I agree. Mine aren't quite as far along as yours but I'm a couple zones colder. It's commonly called wild ginger.

Lantana camara.

Ooops! My mistake! There's a much smaller plant that I've heard referred to as "Japanese Knotweed" but when I looked up Polygonum cuspidatum, I saw that y'all were right! I guess I won't be trying to root any cuttings! Thanks!

Probably, you got a mislabeled one. If it is important to you, and if you still have the receipt, and if you have a photo showing the flower color (the last may or may not be necessary), then I would go back to whoever sold it to you, and complain. They SHOULD give you a pink one or refund most or all of your money. If you don't really care, and it's growing well, enjoy it, and let the matter go. You might want to give the seller a heads up that there is a problem, just for their information, but that's up to you. To my knowledge, soil acidity or lack thereof has no effect on dogwood bloom color - it's only certain hydrangeas that change color according to pH.