One of the least understood aspects of orchid-growing is proper potting and repotting, and an oft-heard question is “What is the best potting medium for [a particular plant]?”  Like most things related to orchids, there simply is no “universal” answer – what works for one may be tantamount to plant torture for another grower.  The best course of action for avoiding repotting woes is to do the following, something that will soon become “second nature” as you gain experience:

1. Understand the needs of the plant. Some need to be constantly moist, while others tolerate drying between waterings.
2. Evaluate and understand your growing conditions. Do factors like light level, humidity, temperature, or air movement favor rapid drying?
3. Consider your personal habits. Do you like to “mess with” your plants, watering frequently, or are you more of an “admire from afar” type of grower.

Once you have taken those into account,

1. Consider the properties of various potting medium components – size, shape, and especially water-holding capacity and drying rate, and
2. Consider how the choice of container can affect those properties, as well.

Then, armed with all that info, choose a combination – including, potentially, a blend of different media components – that works with your growing conditions and watering habits.
Now that you know what to container and medium to use to pot up your plant, how about understanding when to do so?

Without a doubt, the best time to repot any plant, whether changing to a different type of culture or medium, or just replacing old medium with the same stuff, is just as new roots are emerging from the base of the plant. The logic relates to root cells:

1. As roots grow, the cells “tailor” themselves to function optimally in that environment.
2. Once they have grown, those cells cannot change.
3. Moving them into a different environment immediately makes them “sub-optimal” and they will no long function efficiently, and will ultimately fail.

If the repot occurs just as new roots are emerging, those will be optimal for the new environment, and will support the plant as the old roots fail, resulting in a rather “seamless” recovery. So what shall we do if the plant must be repotted immediately (a new plant that came in with decomposing medium, for example), but does not have new roots emerging? That’s pretty straightforward, but it takes a little extra care.

Recognizing that the plant’s root system will not be ideal for the new medium, we’re going to have to “pamper” it a bit, until those new roots emerge, grow, and take over:

1. Unpot the plant and remove as much of the old potting medium as possible, trimming any dead roots as you go.
2. Let the plant sit, unpotted, for a few hours to allow the wounds to dry up.
3. Immerse the bare-root plant in Kelpak solution for an hour or more to stimulate root growth.
4. Premoisten the medium. I like to trickle boiling, or at least very hot water over it, wait 15-30 minutes and repeat; when it’s cool enough to touch, it’s ready to use That double-wetting process “opens up” the medium, allowing it to absorb more water and retain it longer going forward.
5. Make a “mound” of medium in the pot, and spread the roots evenly over it, then add potting medium over the roots to the appropriate depth, pressing it down firmly to make contact with the roots.
6. Stabilize the plant with stakes and/or clips, if necessary, to prevent wobbling.
7. Water it in thoroughly with the Kelpak soaking solution.
8. Invert a clear plastic bag over the plant and pot, simulating a greenhouse. No need to close it. As the plant’s roots will not be optimal for water absorption, but the foliage can still lose water through transpiration, the saturated-humidity environment slows that process and prevents desiccation.
9. Set the plant in a warm, shady location (direct sun will turn that “greenhouse” into a “broil-in-bag” environment instead!).
10. Water as needed, keeping the medium moist, but airy.

In a few weeks, new roots will grow down into the fresh medium, so the bag can be removed and the plant transitioned back to it’s normal growing location.

Speaking of transitioning, there is no such thing when it comes to roots. I have heard of folks wanting to convert from straight bark to semi-hydroponics wanting to make that “transition” by adding sphagnum to the bark, making it a somewhat more moist medium, then switching again to semi-hydro culture at a later date. Another ploy is to switch to LECA and the semi-hydro pot, but let it dry between waterings. Don’t do it! ANY change in root zone conditions means that the root system must be replaced, and that is stressful to the plant. Making an intermediate change just means that you’re subjecting the plant to such stresses twice. “Bite the bullet”, make the change once, and baby the plant while it gets its new root system.