I live in Pa myself and to be honest ... I think pear cacti make for great wall plantings and keep winter green well when suspended above the snow from a wall. Hen and chicks ... compliments this as well and stay green here. Arrum italicum pops up in mid winter but needs flat ground. many types of juniper would suit but one day make the wall dissapear. I get rave reviews from one of my customers ponds when I slip a few dozen burrows tail clones in the four foot wall above the pond I built her each spring even if it dies every winter. I like to put purple foilage clones in as well. I am the only one I know that uses tropical clones and it really does do wonders for the parent plants to be cut back for vigor in my house.

silent1pa, thanks for your response. Do any of those plants, or others, root right in the rock with minimal--or no soil but adequate water? As a pond environment with liner this kind of planting option is limited as far as soil goes. or is this a dumb question? Soil is available at the top of this crevice I would like to plant but I hope to plant lower. Thanks

Opuntia fragilis is known to root in and on rock ledges in minimal soil and my grow best this way. Those I have planted in the cactus garden are very shy bloomers but those I have in pots or troughs in poor to no soil grow slower but bloom well. so these might work for you. They are hardy into northern Canada so hardiness should not be a problem.

Dave
Milwaukee