Everything old is new again. Nowhere is this truer than in fashion, especially the fashion of flowers. The length of the cycle of popularity may vary, but garden favorites always come back. Take the common Zinnia, named the plant of the year by the National Garden Bureau. It has come and gone in popularity so regularly people have lost track of its true origins. Zinnia elegans is native to Mexico and is the most popular of the 20 species of Zinnias known to science. It is an annual herb in the daisy family growing from 18 inches to more than 3 feet tall. Plants are stout-stemmed, freely branched and bushlike in habit if given enough room.