Micro Greens got its name based on the fact that most of these species taste best when micro- sized. Usually harvested when less than a month old, 2.5" tall or shorter. This is especially true of Amaranth which toughens up a bit as it grows.
According to foodie legend, the first Microgreens were produced in Japan, and have been grown there since at least the early 1900s. It is speculated that due to Asian influence in the Pacific Rim the first American growers to sell greens as microgreens were located in Hawaii, around the turn of the millennium.
Hawaiian growers were marketing these tiny tender greens to resorts and restaurants in the area. Soon after, the microgreens trend was seen in Southern California and Australia.