Inconsistent info on dendrobiums in S/H

[Subrosa]

Mine gets night temps as low as the mid 40s, and is kept very dry, a watering about once a month since October. It's starting to develop buds for the first time. Hopefully I don't screw it up!

[Aluka]

I flush once a month and fill all the other weeks never had an issue

[Ray]

Originally Posted by Aluka

I flush once a month and fill all the other weeks never had an issue

No issue....yet.

In S/H culture, the evaporation occurs from the top of the pot. The wicking action of the medium draws the solution up, and the closer it gets to the top, the faster the evaporation rate.

As the solvent (water) evaporates, the dissolved solids in it - mineral nutrients and plant wastes - precipitate out, and get concentrated within and on the medium particles. Flushing monthly only rids the pot of some of those precipitates, so over time, they build and build, ultimately reaching toxic levels.

If, on the other hand, you flush at every watering, the rate of buildup is tremendously reduced.

If you use S/H culture in a humid environment, and always water before any of the medium dries out completely, there will be very little precipitation at all.

[Aluka]

As you know the mineral residue will redissolve in the water when the pot is being flush. If flushed improperly it will only remove some of the residue, but if flushed properly most of it will redissolve and get wash away before any toxic buildup can happen. So if you are having trouble with 1 flush a month, then you should consider changing the way you flush your pots.

Also considering the fact that my aunt has been doing it like this for the last 10+ years without issue (1 flush a month). So if its a matter of not having an issue YET after 10 years, i'll tell you when it becomes an issue in about 10-15 years, if i remember.

[Ray]

Originally Posted by Aluka

As you know the mineral residue will redissolve in the water when the pot is being flush. If flushed improperly it will only remove some of the residue, but if flushed properly most of it will redissolve and get wash away before any toxic buildup can happen. So if you are having trouble with 1 flush a month, then you should consider changing the way you flush your pots.

Also considering the fact that my aunt has been doing it like this for the last 10+ years without issue (1 flush a month). So if its a matter of not having an issue YET after 10 years, i'll tell you when it becomes an issue in about 10-15 years, if i remember.

Aluka, I am not trying to tell you what to do, but am trying to bring some scientific facts to the discussion so folks don't get misconcenptions.

I invented semi-hydro culture for orchids, so probably have the most experience with it, and I also have background in science and engineering, followed by a career in the chemical industry, so can tell you for certain that once many of the minerals in water and fertilizers precipitate, they will not completely redissolve without a very extended soak in a large volume of water. No flush is 100%.

I use RO water, feed with very little fertilizer - 25 ppm N - and flush heavily at every, very frequent watering, and there is mineral and waste buildup in the pots.

Maybe what you perceive as "no issue", I might take differently.

[Aluka]

I guess that is possible. For me no issues means orchid is alive and flowering regularly and producing new growth appropriately. But I guess for you it might mean something different.

Like I said I flush once a month and so far no issues, the person that taught me has being doing it for years and also how also has no issues. However if we ever do we'll report back.

[Ray]

Mineral and waste buildup occurs with all media. We change out organic ones relatively frequently (or should), so we "throw away" the problem. With inorganic media, we can get lulled into thinking that since it doesn't decompose, we don't have to repot, and that is where we get into potential problems with toxicity.

However, there are SO many factors that affect the rate of buildup, that it's impossible to say with certainty when one should do something about it.