Answer: Planting near septic tanks and drain fields is always a concern, and plant selection should be made with caution. Tree and shrub roots naturally seek out water and will grow toward the source. You certainly wouldn't want them to invade your drain field. In general terms, tree roots will extend out one and one-half times the diameter of the canopy. Planting trees that are small at maturity, and keeping them as far away from the septic system as possible will keep your system reasonably safe from invasive roots. Of the trees you mention, Cornus mas, Cupressocyparis leylandii, Pinus flexilis 'Vanderwolf', and Thuja plicata are the best choices. Their mature sizes indicate root spread of less than 30'. Just for good measure, plant them as far away as possible from your septic system!
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