Cattleya's In The US & AUS
 

Somewhere along the journey I have been told to be very careful about making sure the cattleya's root systems are to be very advanced before repotting them into a larger size pot.

My question is why?

Has it anything to do with US virus, bacterias & fungus?

Looks like I need to make sure that the differences between growing in the US & AUS are clear and not muddied as the differences are looking like they could be vastly different.

Regarding root mass and pot size, you want a large root mass relative to pot size to prevent the medium from staying too wet and encouraging fungi/root rot.

I think that advice is just an extension of the advice to not repot until you see new active growth. Most of the time people associate that w/new roots. It doesn't have anything to do w/disease or virus but rather that, typically, the orchid will adjust to it's new set up quicker if it's in active growth. Some catts can actually be set back if repotted at the wrong time. I personally have never had a problem with this but I hear there are some catts that are so fussy about repot schedules taht they can stall out for a year or more if repotted at the wrong time.

BTW - some catts produce the growths before the roots and some do the opposite and because I grow under lights a large portion of the year, I get root growth year round. As a matter of fact, I see a lot of new root development over the winter. For that reason, I mainly stick to a repot schedule that is based on new growths rather than seeing new roots. Well, for most of my catts...sometimes I just have to repot when I can get it done and the growths are farther along than I'd prefer but I've never had any that seemed to mind the less than ideal timing.

While not exact, the rule is that bifoliate Catts should only be repotted when there is sign of new roots developing (about 1/2" long).

The extreme case is Catt amethystoglossa, for which this is a MUST. Root growth starts when the new growth is about 1/3-1/2 of the eventual height. If you miss the window, it will not grow new roots till a year later.

Thank you all very much for taking the time to share!