obviously in plants,particularly the ones that regenerate compleately new organs every year,is different and slower...but think about it...the older plants around known can have 10-15000 years?...There are no plants around that are milions years old....
But the limitation of no million year old plants being around is a result of sexual reproduction and evolution changing the plant to a new species for the progeny that are dispersed.
It could be possible to have an individual plant live a million years if the environment it was located in remained constant. So based on this concept let's assume the plant reproduces vegetative for a million years and never mutates but the earth mutates (erodes) away and no longer provides a home for the un-mutated plant. That is why there are no 1 million year old plants.