Roots invading waste water inspection chamber

[silverfoxcc]

Opening the cover found a blockage. After clearing and releasing the backed up detritus, found small roots coming through the wall and joints.
Cut them back as they were the problem, but fear that hey will need constant watching.
They are above the 'water line' so powder will not get washed off. Therefore i need some form of 'weapons grade' answer to get them to die back....hopefully permanently
Any suggestions or recommendations?