I never used moss before so... For those orchids that like moist (in this case, oncidiums and stanhopeas) what are the signs to look for that tell me it's time to water?
I try to keep it evenly moist by standing it in a shallow tray of water. By allowing it to wick the water up from the bottom, it helps avoid the top-down compression that eventually will suffocate the roots.
I grow my Stanhopea alliance plants in 100% moss, and find that overpotting them makes keeping them moist much easier. Sitting them in a tray would work well for Gongora and others that spike upward from the base of the pseudobulbs and then hang down, but could be problematic with those that spike straight downward.
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I could not grow Oncidium hybrids in sphagnum without wrinkled leaves on new growth until I began standing them in trays of water. I let the tray go dry for a day or so, then add more water to achieve a 2cm water level. I have to refill the tray every 2-5 days. They use a lot more water than Cattleyas.
As a bonus, I now also have some nice ferns that came from the Big Island of Hawaii with the plants.
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Compare the weight of a fully hydrated plant. Then compare the weight of a dehydrated plant. This is one sure way to know it is time to water.
My Oncidium are watered one day after I think they need water. I water less in the winter and then from spring into fall, I water them more often. My Oncidium are wick watered (as Ray advised) regardless of the media. I hate seeing accordion leaves!