Answer: There are several possible causes but my first guess is that it is showing symptoms related to fluoride in your water or to overfertilizing resulting in salts building up in the soil. You could try watering with rain water or distilled water and see if it stops. If you fertilize do so only at the lowest label rate (or even less) and only during the growing season, stop for the winter. If you have been fertilizing a lot you can try to leach the soil clean by drenching it with water until it runs out the bottom of the pot, allow it to drain, then repeat several times in quick succession. Also make sure they are not getting too dry between waterings and that there is ample humidity where they are kept, also do not put them in direct sun. They like bright but indirect light. Once the damage has occurred there is no way to repair it, just wait for new growth to conceal it.
You can propagate it by division in the spring or by rooting tip cuttings -- but neither way will affect the existing foliage.
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