Tree roots growing into fosse

[JohnRoss]

Having cleared another blockage at the entrance to the fosse discovered that fir tree roots had infiltrated into the fosse round the 100mm entry pipe. These are thin, hair like roots from a line of fir trees that run alongside the house and the fosse. I have cut them away where they had squeezed round the pipe where it goes into the concrete fosse but can I put some sodium chlorate around this entry point to stop it happening again or is there something better I can use? Don't want to kill the trees themselves just the roots in that area.............JR