Ask Mr. Smarty Plants

First, we need to tell you we may not be much help-most house plants are non-native to North America. The Lady Bird Johnson Wildflower Center is committed to the growth, propagation and protection of plants native not only to North America but the the areas in which they grow natively. Your lilies and philodendron will not appear in our Native Plant Database. From the Black Thumb Guide to Gardening, How to Care for Potted Plants. From PlantCare.com Plant Care for Indoor House Plants.

Now let us make a couple comments from our own experience. The first is that flowering plants are very nice sent as a memorial because the blooms will last longer than cut flowers will. However, those blooming plants may not have much in the way of roots. Nurseries who provide florists with these flowering plants do not have the time and space to grow a plant that is meant to be a permanent feature. With greenhouses, special lighting and plant foods they can produce lovely plants, but they are only expected to live as long as the blooms last. All the energy in the plant has gone into producing those blooms and none in producing roots or leaves, which manufacture the food for the roots. You have done very well to keep your plants going as long as you have. As for adding new dirt, the plants may very well be undergoing transplant shock, as you have suddenly changed their environment.

One more thought-you might consider attempting to propagate new plants from the ones you have. In the articles we linked you to above, there will probably be instructions to do this with specific plants. Lilies are ordinarily grown from bulbs; since we do not know precisely what kind of lily you have, we can't make any suggestions on that. From lilies.org, here is a very comprehensive site on lilies, including Propagation.