Vegetation in drainage system?

I have an underground drainage system in my back yard to drain off an underground spring. How can I kill the plants and roots that grow inside it?

50 years ago they used copper sulfate crystals. Not sure if there is something better now.

Can't help ya.

Get it above ground and plant with water crest. You'll have salad all year round.

Copper sulfate still works as well as anything. mike