Peace lilies (Spathiphyllum wallisii) are valued for glossy, dark green foliage and occasional creamy white and lightly fragrant flowers. A number of environmental factors, as well as improper care, can damage peace lilies. Identifying the problem and treating the lily accordingly are key to nursing a peace lily back to health.
Peace lilies that have been subjected to extreme cold or extended periods below their ideal temperature range may not be able to recover. Cut back any brown or blackened foliage. If all of the foliage fits this description, cut it all off and let the crown dry out. In spring, set the lily in a well-lit area with indirect sunlight. Begin to water the lily again. If the plant is still alive, new growth will emerge within two or three weeks.
High Salt Content or Overfertilization
If the foliage, including young leaves, of the peace lily has black areas or marks, the plant may be receiving too much fertilizer or could be suffering from being watered with water containing a high salt content. Allow the soil to dry until the lily almost wilts. Completely water the pot with distilled water and pour out any water that drains out of the bottom of the container.
Peace lilies can suffer if they receive too little or too much water. These plants prefer evenly moist soil and typically only need water once per week. In lower light or temperature conditions the peace lily requires less water. The soil should never become soggy. Additionally, peace lilies are sensitive to chlorine, so allow water to stand overnight so that the chlorine can dissipate.
Peace lilies naturally enter a dormant or rest period. This normally occurs after the plant spends a good deal of energy flowering. Do not over-water or fertilize the plant and let the soil dry between waterings. The plant can eventually be divided, re-potted and receive water as usual.
Peace lilies should be fertilized regularly. This plant is particularly sensitive to a magnesium deficiency. In this case, peace lilies may become stunted and leaf tissue between veins may turn yellow while veins stay green. Use a 20-20-20 standard houseplant fertilizer at a quarter of the label-recommended dilution rate. Apply this about once per month when the lily is actively growing. Improperly diluted fertilizer may burn the lily's sensitive root hairs. Over-fertilization can also cause the edges of leaves to turn brown.