I don't know who decided that, or when. I don't think it was Hallmark.
Among other daily poem feeds in April, I get one from Knopf, who provided this one today, from Mark Strand.
The sorrows of the rose were mounting up.
Twisted in a field of weeds, the helpless rose
felt the breeze of paradise just once, then died.
The children cried, “Oh rose, come back.
We love you, rose.” Then someone said that soon
they'd have another rose. “Come, my darlings,
down to the pond, lean over the edge and look
at yourselves looking up. Now do you see it,
its petals open, rising to the surface, turning into you?”
“Oh no,” they said. "We are what we are — nothing else."
How perfect. How ancient. How past repair.