Why am I asking this question?
Because I noticed that the leaves were taking a lot more time to unfold than I have experienced before with this plant.
Secondly, although it has bloomed before, it did it always from green sheaths.
That made me conclude that although the conditions didn't changed with time, probably it was a low humidity issue. This year the high temps and low HR came a little earlier (which means temps over 30ºC / 86 F and HR frequently lower than 30%) so I think it was this the cause.
But I also noticed the sheaths were getting fat and not thin as normally a dry sheath.
Anyway, I think this answers my question:
So, now you know: Laelia briegeri blooms from both, green and dry sheaths.
Some more photos of the plant and its other fat sheaths:
Laelia briegeri - Question
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