From a botanical perspective, the common name "marigold" is applied to two distinct genera within the Asteraceae family, and this distinction is critical for determining toxicity. The first and most common group is the Tagetes genus, which includes the popular French marigold (Tagetes patula) and African marigold (Tagetes erecta). The second is the Calendula genus, most notably Calendula officinalis, often called the pot marigold. Their chemical compositions and effects on pets differ significantly.
Tagetes species are considered mildly toxic to both dogs and cats. The primary irritants are essential oils and phototoxic thiophene derivatives found throughout the plant, with a higher concentration in the roots. If a pet ingests a small amount of these marigolds, the most common symptoms are gastrointestinal upset. This includes vomiting, diarrhea, and drooling. The plant's sap can also cause mild dermal irritation, leading to redness, itching, or a rash upon contact with the skin, particularly around the mouth. While unpleasant, these effects are typically self-limiting and not life-threatening. However, consuming a large quantity of the plant material could lead to more significant stomach distress and may necessitate veterinary care to manage dehydration from vomiting and diarrhea.
In contrast, Calendula officinalis (pot marigold) is generally regarded as non-toxic to dogs and cats. It is widely used in topical ointments and salves for its purported skin-soothing properties. While ingestion of a large amount of any non-food plant can cause a pet mild stomach discomfort simply due to the unusual material, Calendula itself does not contain the same irritating essential oils as Tagetes. The American Society for the Prevention of Cruelty to Animals (ASPCA) lists pot marigolds as non-toxic. Therefore, an animal nibbling on a Calendula flower will likely experience no ill effects, though monitoring is always advised.
The mild toxicity present in Tagetes marigolds is a form of chemical defense evolved by the plant to deter herbivores and pests. The essential oils and thiophenes are secondary metabolites that are not essential for the plant's primary growth and development but serve a protective role. These compounds can irritate the mucous membranes of the mouth and digestive tract of animals that attempt to eat them. Furthermore, some compounds in Tagetes can have an insecticidal effect, which is why these plants are often used as companion plants in gardens to repel nematodes and other insects. This same mechanism that deters insects can cause a negative, though usually minor, reaction in companion animals.
