Why do some vanda leaves turn yellow and drop?

[Keysguy]

OK, so I'm putting this question here in the beginner's forum in spite of my possibly misguided self-esteem that I am no longer a beginner at this crazy hobby. I've done a search and although there are historical threads that touch around my question, none seemed to specifically address what I'm going to ask. So, here goes.

I have maybe somewhere in the neighborhood of 50 vandaceous orchids hanging in my shadehouse here in the Florida Keys.

I have an off and on situation where some of the plants will have bottom leaves turn yellow and eventually drop but it doesn't happen to all of them (maybe 10% +/-). I have several that have never lost a single one. I'm going through a spell right now where there are quite a few actually and the only thing I can think of is we had a little "cold snap" here while I was up north for the holidays. And by that I mean maybe it got to 55F for a couple of hours. I wouldn't think that would be cold enough to cause that kind of stress but what I'm noodling on is whether it was cold enough coupled with watering from the automated misters?

This isn't a catastrophic thing, I'm just trying to figure out "why" and then correct a culture problem if I have one.

Like I said, they don't all do it and I've never been able to put my finger on a cause before but those couple of chilly nights are making me wonder....... if that might be "a" cause or contributing factor. And that's not to say they don't do it other times of the year because they occasionally do.

Anyone have thoughts?

[dbarron]

I'm not sure this is the answer, but was something I was musing this week here too, with asst orchids (non Vandas). I think it's just less than ideal conditions (cooler and darker). I expect in about 2 months a big flush of growth to start (if I remember correctly?). Seasonality....

[estación seca]

Motes mentions in his book that 55 degrees F / 13C is the lowest Vanda hybrids should go. He says to expect serious leaf fall from 50 F / 10C on down.

[Roberta]

I suspect that there's going to be variation in the cold tolerance of Vanda hybrids, depending on ancestry. Even if most of of them are predominantly V. sanderiana, the amount of other species is going to vary - and some are pickier than others. So the little temperature jolt will get laughed off by most, but some will be "offended". Getting down into the weeds of pedigree may help to identify which are the most sensitive ones.

[Toadwally]

Originally Posted by Roberta

I suspect that there's going to be variation in the cold tolerance of Vanda hybrids, depending on ancestry. Even if most of of them are predominantly V. sanderiana, the amount of other species is going to vary - and some are pickier than others. So the little temperature jolt will get laughed off by most, but some will be "offended". Getting down into the weeds of pedigree may help to identify which are the most sensitive ones.

I don't laugh it off. In fact, to add to your comment, the grower can consider placement of plants, too. Windward side of shade house v. in center or lee. Also, consider wind, which will aggravate effects of cold.

[Keysguy]

Excellent points.

When we get a winter cold front it comes from the north and as the shade house is up against the south facing wall of the house it's actually protected from those chillier north winds.

Our prevailing winds are easterlies (from the Bahamas) and the vandas are in the northeast quadrant of the shadehouse under misters that run for 10 minutes, twice a day. First light and 8 hours after which is around 3 PM these days. East, south and western shade house exposure is unrestricted except obviously for the shade cloth itself.

I like the concept pf parentage of the hybrids. I guess to do this properly I should take an inventory of the ones affected and run them down. I'll do that tomorrow if I get a chance and report in. And remember that I said I haven't really correlated this to a temperature thing yet but maybe it is a "thing".

In the meantime......keep "noodling"

[Ray]

While genetics can certainly play a role, I think it’s also worth considering watering. (Speculation, based upon my observations.)

Folks typically water less when the temps drop, and vandaceous plants have a lot of surface area and little in the way of storage, so need to be watered quite a lot. Maybe the plant is ditching “excess transpiration area” to reduce losses.

I used to experience exactly what was earlier described before I found a way to automate watering in my greenhouse while I was away on business travel.

[Ben_in_North_FLA]

I have seen many "palm trees" in my days in South Florida. In the vanda world palm trees are large vanda plants with just a couple or three rows of leaves at top and a long bare trunk between leaves and roots.
This is mostly due to the plant regulating the amount of foliage that it can sustain with the amount of moisture that it gets.
A well grown and hydrated vanda should not loose leaves at bottom unless there is a disease involved.
High humidity and watering commensurate with local conditions (more air movement requires more watering).
Generally well watered vandas with good air movement will dry completely in just a couple of hours.