I know that some plants, such a hydrangea can change colour according to iron content in the soil...but I'm wondering why an Asclepias quadrifolia would change from pale pink to snow white w/out a trace of colour. I have 2 plants at different locations in a disturbed woodland area. They both were pale pink last yr and both are white this year. We had a harsher Winter followed by a wetter Spring but beyond that I can't think of any other differences that could be applied to both plants. Any ideas would be appreciated!