This article is from the Pet Owner's Guide to Common Small Animal Poisons, by Julie Dahlke, DVM, a graduate of the University of Minnesota, College of Veterinary Medicine.
It is difficult to give concise information about plant toxicities as
there are hundreds of plants that are potentially poisonous to
animals(1). However, actual reports of animals getting seriously ill
from eating plants are relatively infrequent compared to reports of
poisonings from household products or drugs. The plants discussed
below can be found in Minnesota and represent among the most dangerous
of poisonous plants. You may notice the conspicuous lack of "holiday
plants" among the list. While many people seem to think poinsettias,
ivy and mistletoe are dangerous plants, and while these plants have
toxic potential, they seldom cause serious clinical signs if eaten.
It is worth noting here that dogs and cats often vomit after chewing
on plants; this probably does not represent "poisoning" or any
dangerous exposure. Only severe or persistent vomiting is a danger
sign in small animals. Sporadic vomiting without accompanying signs of
illness (for instance, diarrhea, depression, loss of appetite) is
rarely a cause for worry, whether associated with plant ingestion or
not. The best advice, however, is to contact your veterinarian if you
have specific concerns.