Help with yellowing cattleya and dendrobium

[Mtf1234]

Hi, new to orchids. Got a few young plants back in August. Most are doing well but two are not. I have them in a greenhouse cabinet that’s 69F/50% RH at night, 75-79F/85-90% RH during the day with fans and lights on for 13 hrs. I soak pots for 15 mins after the medium feels dry, every 5-9 days.

One of my cattleyas has been struggling since January. Its old leaves started to yellow and the new growth it had when I got it hasn’t grown (but has remained green). I added sphagnum to its roots initially in case it was drying out too fast but that hasn’t helped.

One of my dendrobiums started to yellow/drop leaves about a week ago. Not sure what has changed other than it may have gotten warmer outside, but I keep the AC on. Any help would be greatly appreciated. Thanks

[estación seca]

Welcome. They're both severely, severely underwatered. The Cattleya will probably die. In those conditions you might water every 2-3 days. The light intensity is probably too high as well.

[Mtf1234]

Thanks for the tips, that makes sense. I will water them more frequently and see if they can be salvaged. Any advice for knowing when to water?

---------- Post added at 05:40 AM ---------- Previous post was at 04:53 AM ----------

Also is it worth repotting? I’ve noticed that these orchids have far and away grown the fewest new roots since being moved from the plugs they arrived in to bark — was my repotting badly done and preventing their uptake of water? They seem to have shifted around so that more of their roots are exposed to the air.

[Ray]

Originally Posted by Mtf1234

Thanks for the tips, that makes sense. I will water them more frequently and see if they can be salvaged. Any advice for knowing when to water?

As is, unfortunately, all too common, you’ve been taught to think about this backwards and I’ll bet you are being very wary of overwatering.

Water does not cause root rot. “Under airing” - AKA suffocating - the roots by saturating a lousy substrate is the cause of root death and decomposition. That mix you are using appears to be so open and airy that it can likely be kept constantly moist.

Water is the true driving force for growth. The more a plant can take up, the better it will grow. The trick is learning to keep the roots airy at a the same time.

Also is it worth repotting? I’ve noticed that these orchids have far and away grown the fewest new roots since being moved from the plugs they arrived in to bark — was my repotting badly done and preventing their uptake of water? They seem to have shifted around so that more of their roots are exposed to the air.

as roots grow, they “tailor” themselves on a cellular level to function optimally in that environment. Once they have grown, they cannot change. If you change the environment, you have immediately rendered existing roots sub-optimal and they will start to fail. That’s why the best time to repot is just when new roots are emerging from the base of the plant, as those roots will grow optimized for the new conditions and support the plant. (That is also why you cannot just start watering a lot, as the roots that have grown attuned to the drier conditions will suffer.)

The first thing you must do in encourage new root growth, and using a rooting hormone or biostimulant like Kelpak can go a long way toward helping that.

[Mtf1234]

Interesting, thanks for this explanation. So I should not start watering more, but rather use rooting hormone to get new roots? Can I do anything with my watering schedule to help? Once new roots begin to appear should I repot? How can I improve my potting medium/potting technique for next time?

Sorry for asking so many questions.

[Ray]

Personally, I doubt that cattleya has enough stored resources to recover. The dendrobium might, so keep it warm and very humid, and keep the medium moist, not saturated.

[estación seca]

Yes you need to water more frequently. Don't let the roots become completely dry. You can't water by calendar; you need to look at the plants to decide.

In habitat roots are well aerated and stay moist throughout the rainy growing season. Your plants may have no viable roots remaining. They may survive long enough to make more roots and take up more water.