Morning Guys!

These marks could be caused by a number of things but without an in-depth labratory analysis, I guess we'll never really know exactly what it is and although I didn't agree with Des' fertiliser suggestion I still strongly believe he, (like Calvin) is right in that its a chemical reaction not an organic one (mold).
I suppose in the final analysis, our plants can give us enough problems without looking for them, those marks have been around for as long as terra cotta pots have. The plant will tell you if something is wrong, until then why look for problems which may not exist.
I found a couple of articles, extracts reproduced below (hope I don't get nabbed for copyright!), one is from a professional potter, the other a University horticultural lecturer:

"Earthenware pots are quite porus, and, in addition, if they are from an Asiatic source the clay may not have been properly washed/prepared and may contain various salts that may react with your plants as they leach out from watering, especially if the water you are using is rainwater,which tends to be mildly acidic! (Similar to efflorescence you get on some brickwork, it looks like a white powdery coating)!"

"Unglazed clay containers are popular, but can be heavy and also tend to accumulate unsightly salt residue as water evaporates from the surface. This residue is generally not harmful to the plant, unless it accumulates in the soil"

Unfortunately the second extract doesn't hint where the salt residue originates from, the water or the pot and although they may or may not have the answer, both suggest it is still a chemical rather than organic reaction.

I suppose you could always move your collection over to plastic pots Chris? ;)

Pals

Thanks for the research Pals.

I put all of my orchids in the clay because I am a "waterer", but I have just ordered some clear plastic replacements. I will eventually repot everything into the plastic, and will, I believe, solve the problem.

After closer inspection last night, I noticed that it happened on 3/4 of my pots, but not all, lending credence to the "poor clay" theory. The more expensive clay 'orchid' pots are clean.

Thanks to all who responded.

IF that's the solution Chris........you're welcome!
Hope you haven't got toooo many orchids to move over to plastic, that could be a task?
Feeling slightly guilty that we may have hi-jacked Emily's original green? mold topic on top of S/H pellets and turned it into a white line on base of pot topic!
Sorry Emily!!!!, hope you get an answer too!

Pals

:hijacked: Yes, Sorry for the Hijak Emily, although, our problems might be the same. Your mold/mildew is on the clay also from the sound of it, but yours is in the medium where mine is on the outside.
I hope you received a good answer as well.
:goodluck: