gal,

I see the volcanic rock mostly from plants that come from Hawaii (imagine
that from volcanic islands).

It works for them but I find that I have a lot of root loss when I try to
grow in pure rock. Remember that each plant is different and requires more
organic matter (bark, moss chunks, coco husk) than other plants.

The type of pot you grow in also affects the medium you put in the pot.
Plastic pots reduce evaporation so a very dense medium will not dry out as
quickly in plastic as it would in clay.

Try growing your Dendrobium for a while in the rock before you convert the
other plants in your collection. You may find that it worked great for the
grower you bought it from but with your watering schedule it may prove to be
too dry for your culture.

Remember that you don't want to be a slave to your plants. My philosophy is
that if it won't grow under my culture then I shouldn't grow it!

Good growing,
Gene