My orchid has one yellowing leaf. What's gone wrong?

Question

I have an orchid that I bought about a month ago. I'm following the watering directions that came with it (which means it's only been watered once so far) and I believe that it has the appropriate amount of light. But one of its leaves suddenly became yellow all over.

What might have caused that and what can I try to fix the problem?

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Further information: The tag that was attached suggests that it is a Phalaenopsis orchid and the medium it is planted in is mossy and moist to the touch. It has two stems and the flowers on the side with the yellowing leaf have fallen off, but the flowers on the other stem are still in place. There is a much larger leaf above the yellowing leaf that is still green and healthy as are the two leaves on the other side. Does this help?

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Final update: I took courage and poked around in the pot. It seems the medium is entirely moss and that Phalaenopsis prefer bark. Therefore, my plan is to repot the orchid in bark and see how it goes.

Answer 1

What type of orchid is it and what type of potting medium it planted in? If its roots are in bark, I suggest it needs more frequent watering. I water my bark-potted orchids at least once per week, twice if it's hot and dry.

In response to your additional information: Phalaenopsis orchids in their natural habitats typically grow in trees or on rocks and don't like their roots constantly exposed to wet (more than moist) conditions. For that reason, domesticated Phalaenopsis are usually potted into a medium of free-draining bark chips that's kept moist by watering once a week or so, as needed. Try digging around in the moss to see how wet it feels under the surface and if there's much air space in there for water to drain through freely. If it feels like the water really sits in there, it could be that your orchid's roots are too damp. If that's so, I suggest re-potting into orchid bark.