The common name may refer to the size of this plant, a robust, coarse summer annual in the Aster Family. Horseweed grows 1 to 6 feet tall on a stout stem often covered with long white hairs. There are a lot of narrow leaves crowded on the stem, which becomes branched near the top. These flowering branches have many small flower heads, each one with a yellow disk packed with 12-25 florets. The petals (rays) are tiny, less than 1 mm. long, white and inconspicuous. The reproductive flowers are in the central disk, a common pattern with asters.
In early fall the flowers are replaced with seeds (achenes) covered with fine bristles, which are carried by air to new locations.
A very similar plant is Fireweed (Erechtites) but the leaves of Fireweed are larger, with teeth on the edges, and scattered alternately on the stem.
Horseweed grows rapidly in the summer and has a long blooming period, from July through November. The plant thrives on bare soil, will grow on many soils, including those with clay and gravel, and can survive drought. Growing in roadsides, old fields, and disturbed ground, it is found all over North America and in every county in Virginia. First listed in North America in 1640, it was in France 13 years later, perhaps from seeds carried on beaver pelts exported from Canada.
Native Americans and early settlers used a tea from the leaves to treat dysentery and sore throat. Young leafy seedlings and young leaves can be eaten after boiling, and the dried leaves have a flavor similar to that of tarragon. Many herbal uses have been found for this plant. Essential oil from the leaves has been used to stop bleeding, as an astringent to dry mucous membranes and to flavor candy, condiments and soda.
Deer don’t eat Horseweed because the leaves contain a turpentine-like substance that is resinous and bitter to the taste. The foliage can irritate the skin of some people, and the noses of horses that try to feed on it. The flowers are attractive to insect pollinators, including flies and many species of bees.
Past President of the John Clayton Chapter, VNPS
Author: Wildflowers and Grasses of Virginia’s Coastal Plain