When Rea brought the hydrangea parts to Furuya, he didn't think them poisonous as the geezer was munching on them all the time. Didn't they speculate some cultivars might be more toxic than others? Furuya's place's flowers might be of less poisonous variety and thus of less use to zombies. Who knows. I personally think that's why Baby sought out the place where Rea originally found them.
If Furuya has half a brain, he should already know to bring them to Rea.