The question of lifespan of plants is very much a question of definition.
In some cases death is a natural occurence. A flowering shoot is normally terminal in all meanings of the word. Unless there is a side shoot developing, a flowering plant dies with the seed pod. The process is obvious in Sempervivums and Saxifrages. The flowering rosette dies. Southside seedling always dies on me because in my garden every rosette sends up a flower. If a "perennial" young Meconopsis sends up a flowering shoot before having developed a secondary one it dies.
However, if the "terminal shoot death" does not occur, other factors may eventually kill the plant. I have read statements that Liliums will only live a few years. That is true if they are not attended to. However if they a re regularly replanted and cleaned, they live for ever. The triploid and double Lilium lancifolium clones are most probably more than a hundred years old and the same is true for the clone of Lilium bulbiferum that grows in Swedish cottage gardens. I have a Begonia clone that was in the family as early as 1880.
Plants that are left in the same place tend to have a limited life span. They often kill themselves by congestion and by depleting the soil. Recently a 9500 year old Picea excelsa was found in Scandinavia. However it had survived because low branches had been pressed to the ground by snow and struck roots.
My conclusion is that most perennial plants can be kept alive as clones forever unless it is killed by outside causes.
Göte
