! Nobody has posted this yet? =(
The GREEN flower STACKS. That is: you can have the single-resist spells (Resist Poison) or the Talisman (Poison/Disease) SHM spell, NEC spell Dead Man/Men Floating, AND Aura of Green Petals all at once, raising your Poison Resist by:
(40 or 65) + 70 + 50 = 160 or 185 points. You can also throw the bard resist song(s) on top of this, raising it even further. GREEN flower is awesome!
Black flower does NOT stack with Resist Disease or Talisman (SHM) line, so it is good for those times when these buffs aren't available, or as a quick clicky-buff.
I have not tested either one with the POTION resist buffs. I suppose I should. ;-p
I have not yet gotten the Red or Blue flowers (and never probably never get the blue one unless they change/remove the quest in the upcoming revamp, which is possible), so I cannot say for sure if they stack or not. The most popular hearsay is that they do not, or did not, but this may have been changed.
(Also, there are a few items with Flower effects from Vex Thal, which makes them "easier to get" haha - sad, but perhaps true.)
*note* : For those interested - The Plane of Mischief is no longer a bindable zone (you get the "You cannot develop an affinity for this place - try a city instead" message. This was changed recently (around September 1st? roughly) in anticipation of the upcoming revamp.
Edited, Wed Sep 17 14:10:23 2003