What does 18853 (Friends flower) connect to besides 37695 (Plant stem with 3 leaves) or 11609 (Friends accessories star)?
Is there a good way to attach it to common plant stems (3741) like this one:
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Is there a good way to attach it to common plant stems (3741)?
Although strictly a non-official connection, the Technic pneumatic hose (or its rigid variant) can fit over the plant stem, and thus the Friends flower pin. If the connecting piece is cut short enough, the resulting assembly might even look somewhat good.
What does 18853 (Friends flower) connect to?
Most of Friends animals have this type of connection to attach accessories like flower you mention, bow or other. Coule of examples:
Number of hair pieces have attachment point for accessories like the flower:
Tile, Round 1 x 1 with Bar and Pin Holder gives an option to attach these kind of accessories to usual studs, as this has been used several times on this picture:
Is there a good way to attach it to common plant stems (3741)
If you define "good way" such as attaching stud to anti-stud then no. Both ends of these elements are of the same type.