PESTNET — a free service to help combat crop pests and diseases.
Crops Roots & tubers Sweet potatoWeed ?Zinnia linearis, highlands, PNG
March 2010. A plant from the highlands of Papua New Guinea said to be planted around plots of sweetpotato during dry times to keep away sweetpotato weevils.
Suggestions were Tagetes or Zinnia linearis.
See: http://www.hear.org/starr/plants/images/search/?q=tagetes, but also, and more likely:
http://www.armitageimages.net/index.cfm?fuseaction=stock.detail&itemID=12728