Here is an article that explains some of these plants' problems, and this one has suggestions for what to do with brown leaves. As they indicate, brown leaves of this pattern are typically a temperature (too low or too high) problem, or leaf or winter scorch. Since I don't know whether the browning began last summer during the dry weather, or just began over the winter, it is difficult to know its origin. Try cutting out the brown as soon as the weather gets above 45 to 50 during the day (when it should be above freezing at night), and see if it returns. Thanks for your question!