The Baldur's Gate 3 Flowery Key item is a bit of a mystery when you find it on the corpse of a shapeshifter skulking in a cave beneath the Open Hand Temple in Rivington. Smart money would be on it having something to do with the murder that just took place in the temple above that you're likely trying to solve at this very moment.
It isn't clear where exactly to use the key, though if you're paying attention, the description does give you a pretty big hint. The good news is that this key will help you complete two different quests, and give you a way to enter the Lower City of Baldur's Gate. That said, here's where to use the Flower Key in Baldur's Gate 3.
Where to use the Flower Key
If you glanced at the Flowery Key's item description when you looted it off that shapeshifter's corpse, you may have noticed that it mentions a location: Fraygo's Flophouse. You can find this establishment on the central thoroughfare of Wyrm's Crossing, opposite the Sharess' Caress brothel.
This key is an important part of two separate but related quests: Solve the Open Hand Temple Murders and Find the 'Stern Librarian' Ffion. You'll likely already have the first quest if you visited the temple in Rivington, but the second quest is very easy to miss and does have a separate reward. Before you head to Fraygo's Flophouse, talk to Mamzel Amira in Sharess' Caress and ask about the Stern Librarian. She'll explain that Ffion is missing and recruit you to help look for her.
Now you can visit Fraygo's Flophouse. Climb the stairs to the first floor, and then turn into the right side of the room and go up the ladder into the attic. Here, you'll find a locked wardrobe you can open with the Flowery Key, giving you access to a secret room used by the Bhaalist murder cult. Make sure to examine the Temple Plans on the table, and investigate the blood stains by the bed in order to find Ffion's body, progressing both quests.
You can now visit Inspector Valeria on the first floor of Sharess' Caress, who'll ask you to report to Devella and give you a pass letting you enter the fortress and the Lower City. If you tell Mamzel Amira about Ffion's fate and ask for payment, she'll let you sleep with one of the Drow twins for free. Job well done.