I looked it up on the Tortoise table. http://www.thetortoisetable.org.uk/site/plants_19.asp?st=true&mode=main&catID=538 You'll kind of have to ignore the photos on the tortoise table, because there isn't one exactly like yours. There are millions of varieties of dahlias and it would be impossible to include a photo of every kind, but I have zero doubt that you have dahlias.
Tortoise table says "feed in moderation." It sounds like they suspect it's totally fine for tortoises, but haven't tested it out. Try it with your beastie and write back on how he/she does