The Red Mums can be sent straight from Facebook in one of two ways. Either head to the free gifts page and choose the Red Mums from the long list of items available, or look for the small square Mums icon on the FarmVille game screen, which gives you instant access to sending these limited (and therefore presumably rare) flowers to your friends.
If this is like other Limited Time gifts in the game, you'll only have a chance to send these flowers for a matter of hours - a day at most, so make sure to send some soon so you don't miss out, and ask your friends to send them back as well!
Have you collected all of the other Mums in FarmVille? Will you work to add a few of these to your collection as well? Let us know in the comments.