Should I repot my 5-year old peace lily or leave it alone? I water it once a week and it is doing well.
Since your Peace Lily is doing well, there is probably no real need to repot it.
However, the soil it is growing in is probably depleted, or close to depleted, after five years, even if you have been fertilizing the plant over the years.
In general, you should repot house plants when the plant has roots protruding from the bottom of the pot, it is root bound (pot completely crammed with roots), and/or the plant has stopped growing and looks “listless”.
When you do repot a plant, never move it into a pot that is significantly larger than the one it has been growing in. For example, if it has been growing in a 4” (10 cm) pot, you could move it into a 6” (15 cm) pot, but an 8” (20 cm) pot would be too big. Many tropical plants, including the peace lily, prefer to be slightly potbound and may sulk if moved into a much bigger pot.
The basic technique for repotting houseplants is to start with a good quality potting soil. You may want to add a bit extra perlite to a commercial potting soil to increase its porosity, but this isn’t essential. Before repotting a plant, water it well and then wait 1 – 2 hours before repotting. Put a generous layer of fresh potting soil in the bottom of the new pot. Once this is ready, remove the plant from its existing pot – e.g., by tipping the pot on its side or upside down and giving the pot a solid bang on its bottom to release the plant, its rootball and the existing soil. Examine the root ball and carefully cut off any damaged, dead or rotten (soft) roots. Then place the entire root ball into the new pot and backfill around the root ball with fresh potting soil. Next, thoroughly water the newly repotted plant. You may need to top up the new potting soil around the edges after watering.
Don’t fertilize newly repotted houseplants for a number of weeks. Peace Lilies should only be fertilized every 2 – 3 months in any case, and then only with half-strength balanced fertilizer.