Answer: Brown edges on houseplant leaves are usually caused by one of two things: overwatering or salt burn. Peace lilies (spathiphyllum) are particularly susceptible to brown edges caused by overwatering because they require consistently moist soil. As you may have noticed, if the soil dries out too much, the plant wilts dramatically. For most houseplants, however, it's a good idea to let the soil dry out somewhat between waterings. If the edges of new leaves are browning, I would suspect overwatering.
Salt burn occurs when various mineral salts, from water and fertilizers, build up in the soil over time. Browning usually occurs on the old leaves first. The excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. To prevent this from happening, water plants deeply and slowly. Then, about once a month, flush accumulated salts from the soil by watering plant thoroughly, letting excess drain, and watering again. Similar symptoms occur when too much fertilizer has been applied, or when the fertilizer wasn't properly diluted. Always water plants thoroughly before applying fertilizer to help prevent burn.
Finally, very dry air -- a common problem in heated homes in the winter -- can cause brown leaf edges. Try increasing the humidity around the plant by placing it on a tray filled with pebbles and water -- make sure the plant isn't sitting right in the water. As the water evaporates, it will increase the humidity around the plant.
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