In my front yard, I found that there was only about 4-6" of soil over concrete down the side of my house (which was a BIG dissapointment for us when we tried to put in an azalea and discovered this!) I have been succesful planting carpet phlox and a couple of other low ground covers. The phlox gives it color and has survived over the winter there. I am going to try to put tiger lillies there this year also. Good luck!

It occurs to me that the plants used on "roof gardens" might work, so I'm attaching a link to a page at Timber Press on the subject.

A couple of details that will make a big difference are whether the rocks are really a solid mass, or if there are gaps where roots can penetrate underneath, and whether the area is always damp or dries out thoroughly. And, I guess, if it's very dry naturally, whether you'd be willing to irrigate the area.

Here is a link that might be useful: Timber Press: roof plants

If these are big chunks of concrete (like construction debris), then the cement in them may make the soil alkaline. In that case, you should avoid plants that like acidic soil.

Claire

Creeping sedums may do well here. I have a spot in my yard that is pretty much solid rock ledge. It has some very shallow pockets of soil (maybe an inch deep or so). There are some hens and chicks that do very well in one spot. I always wonder who put them there as the previous (and only) owner did not seem to be much of a gardener, but I'm glad they did, because I really enjoy them.

:)
Dee

I am going to say hostas. My yard is hosta land - I have my pretty favorites, but I have about 75 to 100 of those plain green ones that I inherited when I bought the house. Last fall, I dug up some of those plain old hostas intending to find them a new home and never planted them or gave them away...whoops.
And this spring - they are looking better then the ones in the ground and the first to sprout leaves. If you want some of these - and they are just plain, no frills hostas - come on over and take them. I am fairly sure they cannot be killed and they seem to need no soil...ha ha. I do like them as fillers with other more decorative plants.
Kathy

I had bearded iris on my rock ledge. The dwarfs in particular were in extremely little soil.

Right now I'm in the process of quarrying the rock ledge for paving stone.