The term “dog weed” is a common, non-botanical name used across different regions for several distinct plants, creating confusion about which specific weed is being discussed and its potential hazard to pets. To clarify this issue, it is necessary to identify the common plants labeled with this name, understand the specific toxins they contain, and outline safe methods for managing them.
Identifying the Plant Behind the Name
The confusion surrounding the term “dog weed” stems from its application to at least two primary, unrelated plant species. The most common culprit is Prostrate Spurge, scientifically known as Chamaesyce prostrata or Euphorbia prostrata. This is a low-growing, mat-forming annual weed found in lawns and sidewalk cracks. It is recognizable by its small, oval, pale green leaves and a central taproot. It is identified by the milky white sap it exudes when broken.
The second plant often called dog weed, particularly in the southeastern United States, is Dog Fennel, or Eupatorium capillifolium. This is a tall, perennial herb that can grow up to six feet, distinguished by its finely dissected, feathery foliage. When the leaves of Dog Fennel are crushed, they release a strong, pungent, and often unpleasant odor, which is the source of its common name.
A third, less frequent plant referred to as dog weed is Mayweed, or Stinking Chamomile (Anthemis cotula), which features daisy-like flowers with white petals and a yellow center. The use of a single vernacular name for these three plants is purely regional. Knowing the specific physical characteristics of the weed in your yard is the first step toward determining the appropriate course of action.
Toxicity and Safety Concerns for Pets
The level of danger posed to pets varies significantly between the plants commonly called “dog weed,” primarily due to different toxic compounds. Prostrate Spurge contains a milky latex sap that holds a group of irritating chemicals called diterpenoids. If ingested, this sap can cause immediate symptoms like inflammation and blistering of the mouth, skin, and eyes.
Although Spurge is considered a low-severity toxin, its sap can lead to significant gastrointestinal upset, including vomiting and diarrhea. Dog Fennel, on the other hand, contains pyrrolizidine alkaloids, which pose a more moderate, but potentially serious, risk to a dog’s health. These alkaloids are hepatotoxic, meaning they can cause liver damage or liver failure, especially if ingested consistently over time or in a large quantity.
The Mayweed variety of “dog weed” contains acrid substances that are known to cause intense irritation and swelling in the mouth and throat upon ingestion. If you suspect your pet has eaten any plant labeled “dog weed,” observing the animal for symptoms like lethargy, persistent vomiting, or signs of abdominal pain is important. Immediate consultation with a veterinarian or animal poison control center is the safest course of action.
Common Misconceptions and Traditional Uses
A persistent misconception is that “dog weed” is related to Cannabis or marijuana, a confusion arising from the common slang term “weed.” These plants are not botanically related to Cannabis and do not contain psychoactive compounds like tetrahydrocannabinol (THC). Cannabis is highly toxic to dogs, causing neurological symptoms such as incoordination, tremors, and severe depression of the central nervous system, which is a far different and more severe reaction than that caused by true dog weeds.
The plants’ traditional uses offer context for their names, though these historical practices are not recommended. Dog Fennel was historically used by some to repel insects; the strong odor released when its foliage is crushed was said to deter fleas and other pests from dog kennels. Some species of Spurge have been noted in traditional medicine practices, such as in India, where extracts from Euphorbia prostrata were used as a treatment for hemorrhoids.
Managing Dog Weed in the Yard
The most effective method for managing these weeds involves maintaining a dense, healthy lawn, which naturally crowds out opportunistic weeds. For small infestations of Prostrate Spurge, hand-pulling is effective, but gardeners should wear gloves to protect their skin from the irritating milky sap and ensure the entire taproot is removed to prevent regrowth. Dog Fennel, with its fibrous root system, is also best removed by digging it out, especially when the plants are young.
For larger areas, homeowners can opt for mechanical control methods like covering the area with a light-excluding barrier such as thick cardboard and mulch to smother the weeds.
Chemical Control Options
Chemical control requires careful selection; while broad-spectrum herbicides containing glyphosate are effective on both Spurge and Dog Fennel, they are non-selective and will kill any plant they touch. A less toxic alternative for spot treatment is concentrated horticultural vinegar or commercially available pet-safe herbicides based on acetic acid or plant oils.
When using any herbicide, even those marketed as pet-safe, it is important to follow all label instructions and keep pets off the treated area until the product has completely dried. Applying a pre-emergent herbicide in the early spring can help prevent the germination of Spurge seeds. For Dog Fennel, which is a perennial, post-emergent herbicides are necessary, and they are most effective when the plant is less than 20 inches tall.