I like to wilt all the leaves when I torch. Following her dad's instructions, she pointed at the base of the plant before squeezing the handle to blast it.
The idea is to heat the water in the plant so hot that it damages the cells. It's not only fairly effective, it's lots of fun!
A lot of things aren't killed with the first try. I'm currently trying to eradicate some daylilies and tall phlox. I've torched them 4 or 5 times this spring and they just. won't. die. I'm not giving up, though. I have too much fun trying.