It will hold some water yes, somewhat similar to pumice.

It's lava rock, right? I think the little pockets in it would hold some water which would evaporate eventually, depending on the weather and location of the garden. Seems to me if you water enough, it will go down into the soil. I think bark would hold some water as well. What plants are you putting it around?

Socks, I would be top-dressing succulents, mostly gasteria and haworthia. Most of mine are in some form of gritty mix (often without bark), and I do water until it drains out. But we all know how fast that happens with a gritty mix. Maybe I'll try it on two or three (since the bags are small anyway) and see what happens. Thanks for the advice.

If you water till it runs out, there should be no problem. Scoria will hold a bit of water, but it does not keep it from getting to the roots, same way that some extra gritty mix would hold some water, without robbing the plant. The water flows through the mix, which will absorb as much as before. The only difference would be a somewhat slower rate of evaporation, as it covers the mix the roots grow in. Should not be a problem either.

Wanted to quickly mention if the scoria is of a darker color be cautious when using it as a top dressing, it can easily roast the roots and the plant on a hot day. Something I unfortunately learned the hard way. Keeping pumice or any other light colored sand on top etc is a good idea.

Robin, I have used scoria for top dressing but I have trouble finding the smaller size. Most of mine is between 0.5 to 1 inch, like in this picture -

A top dressing helps keep the perlite from blowing out of my pots, but some of the decorative rocks hold in too much water. The lava rock allows it to evaporate better. I do have an interesting little problem. The anoles like to knock the lava rocks out of the pot! LOL

Off topic question, Robin -

How much rain have you gotten this past week? That low is stalled out over us in Cape Coral. Since it started on Thursday night we have gotten 24 inches of rain!

Thanks, Crenda. I really like the look of your plant with scoria as top dressing. I bought 2 small bags on sale at HD because they were $3 each. They seem to be about one-half to three-quarters of an inch, so probably like yours. I love the look, so I might as well try them. If they allowed evaporation more easily than decorative rocks, I'd be happy.

Wow -- 24 inches! I'm north of you, up in Venice. Haven't monitored exactly how much rain. We are so lucky here -- we're between Sarasota and Port Charlotte and we never seem to get pelted as badly as they do. But we have had a lot. Sometimes I get bored with the FL sun, but -- I'd be glad to see some now!

Plantspace -- thank you. I had not really thought about the color. I will have to give that consideration too. Appreciate the advice.

Wow we've got masses of scoria here! I'm sure it's sold in bags too, but I see it in places where you buy soil rocks sand mulch etc by the ton. People can come in with trucks.

I've been wanting to ask about it in here but because I've had a lot of trouble with names of products meaning different things between countries I didn't bother. I will look into it.

There's a place I go to where we can fill a large strong plastic bag they provide with whith whatever we want for $8. I get a variety of things, taking my own bags to keep them separate. Now I can add scoria to the list.