When you contemplate repotting an orchid, you must develop the ability to look at your plant, see the root system and learn to interpret and understand exactly what you are seeing. Developing that skill is a difficult thing to master.
Number one. You must remember the simple fact that these are epiphytic plants. The new potting mix that you use has to take into account the root system of that particular orchid. An orchid with either short skinny roots or longer and slender roots does not belong in a mix of coarse particle size. That is a bad direction to go.
Number two. The opposite side of that coin is also generally true. Any orchid with pencil thick roots does not belong in a fine mix featuring very small sized pieces. You should always look at the roots to determine the proper mix. This ability is very hard to develop. Haphazardly repotting your orchids into "whatever mix is on hand" is not really the best idea!
The key to good healthy orchids is the root system. Your orchid pant can be growing in an ideal set of conditions, the best light, the best temperatures, yet without a healthy root system your orchid may never be happy. Also let me add here that being afraid to repot, or being neglectful about repotting, or turning a "blind eye" to the situation, is also a bad idea.
I am a person who believes in making custom mixes for all of my orchids! And here I go again, orchids are fairly complex plants. As Epiphytes, they grow onto a host plant for support only, they are not parasites. Orchid roots do not penetrate the host plant taking nutrients and moisture from the host.
I will bore you with a simple analogy, ALL orchids are plants, BUT not all plants are orchids! Just reflect upon that statement. It is so true. And yes, it becomes very hard for a great many plant growers to suspend their beliefs, or to forget their experiences with non orchid plants and not to apply those practices to orchid care.
Now for my last point. Failure to properly take care of your orchids potting and root system can lead to limited growing success in the beginning of your little orchids life but long term neglect will lead to declining growth, failing roots and ultimately the death of your plant. Well, not always death, but a plant becomes so sick, so downtrodden, it becomes very difficult to bring it back to health. [ There are many cases where it might be best just to toss out that orchid and start over]
One of my recommendations is for beginners growing orchids is for them NOT to use sphagnum moss! Why? Because in my opinion, it takes some level of experience to learn how to pot with it correctly. It is much easier, or much more common, for beginners to use sphagnum and end up killing their plant. The moss is either packed in too loosely, causing a problem by not holding enough water, OR it is packed in too tightly keeping the water from flowing through the media properly. If the moss is packed too firmly, the roots can suffocate or not get access to enough moisture.
