Why is my plant dropping leaves?
Sudden leaf drop is nearly always a change the plant is reacting to, not a disease. The question to answer first is: what changed in the last three weeks?
The causes, and how to tell them apart
Relocation or environment change most common
How to tell: Started within days of coming home from the shop, being moved to another room, or the heating coming on.
What to do: Pick a spot and leave it there. Most plants stop dropping within three weeks once conditions are stable. Figs and crotons are the worst offenders and the slowest to settle.
Overwatering common
How to tell: Leaves yellow before they fall, soil is persistently damp, drop starts from the bottom.
What to do: Let the soil dry down and check drainage.
Cold draft seasonal
How to tell: Drop concentrated on the side facing a door, a window, or an air vent, often after a cold night.
What to do: Move it away from the draft. Cold air pouring off winter window glass is enough on its own.
Severe drought common
How to tell: Leaves go crisp and brown before falling, and the pot is very light.
What to do: Rehydrate by bottom-watering, then keep to a consistent schedule.
By species
The likely answer differs by plant — these species have their own version of this problem:
Plantroster compares this month's photo against last month's, so slow changes — the ones you stop noticing because you see the plant every day — get flagged.
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General horticultural guidance, not a diagnosis of your specific plant.