Q.Could tree cut down many years ago still have roots grwoing underground?
A tree was cut down at the back of my house around 25 years ago. A cross was cut on top of the stump and it was paved over. For a number of years I’ve been getting lots of white marks on my paving (even at the front of the house now). I believe the roots of this long expired tree are still growing? Is that possible? If I pour root killer between the slabs at the back of my house would that sort it? Any advice would be greatly appreciated.

Tree roots can not survive 25 years without photosynthesis, the source of their power. Tree photosynthesis only occurs in leaves. The roots are well on their way to becoming compost and not affecting your paved areas.