They retain moisture, cool with the air temps, and fertilize plants naturally. I don't use lava rock for bedding, I can see the problems with that. But as a planter? The best planter I have ever had. AND, no maintenance, and years of use looking great.

  • Nancy Keith
    4 years ago

    I love the red lava rock, it has been great for my flower beds. I love the look as well.

    I am buying more to finish the new flower bed I started this year.

  • Valkyrie Elyn
    3 years ago
    last modified: 3 years ago

    We put down red lava rock in a 4x30 bed last year instead of the annual red pine mulch that bleached out and burns up in the Florida sun. We completely removed the mulch, dug down about a foot and removed any weeds. Put down one of the most expensive landscaping cloths, not black plastic, as there were already plants in the bed. Black Edging and 3-4 inches of lava rock put down. Within 1 week there was St Augustine grass coming up thru the fabric and rock in the center of the 4 ft bed. The stuff is a nightmare to try and maintain. You’ll destroy the fingers of any garden gloves weekly. It DOES RETAIN HEAT and my plumerias lose leaves daily from being baked. It’s also breaking down, with no car or foot traffic in the bed and there’s almost an inch of red rock sand now trapping weed seeds also. We’re in the brutal heat/humidity part of the year right now. In a couple of months, we’re removing it all. Not sure what the replacement will be.

  • Denise Bermudes
    2 years ago

    Depends where you are..I am in zone 8 and have lava rock around some lavender and a couple of ferns. I don't have an issue. In fact they are doing quite well with the lava rock. Another area that only gets morning sun and afternoon shade the plants are doing well. And yes I have landscaping plastic under the rock. My plants are thriving. And since I placed it down in the spring time I have yet to have any weeds.