Dog Fennel Collage

Dog Fennel's thin, feathery green leaves will gain silver tones in the fall and the tall, green stems will take on a red hue, adding to the show of color along lake roadsides and edges of open woods.

The distinctive, lacy green plumes of Dog Fennel are easy to spot, even at 55 miles per hour.

Late summer is a great time to look for this tall native weed. And the shoulders of roads are a good place to start looking, since Dog Fennel likes roadsides, clear-cuts, recently burned woodlands and forest edges. It’s a prolific early colonizer of newly disturbed soil, and it tends to prefer moist soils.

Dog Fennel can grow to more than 9 feet tall – another reason it’s easy to spot through the windshield at high speed.

Its leaves are very thin and feathery, almost hair-like. The leaves begin the season a bright, yellowish green color but change to silver tones during the fall.

The long, tall stems of Dog Fennel change from spring and summer green to a more red color in autumn.

The tiny white daisy-like Dog Fennel flowers bloom in September and give way to tiny red berries in the fall. These berries produce hairy seeds that catch the wind to help spread this plant.

Dog Fennel is native to the eastern and southern U.S., from Massachusetts and Missouri to Texas and Florida. It is also found on the nearby islands of Cuba and the Bahamas.

Eupatorium capillifolium is the botanical name of this plant. “Capillifolium” aptly means “hair-leaf” in Latin.

The common name Dog Fennel came about because this plant looks a little like the herb fennel, which is used in many cooking cultures, as well as the fact that this weed was once commonly placed on the floor of dog kennels to ward off fleas and ticks.

That kennel trick worked because Dog Fennel contains the chemical pyrrolizidine, which is a toxic alkaloid. If consumed by birds and mammals, pyrrolizidine could cause liver damage – the liver clogs and begins to retain fluid – and may even cause death.

That’s why you will often see Dog Fennel standing tall in fields that otherwise are being grazed low by cattle: Livestock know better than to eat it.

Scientists think the plant produces pyrrolizidine as a defense against getting eaten, and they have good reasons to believe that.

One is the fascinating relationship between beautiful Scarlet-Bodied Wasp Moth and Dog Fennel. 

When it comes time to mate, a male Scarlet-Bodied Wasp Moth will land on a Dog Fennel plant and throw up a chemical that dissolves the leaves. He then laps up the pyrrolizidine-laden liquid and goes searching for a mate. When he finds her, he will begin a nine-hour mating process that involves coating his date with Dog Fennel juice and transferring some to the eggs as they are fertilized. As a result of painting both his girlfriend and her eggs with a toxic chemical, his offspring have an improved chance of survival.

Scientists testing this observation took a female Scarlet-Bodied Wasp Moth that had been doused with pyrrolizidine and put her in the web of a banana spider. The spider promptly cut the moth from its web and freed it.

People can also benefit from this chemical by rubbing crushed Dog Fennel on their clothing as an insect repellent. Just don’t eat it. Although there have been reports of some people using Dog Fennel as a cooking herb with no obvious ill effects, there have also been reports of people who have permanently damaged their livers by consuming small amounts.

Dog Fennel played an interesting role in the life of early American legend Johnny Appleseed, whose real name was John Chapman. In 1802, Chapman lashed two canoes together and floated 16 bushels of apple seeds down the Ohio River, planting apples and preaching at homesteads across rural America. He charged a few cents per tree and used that money to buy more seed to continue his mission. He was well loved for his work with apple trees but not so much for also planting Dog Fennel on each farm he visited, which he thought provided protection against malaria. As it turns out, Dog Fennel didn’t ward off malaria; however, it is invasive and within a few seasons, Dog Fennel spread to the pastures and newly tilled fields on the farms that Chapman visited.


Information for this article came from the Virginia Native Plant Society, the South Carolina Native Plant Society, Eat the Weeds, and