The best answer is to remove the lava rock now before you further compound a problem that will be even harder (or more expensive if you have it done by others) to correct in the future. IMHO.
hortster

I have sunflowers thriving in lava rock. Seed were dropped by birds from the bird feeder.

A neighbor has iris growing in lava rock.

My spouse uses lava rock in her filter for the koi pond.

The difficulty of removing it may depend upon what is under it. The sunflower and iris mentioned above are in lava rock over landscaping fabric and could be removed if we wanted to bother. The rock at the bottom, on the fabric has been ground by the elements into red dust.

Many organic gardeners highly recommend lava rock for it's mineral properties for the healthy growth of plants. It breaks down very slowly, but provides all the essential nutrients that plants require. I add lava rock and lava sand to all of my potting mixes, both for the mineral properties and also for the texture and drainage it adds to the soil.

If your lava rock is sitting right on top of the soil around the trees, I would leave it there and top it with mulch, just as you suggest. If it's sitting on top of a weed stop cloth, it will still add to the quality of the soil around those trees. If it's on plastic mulch it would be better to remove as much of the plastic as possible.

I am viewing this from the perspective of a landscape professional. I have recently looked at not lava rock, but "expanded shale" as a soil modifier. I agree that, finely ground, porous materials such as this hold oxygen in the soil, helping in a way to aerate it. But, as someone that deals with the previous situation described because people were too lazy to remove the lava rock before applying pure organic materials like mulch or compost, and charging others for it because it has become an encumberance to effective re-landscaping, I still make my recommendation to remove it first. I guess it becomes a view of the correct thing to do, "short term" or "long term." My neighbor has this very situation, and won't do a thing in his yard because it is too much work to dig through the soil/mulch/lava mix. I guess it becomes quite subjective.
hortster

That Lava Rock is a mulch, but covering it with a vegetative type of mulch is not a very good idea since the vegetative material will be digested much sooner then the rock would and you will have this same discusssion again then. Better to remove it now rather than later.

In a similar situation (river rock / no weed barrier / rock fully integrant with soil surface) I covered the rock with weed barrier and put shredded hardwood over it. Works fine, you will soon forget the rock is there.

Thanks for everyone's suggestions. I could remove the lava rock but it would be a lot of work. Not that I am against work....but with only weekends to do it, I need to pick and choose my work carefully. It does have some kind of landscape fabric underneath it but the rock has been there for ~15 years and has still managed to embedded itself into the ground. Using a rake works partially but much of it will need to be done by hand. I have about an 3/4 of an acre with multiple beds of this rock. I really do not want to pay a landscaper to come in crawl on the ground and pick rocks out for a week if there is an alternative way of taking care of this....

Thanks Again

Yardmann, how do you eat an elephant?

(answer: one bite at a time)

Try not to look at it as a whole. Take one bed at a time, or even one section of a bed at a time. Another alternative to paying landscaper prices is to find some kids out of school for the summer who want to earn some money. This works well for kids who are too young to get another kind of job (under 16). Obviously you should check with their parents. Don't pay them by the hour or you will go broke. Offer them a set amount for clearing one whole bed. See how they do and if you are happy, and if budget permits, have them work on another one.

I agree cover them up with your perfered mulch and forget about why bother digging them up and finding a way to dispose of them if somebody else worries that there are some rocks under 8 inches of wood chips let them deal with it(:

This question reminds me a bit about people asking on the home decor forums about painting over wallpaper. Some people do and some people don't. I'm one of the ones who won't either paint over wallpaper or mulch with organic mulch over lava rock. But I'm sure many people do both and if you think you can be happy with that decision, go for it. Of course I also don't use landscape fabric as I always think it's easier to avoid problems than have to solve them in the future.

I'd pay kids (and have done so) to remove, then I'd remove the landscape fabric, lay down cardboard, then organic mulch. Might take a year or two to complete this in a large garden but you'll have a property you can be proud of.

One of the interesting things about these Gardenweb forums is that we gain a perspective about the other members and their personalities and their ways of thinking and living life - whether in agreement or not. 'Nuff said.
hortster

The best is to remove as much of the rock as you can but I didn't in the large beds in front of our house. (I did remove the lava rock from some smaller areas in the back yard though.)

FWIW, in the large area in front of the house, I removed as much of the landscape fabric under the lava rock as I could (probably got about 90 percent of it out).

(I did this on a real hot day a few years ago. I was NOT a pretty sight when I was finished!! It is hard, dirty work IMHO.)

Then I moved the lava rock away from the new plantings for what I estimated would be the drip line for the new plantings and place wood chips, compost, etc., over that area every year.

The wife makes me use red colored wood mulch because it matches the lava rock color. (I don't argue with her about that even though red mulch ain't my favorite mulch.)

The most important thing I think you should do is to, at least, remove as much of the landscape fabric as you can. (Remove as much fabric as you can even if it is in areas where the lava rock will remain on top of the soil IMHO.)

It can be lifted up fairly easily and the lava rock will have to be moved back to re-level the area, (i.e., the lava rock gets displaced as you lift out as much landscape fabric as you can on each section you are tackling).

It is heavy, sweaty work but it will, I assume, make the area more "breathable", and that is a good thing IMHO. Plus, removing the fabric will allow the organic mulches to directly touch the soil. That is better overall IMHO.

The plantings are doing very well.

(Just my personal opinion.....I think landscape fabric is one of the WORST gardening inventions to ever come down the pike.)

Good luck. You'll get some GOOD exercise - LOL!!

Just as an end to this string, I thought I would update you all on what we did this weekend. After some good suggestions from this forum, I rented a front-end loader and removed all the lava rock from my two big beds before mulching. I elicited the help from my two young sons and we knocked it out....It was not easy but we did it. If any of you find yourself in a similar situation, I would suggest using a loader. We were able to put the bucket at ground level and sweep the rock directly into the bucket with our hands and feet. We also used a sweep rake with limited success since the lava rock was light.

Thanks!

I know this is an old thread but I wanted to update what I did over time to get "rid" of the lava rock problem that we had.

This last week, I finally scraped alot of the remaining rock out of the large bed and spaded topsoil, compost and some organic fertilizer into the soil. However quite a bit of the lava rock remained and I spaded that in along with the amendments. I simply could not get all the rocks out with a flat type of coal shovel.

I double dug the areas adding amendments each time along with organic fertilzer, soy pellets, etc. (Not alot of fertilizer but a sprinkling to get things cooking along with the compost in there.)

Of course this was heavy, sweaty work but it is finally finished off. I wish I had been able to get almost all of the lava rock out of there but too much of it had already been mixed in with soil over the years and I simply couldn't get it all out without removing ALOT of the soil along with it.

But I was able to spade the vast majority of it back into the amended soil before adding a final layer of topsoil, compost, soy pellets and wood mulch. (In the end I probably added back 4 inches of amendments to make up for the removed lava rock and put another 3 inches of wood mulch on top of all of that.)

So now most of the remaining lava rock has finally been worked into the soil where it can hopefully decompose more quickly and become part of the soil.

I NEVER recommend lava rock or landscape fabric to anyone but if you move into a house that has them you can probably get away with leaving some to dig into the soil.

I say this because after all the amendments and digging in of the remaining lava rock (which was still a fair amount of lava rock) the final "soil" drains well. (But it took ALOT of work that wouldn't have been necessary if I had never had the lava rock in the first place.)

The plantings are doing very well but I'll know more this summer when the wife plants flowers into this newly amended soil with some lava rock spaded in.

Time will tell.

In the end though, I WILL NEVER use lava rock or weed barrier fabric on my garden ever again. These were the two biggest mistakes I have ever made in all my years of gardening. Just my personal opinion.

(The reason I got rid of mine and/or incorporated the remaining lava rock into the soil was it had become unsightly over time and some parts stayed too damp and a small bit of moss was growing on some of them. This situation had to be corrected because it was really bugging me. LOL!)

Finally, I was surprised to find quite a few earthworms in this old "lava rock" soil when spading in all the new stuff. That did really surprise me, I didn't expect to see earthworms in this old stuff.

Now the earthworms have a good soil to live in!! (I hope!!)

I know this thread is way old, but I just came across it because we are now encountering this situation. Bought a house 2 years ago and am redoing the horrible landscaping. We are trying to redo a bed and are finding that the entire bed has dirt and mulch piled on top on tons of river rock. Not cool. Am SOOO angry with the former owner for this kind of shoddy work and wonder what other surprises they left for us. I hate when people don't do things the right way or consider the impact of their decisions down the road.

We had small lava rock in our beds when we bought our house. Couldn't stand it, put an ad on Craigslist for it free. Next day, no lava rock in my beds.

Problem solved.

to easily separate lava rock shovel it into a Milk crate and shake. it filters out the lava rock from dirt. Pretty fast.