If a person runs to seed (or goes to seed) they stop taking care of themselves and their appearance.
If a place has gone to seed it has become run-down and neglected.
This idiom has its origins in flowering plants that fade and become less attractive as they produce seeds and then die.
Examples of use:
1. Our old teacher has gone to seed since we last him. He used to dress smartly and always wore a tie – now he looks untidy and dirty.
2. The town I used to live in has really gone to seed. The shops are boarded up and there's litter everywhere.
Image by dmswart