I was about to throw them out, but since the flower/plant has such significance in the East, I just had to try. I cooked up flower heads and leaves separately, here is what I got: top left – leaves with alum bottom left with alum and iron, top right – flower heads with alum, bottom right – with alum and iron. These were white hydrangea exactly the same as in the photo.
Take-away: leaves with alum are the more potent source of yellow that turns green-ish with iron. Would be interesting whether differently coloured plant (not white blooming) would have different results, and since hydrangea themselves are quite ph sensitive, so might be the dyes.
Try them yourselves if you have any!