Try stuffing the holes with long-fiber sphagnum moss; the kind you use in hanging baskets and such. Then stick the plant, with it's soil and rootball into the moss. Keeping them watered will be your biggest issue. The more moss you can get into the crevices, the better it'll hold moisture. I've also seen it done with OASIS® Floral Foam.

What a great idea! And perennials will winter over this way, with just a little soil around the root ball? This sounds easy and would be a great trick. Thanks so much.

I'm guessing, but I'd say this would be the functional equivalent of growing something in a container, meaning you'd lose 2 zones of hardiness being above ground like this. Plants hardy to zone 4 and colder, should be ok. It's also possible the stones will retain enough of the day's warmth to sustain plants.

It also a good way to grow annuals like things which could cascade down the wall.

You're right, of course. If the perennials don't make it, I could restrict myself to annuals in this particular part of the garden. You've been very helpful. Thanks so much.

While I like the peat moss idea, I would still try to get soil in the crevices. Peat moss dries out pretty quickly by itself. I use a good organic top soil (not potting soil). The top soil is heavier and holds together better. If you have trouble with it disappearing into the rocks, incorporate some peat moss along with the soil. You'll have better luck if you use rock garden plants with low water requirements. Do a search on the internet and you'll come up with some ideas. Definitely use the mist setting on your hose because you don't want to wash away all your hard work.

Thanks, Nancyd. I think this is important enough to be worth an experiment -- plant some with top soil, some with
sphagnum moss, and some with a 50/50 mix. I probably won't write an academic paper, but I could post the results next summer! Thanks to all.

I've used old onion mesh bags or the black weed cloth formed into pockets to hold soil in a rip wrap stone retaining wall. Sedums, sempervirens, etc. love these conditions and can be planted in these pockets quite nicely. You could try putting some of those moisture retaining crystals in the soil too.

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Here is a link that might be useful: Photos of plantings

Thanks for the great tips -- and for the link! Those pictures show exactly the look I want. I have bigger stones and fewer courses, but that's the look. I'm beginning to think the succulents like sedum are going to be easier than some of the campanulas and veronicas I also hoped to include. I'm still going to try Serbian bellflower because I really love it.

"I've used old onion mesh bags..."

Yup. I was gonna say I use old stockings, fill 'em with good potting soil then poke a hole where your plant goes. I plant small ferns in my walls this way. Haven't had a problem with things getting root-bound but I suppose big stuff could. Brown or black work best 'cause you're less likely to see 'em.

What a creative bunch of people! I have lots of ideas to experiment with.