Orchids need re-potting every 1 to 2 years. This is because either there are too many roots in the pot and/or the medium that the orchid is growing in is breaking down. When the orchid medium breaks down, the orchid's root will suffocate from the lack of oxygen and will be saturated in water thus leading to root rot. root rot = dead orchid. So, here is how to repot a common phalaenopsis orchid, one of the easiest orchids too repot because of its thick roots.
This is the Phalaenopsis orchid that needs repotting.The best time to repot an orchid is either after flowering or when you see new roots growing. This is usually in the spring or fall. I don't recommend repotting an orchid when it is in flower, unless it is a phalaenopsis. I've caused many orchids to go tease with death from the stress repotting causes. Anyways, the reason I'm repotting this orchid is because it is growing in a sphagnum moss medium that does not fit with my watering schedule, I don't like sphagnum moss, and it just finished flowering. Like I said in a previous post, sphagnum moss scares me....it holds tooo much water.
Here are my supplies. I need newspaper to help contain the mess. 70% rubbing alcohol to disinfect my tools. A plastic pot. And a fine fir bark mix with perlite that can be easily found in a Lowes or Home Depot.
I cut the plastic wrapping that is the orchid's pot and remove the orchid. Since the orchid is growing in a plastic container, it is easier to remove and comes out in a solid mass of sphagnum and roots. In clay pots, you might have to even break the pot to remove the orchid roots safely. Then remove the sphagnum moss from the the root ball.
Then, using your sterilized scissors, trim off any dead roots. For phalaenopsis orchids, the roots should be nice and firm. They should not be hollow or squishy, so any hollow or squishy roots must be removed to prevent bacteria from causing any future damage. Once the trimming is done, you should dip the orchid roots in water to make it more flexible which makes it easier to place it into a pot. Also, this helps remove any leftover sphagnum moss from the roots.
Once the olive colored roots have turned into a grassy green, you can remove the orchid from the water and start finding a right pot for it. The orchid pot should be just large enough to hold the orchid's root. Any bigger pot might lead to root rot because of the extra water surrounding it. My rule of thumb is to find a pot that all the roots can touch all the sides of the pot, which is the safest for me. You can also add packing peanuts to make the large pot smaller, but the same rule applies: the roots should touch the bottom and the sides.
Once a tight fitting pot is found. You should place a thin layer of bark at the bottom of the pot. Just enough to cover the bottom. Place the orchid in the pot and squeeze any aerial roots into the pot. It is normal if you accidentally break any roots, the orchid will grow new roots. Finally add the orchid bark into the pot. With each scoop of orchid bark, shake the pot or tap it on the table to let the bark get in between the roots. You are done when the orchid bark has reached the top of the pot. The last step is the simplest.
Place your orchid in a bucket. Then water the orchid until the water reaches the brim of the pot, and let the orchid stay in the water for five minutes so that the medium can settle around the roots and the bark can retain water. Then YOU'RE DONE!!! YAY!!!!
After repotting care: If the orchid has a healthy root system I just place it back to where it was originally growing. If the orchid is not in active growth and/or has sustained a lot of root loss, place the orchid in a warm, shady location with lots of humidity. This help the orchid grow roots faster and regain it's strength for future flowering.
Precautions: Sterilize everything with bleach or alcohol or fire. This helps prevents virus or fungus or bacteria or unwanted pests from traveling from one orchid to another. Also, if you're repotting several orchids at once, wear gloves in between each orchid to prevent viruses from spreading by your hands. Repot in the morning so any water still between the leaves can evaporate by nightfall.
My miltoniopsis maui mist "golden gate" is a pain in the butt to grow. After it finished blooming, I discovered that only the top layer of roots was alive while the bottom mass of roots in the pot had dessicated and died. I sheared off all of the dead roots, potted it in bark, placed it in a warm and humid place, and prayed to the orchid gods for it to grow new roots. 
