Dogs are man’s best friend, according to Ogden Nash. The mid-20th century poet and humorist focused one of his many amusing rhymes on the numerous positive attributes of contemporary canine companions.
So it is curious a noxious native weed, dogfennel, is identified with this faithful friend. Dogfennel (Eupatorium capillifolium) is not in the same plant family with the culinary herb fennel, but its leaves are similar in appearance.
The link between the two is a toxic compound produced by this plant which was once used as a means of flea and tick control in dog pens, and so the appearance and use combined to form the common name.
Dogfennel shoots are emerging from roots established in previous years. This native weed has an appearance similar to the culinary herb fennel, but it is not a usable in the kitchen.
In north Florida’s late autumn, it is one of the few native plants which produces big clusters of tiny white flowers. The perennial dogfennel adds a cloudlike texture to open areas in in the panhandle and many untended locales in the southeast.
This time of year dogfennel is emerging from its winter hiatus and its new growth is commonly less than a foot in height. The stems and base are covered in fine leaves which resemble branching green threads projecting outward in a delicate drooping mass.
Fallow fields, road shoulders, and other disturbed sites are ideal spots for dogfennel to flourish. This plant’s population is denser in full sun, but it will grow in partial shade.
When the thick foliage and flowers are crushed, they produce an unpleasant acrid odor. This negative trait and quickly wilting blooms have eliminated this plant’s potential use as a cut flower source.
Dogfennel is exclusive to North America and several Caribbean islands, but has been introduced out of its native range as an ornamental. While it is normally considered a nuisance weed to agricultural producers, it does have its place in folklore and the native environment.
There is the belief that the first frost of the season is just a few weeks away when dogfennel blooms. The excellent odds of this being factual is based on dogfennel’s late season flowering genetics, not some unknown stimulus which causes a weather forecast from this weed.
The environmental importance of dogfennel is its use by the Scarlet-bodied Wasp Moth, (Cosmosoma myrodora). The males of this insect species consume this weed as a means of repelling hungry prey.
The plant tissue of dogfennel contains an alkaloid toxin, pyrrolizidine. In mammals this compound causes liver damage and potentially fatal fluid retention if consumed in large quantities.
A bitter taste is present in the adult male Scarlet-bodied Wasp Moths which consume the toxin laden leaf tissues. The female moths do not eat dogfennel, but they are showered with the chemical from the males as part of the reproductive process.
Even this moth’s eggs are infused with the foul tasting compound during its incubation period.
These insects are potentially on the menu for large spiders, birds and small mammals. If dogfennel was not consumed by this brightly colored and gregarious moth, many more would likely fall prey to hungry predators.
Soon this native weed will dominate its territory and shade out competitors. Its growth characteristics are as dependable as the dogs they once served.
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