How much humidity does narrow-leaved water plantain need?

Narrow-leaved Water Plantain genuinely is one of the humidity-hungry minority, wanting 60–100%. That is above what a normal room provides, so this is a case where intervention is worth it — dry air shows up as crispy leaf edges and browning tips. Only around one plant in six actually needs this, and narrow-leaved water plantain is one of them.

Yes — raise humidity around the plant deliberately. What works, in order: a small humidifier nearby (by far the most effective), grouping plants together so they share transpired moisture, or standing the pot on a tray of damp pebbles (the pot base must sit above the water, not in it). A bathroom or kitchen with a window often suits narrow-leaved water plantain better than a living room.

Relying on misting. It lifts humidity for a few minutes at most, then it is gone — it cannot hold narrow-leaved water plantain at 60–100%, and wet leaves overnight encourage fungal problems. A humidifier or grouping does what misting only appears to do.

Crisp brown leaf edges and browning tips are the reliable tell. Yellowing whole leaves is usually a watering issue rather than dry air, and slow growth in winter is normally low light — humidity gets blamed for a lot it does not cause.