Oncidium leaf turned yellow overnight!

[estación seca]

Yes, now is the time to repot that orchid. It may have new roots just at the base inside the pot, or it might be about to make some.

Rain water is great for orchids. Careful you don't wind up farming mosquitos.

Orchids that grow on trees (epiphytes) are accustomed to having roots always well-ventilated, and usually moist. When an orchid's leaves shrivel, it's not getting enough water. For some plants pseudobulbs always shrivel. With Oncidiums the thing to pay attention to is the leaves: Do they look good? When Oncidiums aren't getting enough water as leaves grow, they develop horizontal pleating on the leaves. This never goes away, but you can prevent it next time.

Shriveling can be due to not having water put on good roots, or not having good roots to absorb water.

Light has something to do with water use. Most Oncidiums don't want full tropical sun. In Miami they would do well under the canopy of a tree, or with perhaps only a few hours of early morning sun on a patio. When they get too much light they turn light yellow, or even develop brow sunburn spots on the leaves. What light exposure do your outdoor orchids get?

Are your outdoor orchids exposed to rain? Are they in 100% coir? Coir retains a very large amount of water. It may have no air spaces for the roots. I would not use coir for orchids outside in a rainy climate. I probably would not use 100% coir for any orchids other than perhaps bog orchids that need to stay swampy. It can be used in potting mixes that need to retain water, but I wouldn't think you'd need a mix like that in Miami.

Long-fiber sphagnum moss is a very good growing medium for Oncidiums if the grower can control the water carefully. I would not use it for plants outdoors exposed to rain in a wet climate. It would stay wet, and there would be no air at the roots. Inside the house it is a great medium; once you learn how to use it, you might only need to water your plant every 1-2 weeks.

For any epiphytic orchid, like Oncidiums, the mix has to be chosen so the roots have plenty of air, but they can stay moist. Some orchids really do like roots do dry between watering, but Oncidiums generally aren't among these. People tend to use bark chunks, perlite chunks, charcoal chunks and fired clay balls called LECA. You can get most of these at a good garden center. Hydroponics shops carry LECA and very large chunk perlite. People tend to use smaller chunks of things for orchids with narrower individual roots, like Oncidiums.

So decide how you want to grow your orchids. Always outside? Always inside? Both? Then pick a potting medium. For outside in Miami I would use something with fairly large particle chunks, like medium bark, LECA or perlite. The larger the chunks of medium, the more air the roots will have, and the more frequently you can water. If you can run water into a pot and it never pools in the pot, the roots will have plenty of air. This is good for people who tend to overwater, or for growing outdoors in rainy climates.

If your outdoor orchids are in 100% coir - take the one that you worry about the most, and pull it out of the pot. How do the roots look? Maybe post photos of the plant here. Oncidiums don't usually mind being pulled out of the pot for a look.

Also, any organic potting medium, like bark, moss or coir, breaks down with time. It happens much faster in a wet climate. Most orchids in media like these need to be repotted at least every 2 years. If your orchids are growing well, they will outgrow the pot in about 2 years anyway, and need a bigger pot. If you use inorganic media, like LECA or large-chunk perlite, it will not break down, and you can put off repotting until the plant really outgrows the container.

As to repotting the plant you showed us... find a pot about 4" / 10cm larger in diameter. Decide what medium to use. Unpot the plant. Soak the root ball in water for a while to loosen the old sphagnum. Pull off the old sphagnum. Try to be gentle to the roots, but you will break a lot of them. Strand by strand works well. It is going to take some time. Repot the plant in your new pot. Set it against one edge, with the smallest/oldest pseudobulb as close to the edge as you can get it, and the newer part pointing towards the far edge of the pot. Fill with your chosen media, using your fingers to push it into the old root ball. Try to get the base of the plant at the same level it was in the old pot. Stake your plant if it's wobbly. Water and watch it grow.