rock salt will work, although rootX will work better as it foams to reach the roots. (usually in upper area of pipe)
rock salt will sit along the bottom awhile and kill roots, which then rot, so long as you do not use much/if any water for a while, but it will rinse away, and may not kill as much as you'd like. some people put it down their tanks regularly. we have no trees in our septic area, so we don't need this solution, but i know some folks around here that do it.