Flowers are sent in the highest volumes on Valentines day and Mother's day.
Both graphs have spikes in February. In Britian, Mother's day is in March. In the US it is in May. These correspond directly to the spikes presented.
Also, on the US graph there's also a discernable a mini-spike right on or just before the first quarter. I can't come up with an answer for that. One guess is that people are looking into planting flowers for the springtime around then.
Thanks for preventing me from having this going on in the back of my head all day.