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Rob, I believe all E. grandiflorums are yellow. One source says that parviflorum has white or light yellow anthers.Another clue with E. grandiflorum is that it can be very picky about where it will grow. Even in the Pacific NW, it is common to have 100% losses with them, planted right next to many other species that are doing well. I believe it's because it comes from a dry summer climate with cold winters. Maybe Ed can help with this.
Sorry, don't know how to turn the pics
I've just come across an old article claiming there is carbon transfer between E. americanum and sugar maple through arbuscular mycorrhizal fungi: http://www.jstor.org/stable/4223326It is a wild guess, but I wonder if this may explain why it rarely flowers. Mine never flowered. Perhaps it has to grow under specific trees. In your experience, could this be the case?