I was just passing through to get another question answered and am certainly not an expert here; But at least with what is growing between my stones, roundup does exactly what you want. Of course, you may have a different thing growing where you are.

Thanks Chas045. Roundup kills everything, I think - I want to keep the moss but not the grass.

Roundup does not kill everything!! Your question was perfectly clear. Admittedly (as I said above) I don't know exactly what I have growing between my stones but it looks like green moss. It remains on the soil and does not actually cover the stones. It is a very attractive green and it is very much alive and it has had years of roundup sprayed on it.

Has anyone tried Fertiloam "Over the Top" grass killer...it is supposed to kill grass only, but I am afraid to experiment with it on my moss...It works great killing the grass. Anybody?

I've used Roundup on my moss to kill the grasses, but I don't know if it is safe for all mosses. I once used another weed killer (Ortho) and hurt the moss severely, so I am careful to do a test run on a small spot to see what it will do before I spray a large area.

Never tried Over the Top.

Whatever you try, you don't want to saturate the moss, just a light spray on the weeds and grasses. You still might get a slight yellowing of the mosss, but it should come back.

Depending upon the species of grass aluminum sulfate can kill off the grass as long as it isn't tolerant of acid soils. The added benefit is that moss loves acid soil and will thank you for it.