Question
Hello Jim,
I just bought an Oncidium orchid at an orchid show 4 days ago, it is potted in a fine medium dark brown with little white particles. I watered it the day after I got it with tap water (I am on a wheel and I use the same water for my phal and this one is doing great).
My question is that after 4 days there is a white mold growing on the medium, when I brought it home the medium was almost dry and finely packed. How do I get ride of the mold. I am afraid of repottong, because I have never done it before, also I just got it, so I was expecting the medium to be ok. Do you think if I get a little fan to increase air circulation that will help with the mold, if I keep the tot of the medium dry? Is there another solution for this that does not involve reppoting that I can try before?
I apreciate any help.
Veronica

Answer
Hi Veronica,

From your description of the potting medium, it sounds like it may have some fine grade perlite or sponge rock in it, what the dark brown stuff is I'm not certain of at this time without more information about it.  

It may be that the mold, if it is mold, was present when you bought it but was not evident until you watered it.  If it is mold, it needs to be taken care of soon before it gets worse.  Unless the orchid had obviously been recently repotted, it is difficult to know how long it has been since it had last been repotted.  

There really isn't much to repotting an orchid, it just takes a little time and effort amd gets easier each time you do it.  If the oncidium is in bloom, I'd wait until it is finished before repotting but if not, you can repot just about anytime with early spring the best.  If you like, here are some instructions on how to repot.  

First, get the necessary repotting supplies together such as orchid potting mix (fine grade is best for oncidiums because of the thin roots), new pot that is just a little larger than the current size, cutting shears.  Once you've got things ready, place several layers of newspaper on a table, remove the onc. from the pot, gently remove the loose old potting mix from the roots (if it is tightly packed leave it alone), remove any obviously soft rotten roots.  Put a little potting mix in the new pot and set the onc. down into it so the bottom is just below the pot rim then fill around it with more mix.  Once full, gently press down on the mix to compact it and add more mix.  Set the pot in a warm shady place for 2 to 3 weeks, water only enough to keep it barely moist.  Put it out where it will receive good morning sunlight with mostly afternoon semi-shade.  

Having a small fan to circulate the air is a good idea as orchids love it.  It is best to not direct the fan constantly at the orchids but around them, if that makes sense.  An oscillating or one that rotates is ideal for this purpose.  A fan won't reduce the mold once it is established but it should help prevent future molds.  

Hope this has helped.  Good luck and good growing.

Jim Kawasaki
San Jose, Ca.