go to seed

go to seed and run to seed (to stop taking care of your appearance so that you no longer look attractive;to get into a much worse condition) — махнуть на себя рукой; ~ поставить на себе крест; перестать за собой следить; ~опуститься
If a person or place goes or runs to seed, their physical appearance becomes worse because no one cares for them.
If we say that a person has gone to seed we mean that they have stopped caring for themselves.

Example 1: When/After she retired, she really went to seed: she never went to the hairdresser and she always wore old clothes. — Выйдя на пенсию, она действительно махнула на себя рукой / опустилась / плюнула на себя / поставила на себе крест: не ходила в парикмахерскую, носила старые вещи.

Example 2: I almost didn't recognize John. He's really gone to seed since his wife left him. (Cambridge Idioms Dictionary)

Etymology: based on the literal meaning of plants that go to seed (stop producing flowers and start producing seeds)

[let oneself go]

[катиться по наклонной плоскости]
[плюнуть на себя]

go to the dogs
go to the pack
go to the pot