Yes, Horseweed is toxic to dogs!
Dogs accidentally consuming the Horseweed plants can show the following clinical symptoms: Vomiting, Prolonged Depression, Incoordination, Hypersalivaton, Sleepiness Or Excitation, Dilated Pupils, Low Body Temperature, Low Blood Pressure, Coma, Seizure And Death (In Rare Cases).
Horseweed consumption is also toxic to Cats and Horses.
Toxic Principles Of Horseweed: Unknown.
More Info About The Horseweed Plant
Scientific Name: Erigeron speciosus
Family: Asteraceae.
Also known as: Showy Daisy, Fleabane, Seaside Daisy.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.