I went with Point-settee-a. Named after some dude.
I go with poin-SET-cha. Not too much “CH” on that last syllable; just enough to nod to the “tia.”
i pronounce it poisensetting
How it *is *pronounced and how it should be pronounced are two different things.
Poinsettia. Pronounced “ThroatWarbler Mangrove.”
Depends on whom I’m talking to. Gauge people by the language they expect, and then speak it.
This.
Anything with a “point” in it is flat wrong. Ref the descriptivists above, not that that’s never heard.
I say poin-sett-a.
Fun fact! A poinsettia’s flowers are yellow. What most people think of as their flowers are called “bracts”. What Color Are the Flowers on a Poinsettia Plant? | Home Guides | SF Gate
Next up: Bougainvillea.
Depends on the location.
Is your dog a pointer-setter mix? 
When I moved here, I pronounced the nearby town Lovilia as it looks: lo-vil-E-ah. She laughed at me and told me it’s pronounced lo-vil-ah. I tried to explain it was likely a local bastardization and that my pronunciation was probably the original intention. She still gets mad and thinks I’m full of shit. 
Normally I just say “that red Christmas flower thingy” but of the options given I went with the second.
P.S. I also sometimes say FOIL-age.
Three years ago my gf texted me to buy a couple of those xmas plants.
We still have the two christmas cactuses I brought home and she hasn’t tasked me with any horticultural chores since.
I’m with my stepfather on this one. He called everything a “posey”.
Most likely IME, it is a pelargonium. 
Julestjerne - yool-eh-styearn-eh
Well, you shouldn’t have called her a local bastard.
Around here it’s a poyn-SET-uh. And it has FOAL-edge. And it’ll kill your DAWG. Dammit.
ETA: And it’s puh-CAHN. The PEE-can is located in the BATH-room.