All day, any day, but especially at night they prowl the park. They say, "smoke"? "Yo, smoke"? If you say or nod 'no', they leave you alone. If you say yes, any number of things can happen, but usually it's either you make a transaction and buy crappy weed, or for the less savvy, they simply take your money and fail to give you anything in return, basically robbing you and leaving you with no recourse.
It seems utterly ridiculous that the police and justice system can't do more than they do to keep these criminals from prowling my neighborhood park.