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I have two mature douglas fir trees (40') on my lot. How close to them can a septic tank or leach field be placed safely to avoid root invasion?

Joe
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This answer from J. Musser is detailed and extensive and refers to this site which notes

plant trees at least as far away as their estimated root spread at maturity. There are two methods used to estimate tree root spread. One rule of thumb is that roots extend out from the tree two to four times the diameter of the canopy. Another is that tree roots spread out one to three times the height of the tree. These estimates should be considered a bare minimum, and to reduce the risk, the trees should be planted even further away from the drain field.

Given the possible height of a redwood I would say that the answer to your question is that redwoods should be located two hundred to three hundred feet or more away from any septic system.

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