I'm in zone 5a, so it's not exactly the same. That being said, I've had very good luck with elfin thyme. It's very, very low and doesn't seem affected by stepping on it. I used it on a path built on pure sand with about an inch of topsoil between stones. For several years I'd cut off pieces and move them to the parts that were bare. Now the whole path is covered. I can't even see the stones, so I have to do some serious thyme removal. My rocks are much smaller, so I don't think you'll have as much of a problem.

{{gwi:285820}}

I plan to move my extra thyme pieces to a patio I'm building with rocks similar to yours. My problem is that at least half of my patio is in shade, so I don't expect the thyme to do well there.

Here is a link that might be useful: More garden pictures

Wow that looks great! That's the look I'm going for, so I think I'll give the elfin thyme a try. That fire pit patio's going to look really nice when you finish, too. Thanks.

I am using purple Alyssum in between my flagstone deck. They are doing well here in Z9. Surprisingly, they can take a bit of traffic too, the wheels of my wheelbarrow and yard waste bin haven't been doing much damage, if any.

I am waiting to see if they self seed. If so, I am sold on it.

I figure I'll keep updating this to see what works in case anyone happens upon this via google.

8/31
I picked up about 20 3"x3" (guessing) pots of the following over the weekend at a 1/2 price sale:
Thumus Praecox 'albus', Sedum rubrotinctum 'mini me', thymus praecox 'purple dwarf', sedum acre 'aureum', and thumus praecox 'pink chintz'.
I cut each pot into 3 or 4 plugs and planted as follows: dig out sand, add base of leaf mulch, plant plug and fill in with some pure leaf mulch, pour in a few oz. of Bio-Tone Starter Plus, top off with a layer of sand/leaf mulch mix, one more dump of the Bio-Tone mix.

A few hours after later after I planted approx. 80 plugs, I gave the whole patio a good soaking with the hose.

That's where I'm at right now. Tried to mix up everything in differents spots like full sun and shade, high traffic and low traffic. Will post some pics soon.

Don't use pea gravel in joints - it does NOT pack and stay packed, so it does not stay in the joints.

Use crushed stone with sharp edges that can interlock. AZ uses crushed granite, dunno what Philly has locally available.

Your idea of mixing it up and letting them fight it out is great. You will get a natural mix.

I was looking at it today, and I think I'm gonna double up on the planting and then looking at crushed stone options. Are you saying that once I'm finished planting, to fill in the rest of the joints with the crushed stone? yours are much thicker stones, but I was thinking about spooning out about an 1" of sand if needed so ensure at least that much gravel in each joint.

BTW, we're in the middle of a week+ stretch of dry weather here, so I did a quick soak of everything in the morning and then after work. I figure keep them a little moist until the dry spell passes and the wet fall comes?