Like any houseplant, orchids need to be repotted occasionally, especially when their potting mix has started to decompose (notably, the pieces of bark it contains will seem spongy to the touch), probably after 2 or 3 years of growth. The best time to do this is just before or as the orchid starts a new growth cycle, usually at the end of winter. If however your orchid is in bud or bloom at that season, delay potting until it finishes flowering… or snip off the flower stems.
The first step in repotting is… to find a suitable growing medium. Very few orchids can grow in a standard potting soil: you’ll need a mix designed for orchids, often composed of pieces of bark, coconut fiber, sphagnum moss, perlite, clay pebbles, charcoal, etc. You can find orchid potting mixes in any good garden center, but also at orchid shows and orchid society meetings.
Unlike many plants, it is not always necessary to repot into a larger pot, unless the pot is actually full of roots. This is especially true if your plant is a monopodial orchid (single stem orchid), such as the very popular phalaenopsis. Since sympodial orchids (orchids with pseudobulbs or multiple stems) produce offsets at their base over time, a larger pot may be needed… unless you prefer to divide the plant, which is also an option.
Note that plastic pots are better suited to orchids than ceramic or terra-cotta ones. If you use the same pot, clean it thoroughly before reusing.
Repotting a phalaenopsis
Since potting techniques varies slightly depending on the type of orchid, let’s focus here on phalaenopsis orchids (Phalaenopsis), which after all make up about 95% of all the orchids sold as houseplants. Therefore your orchid is mostly likely a phalaenopsis.
The day before repotting, water your orchid: that will make it easier to remove it from its pot. Also pour a potful of orchid mix into a bowl and soak it overnight to absorb some moisture. Simply let it drain before starting.
Remove the orchid from its pot without breaking the roots. To learn how, see No Damage Pot Removal.
With your fingers, remove as much of the old mix as possible, then cut off any dead, rotten or broken roots (they will appear brown or soft) with pruning shears, disinfecting them with rubbing alcohol between each cut to avoid spreading disease. Also remove any yellowing leaves or dead flower stalks. You can rinse the roots with clean water to remove any dirt.
Add some moist orchid mix to the bottom of the pot (no drainage layer is required). Center the plant in the pot at the same height it was in its original pot, spreading out the roots as evenly as possible. This includes the aerial roots which were probably sprouting from the pot and stretching in all directions. You can now direct them into the pot and cover them with mix if you prefer. Now work orchid mix evenly among the roots, filling the pot nearly to the brim, pushing and pressing down with your fingers as you go so that the plant is firmly seated. Finally, spray the growing mix water with warm water.
During the first weeks, go easy on the watering. Often a daily spritz is enough. When you see signs of recovery (usually after 2 or 3 weeks), begin watering as you did before. You can start to fertilize the freshly repotted orchid in about a month.