The easy answer is “not close at all” to any drain, including modern drains, or old house. If there is a way in, willow will almost certainly find it.
Yes, we also have a friend who had an uncle whose dentist had a willow next to her well, and it was fine, but these exceptions only prove the rule.<\/p>
A golden weeping willow<\/a> is as beautiful and delicate in the breeze, but there is a monster lurking below.<\/p> Willows<\/a> have to be the all-time root producing world record holders, and Alder<\/a> and Poplar<\/a> aren’t far behind. Their tentacles will sniff out water and drainage pipes at long range. <\/p> Even if the roots cannot get in at first, there will be condensation on the pipe that encourages the roots to wrap themselves around the pipe like a sleeve. Perhaps the roots dowse for the water, who knows. Without knowing for sure what their maximum reach is, I think it’s wise to leave at least 10 metres between tree and drain<\/strong>: 15 metres feels better. <\/p>
This should not an issue if the willow is squat and stable, but a tall tree that sways in the wind will tug at pipes wrapped in its roots, potentially working a gap loose for them to enter the pipe.
And if that tree falls over in a storm … messy.<\/p>