While Pussy willows have a fine fibrous root system that stays near the surface, they do have some larger structural roots that can become quite large. Further more they are strongly attracted to soil water. Often the basement walls or foundation of a structure (house) provide a drainage channel for excessive moisture coming off the eaves and will attract and provide a channel for the main roots to go deep..... right next to the supporting structure.
I would want to be at least 20 to 30 feet from the house and not over a drain field. While often prunned to be shrubs, pussy willows are infact small to medium trees and the roots will continue to enlarge even if the top is prunned to shrub form.
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