My daughter standing in front of my tallest giant amaranth (Amaranthus australis) in the garden. I have about 40 giant amaranths in the garden this year. This one is the tallest, but I have some really hefty plants that were started later that will probably surpass this plant later in the year. These amaranth should continue to grow until late September to early October before flowering. If I can avoid any major wind storms then I hope to get some low to mid 20 ft. plants.