If you kill them with copper sulfate or other chemicals it will take some time for them to break down and no longer cause a restriction.
Chemicals are usually for maintenance.
And some can kill the good bacteria in the septic tank.
This is why they like it more than Copper Sulfate.
"Copper sulfate was a common root killer in the past, but is used less frequently now. ... Foaming tree root killer is a more modern invention and is now preferred by many homeowners because the foaming action coats the inside of the pipe with a herbicide that prevents tree roots from returning."
I've been using it every 6 months or so because it cost me $500 to have my sewer line hydrojetted for more than 4 blockages a year ago. So far, so good. (fingers crossed)