How to gauge watering of a potted, rock mounted orchid with roots below substrate?

[springlizard]

A friend of mine gave me a Dendrobium kingianum and told me that it was quite hardy and forgiving. However, I haven't figure out yet how exactly to tell when it truly needs watering or if I am over-watering. The problem is that it is rock-mounted and potted in a clay pot. Also, there is mulch substrate to fill the cracks and retain moisture, so I cannot look for green/white/brown roots. I'm sure this is great for a species that is normally a lithophyte, but it also means that I cannot base my watering regime on how light/heavy the plant feels, which is how I normally do. My second method is normally to look at the roots and water them if when they are totally white, but I can't do that when I can't see the roots. I recently noticed that a leaf was senescing and I wondered if it was rotting or normal growth and death. The canes seem ok, and the plant is actively growing, so I think everything is fine. However, temps are cooling down and I want to make sure that I do not overwater this thing before I subject it to its winter rest. As of now, I water it once or twice a week and it is grown under fluorescent lights. During the summer, it is an outdoor critter.

Here are a few photos of the problem leaf and also the canes.

Dendrobiumkingianum by Kyle JW, on Flickr

Dendrobiumkingianum by Kyle JW, on Flickr

I welcome any culture tips or criticisms.

 

[afrodisa]

Normally water on leaf + sun = burned spots on the leaf but you are growing under lights? The yellowing, shriveled leaf is normal for old leaves.

 

[springlizard]

Ok good to hear. That yellowing, shriveled leaf was the one I was worried about. As for the burn spots, that's from being outside during the warm season.