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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?

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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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  • Douglas Fir have extensive root systems and 40' is probably too close. You're talking about something that can grow to 75' or more. – Fiasco Labs Dec 31 '14 at 2:36
  • @FiascoLabs the trees are 40' tall and I recommended 200-300 feet away. Reading too fast perhaps? – kevinsky Dec 31 '14 at 12:57
  • No, pretty much agreeing with the specs you laid out. Taking the height rule of thumb you mention, that Douglas Fir in the question would be 75' to 225' at 75' tall and in a good location I'd figure on 90' tall at max growth. Given the rich nutrients available, once the tree sensed the presence, it will put out feeder roots so I'd go for the top end. Now switching to your Redwood, I will state from experience that if you had a quarter acre lot, plant it dead center because it will be needing all that lot and somewhere in 50 years start encroaching on surrounding properties. Sequoia are worse. – Fiasco Labs Dec 31 '14 at 16:38

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