I am familiar with the annual plant Amaranthus V. Joseph's coat. It grows 5' tall outside. Could you have a hanging variety of this? I am not familiar with Alternanthera.
Since you have no other replies try a visit to www.horticulturalhelp.com.
The company owner is a houseplant expert. If anyone can help its probably him. Its getting time to acclimate outdoor plants going back inside for the winter so hope you find the information you're looking for.