I was also interested and looked around. I couldn't find a myth about it. All I got to know is that the term "Flowers of the Forest" was used for the first time after the battle of Flodden in 1513. It really seems to be "not more" than a very expressive analogy as there is no historical significance known.
So everytime a beloved person dies it becomes a new member of the Flowers of the Forest, immortal in the memories of those they had left behind.
"Now here's my secret", said the fox, "it is very simple. It is only with ones heart that one can see clearly. What is essential is invisible to the eye."
("The Little Prince" by Antoine de Saint-Exupery)
"The soul would have no rainbow, if the eye had no tears."
(Native American Proverb)