Answer: Most sources indicate no tree should be planted close to the drain field as their roots will infiltrate the area and in times of drought grow into the pipes in search of moisture and clog or possibly even break them. Typical distance suggested is about 50 feet although willows should be even wider spaced. Unfortunately all of the trees you listed are somewhat notorious for having spreading, aggressive root systems and also for preferring a damp soil. Keep in mind that the typical tree will send roots out about twice as wide as its branch spread. You might try using large shrubs or much smaller trees or, to be on the safe side move these much further away. (Wait until fall to do this, mid-summer is not a good time to transplant.) Your local county extension may have suggestions of trees you could try, but the rule of thumb is lawn or shallow rooted plants only near the drainfield. I'm sorry.
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