Answer: As a general rule willows are very thirsty plants and will send roots in search of water, especially in a dry season. If there is a weakness in the pipe then the roots could potentially reach that and exploit it and consequently make it worse. It is usually not a good idea to plant any shrub or tree so close to a pipe of any kind.
Although termed dwarf, this plant does grow over time, typically about five feet tall and four feet wide. The root system usually equals the top portion of a tree or shrub in terms of overall mass, and most root systems will reach outward up to half again as wide as the branch spread.
The conservative answer to your question is that yes it could possibly be of concern in terms of the pipes. Luckily, this shrub is very easy to transplant in early spring if you dig it with as many roots intact as possible and replant immediately. Keep it well watered for the first few years while it becomes established in the new location. A mulch will also help to keep the soil moist.
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