The Latin name for garden mums is Chrysanthemum morifolium and that is listed as a toxic plant for pets because the sap (in leaves and stem) contains oxalic salts.

From: http://www.petplace.com/dogs/fall-an...ogs/page1.aspx

"Chrysanthemum (Chrysanthemum morifolium). Ingestion results in vomiting, diarrhea, depression, drooling and lack of appetite."