How to know if an old lily plant should be replaced
Performance is your best indicator of vigor. If it looks and acts old and tired with fewer leaves and blooms it may be time to dig it up. Most often, it just doesn't come up one spring. Your lily may be suffering from a soil disease that rots the bulb or it may just be in need of division. So dig it up when it has mostly died down and examine the bulb and soil. Leaf spot and rotting of the bulb indicate disease and the plant should be removed with all the soil that is nearby. Don't plant replacement lilies in that area of your garden. Regular division will keep plants young and give you an opportunity to add compost or manure to the soil.