chop them up into a fine pile and flush down the toilet. MAKE SURE NOT TO CLOG the toilet, as you don't want t plumber to snake out a bunch of weed plants...

if you have a lot of plant matter to get rid of, double bag em and toss in an inconspicuous trash dumpster. make sure there is nothing else in the bags to go back to you. also try not to use a private dumpster as sometimes they have cameras. Recycling drop off points work well IMO. That or I'll dump at a church (not mine) but make sure to leave a donation for their help.