The most common repotting mistake isn't technique, it's timing — moving a plant into a new pot far more often than it needs. A plant is generally ready for a larger pot when roots are visibly circling the inside of the nursery pot, growing out of the drainage hole, or when the plant needs watering unusually often because there's more root than soil left to hold moisture. Absent those signs, an annual repot isn't necessary and can do more harm than good, especially to slow growers like ZZ plants, which actually prefer being slightly snug. The check is worth doing rather than guessing at. Water the plant the day before, then tip it out of the pot with a hand over the soil surface and look at the sides of the root ball. Roots wound into a spiral, or a mat dense enough that the soil holds its shape on its own, means it is ready. Soil that crumbles away with roots still running through the middle of it does not, and the plant goes back into the same pot with nothing lost. A plant that is growing well and holding its water for a normal stretch doesn't need a bigger pot to keep doing it. Replacing the top inch of tired mix each spring, and feeding through the growing season, carries most houseplants for years in the pot they are already in. When it is time, go up only one pot size — roughly two inches in diameter larger than the current pot. A pot that's dramatically bigger holds far more soil than the roots can use, and that excess soil stays wet long after the plant needs water, which is a common cause of root rot after a well-intentioned repot. Gently loosen circling roots at the bottom and sides before setting the plant in new mix, water it in well, and expect a short adjustment stall — a week or two of little visible growth is normal and not a sign of failure. Gravel or broken shards in the bottom of the pot do not improve drainage. Water is held at the boundary between two materials rather than crossing it freely, so a gravel layer lifts the wet zone up into the roots instead of draining it away. A drainage hole and a mix that lets water through are the whole of it. Set the plant at the depth it was already growing at, no deeper, and keep it out of direct sun and off fertilizer for a week or two afterwards while the disturbed roots catch up. The best time to repot most houseplants is during active growth, spring into summer, rather than winter, when roots recover more slowly from any disturbance.
Repotting Without Killing Anything
Most houseplants want to be repotted less often than people think.