I suspect that the most significant thing a new orchid grower needs to understand is the need for water and air at the roots.
One of the more significant evolutionary adaptations of epiphytes is the use of thick, waxy cuticle layers on the vegetation, a strategy intended to reduce transpirational water loss. As that greatly limits foliar gas exchange as well, much of that has shifted to the root system.
When we water an orchid, most of it simply pours right through, some is immediately absorbed by the plant and the potting medium, and some is held by surface tension in the void spaces in the medium. If the potting medium is too dense, whether that be due to the use of too fine of a particle size, decomposition of organics resulting in smaller particles, or simple compacting of the medium (most significant with sphagnum moss), then a significant number of those voids can become "clogged" with water, cutting off the air flow to the root system, suffocating them. They then die and rot.
That means that we orchid growers must find a potting medium and container combination that holds enough moisture to make the plant happy under their individual growing conditions and personal tendencies to "mess with" their plants or not, but not so much that it suffocates it, and be sufficiently attentive to repot when the existing potting medium starts to become a problem.
It looks as if your dendrobium is there now, and has been for a while.
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