Jan
I'n no expert on hardy Pings but I have seen two alternatives regarding
vertical soils. First, for european species actually mounted on a
vertical surface, the water was dribbled down from above but not
directly onto pots. Instead the plants were potted into soil in clay
pots and the pots placed in holes on a vertical mount (actually slightly
sloping). This way all water enters only through the clay pots so soil
can not wash away.
As to the alternative, I grow tropical plants that are natural growers
on vertical rock. P. jackii is the perfect example. I grow it
horizontally which will only really affect the angle at which any
flowers are eventually produced (I should be so lucky!). To avoid
overwatering, although the plant pot is standing in water, the plant is
mounted on rock bedded into rock powder so all water must be sucked up
to the rock by diffusion and the porosity of the rock ensures there is
never too much water.
hope this helps a little.
Regards
Paul