expression, godan verb (-tsu)
go to seed; become tough and fibrous
Literally describes vegetables (especially leafy greens like spinach or rapeseed) that have bolted and become too tough to eat. Used for plants that have passed their edible stage.
ほうれん草は薹が立つと筋っぽくなる。
Spinach becomes stringy when it goes to seed.
expression, godan verb (-tsu)
be past one's prime; lose the bloom of youth
Figurative extension of the plant sense, applied to people (especially women) who are considered past the peak of youth or beauty. Can be seen as dated or mildly sexist in modern usage.
彼女はまだ若いのに、周りから薹が立ったと言われるのが嫌だそうだ。
She's still young, but she says she hates being told she's past her prime.