The flower spikes of ragweed are a not-so-showy yellow-green. George Weigel
Getting to know ragweed
Common ragweed (Ambrosia artemisiifolia) is a summer annual weed that’s native to the central and eastern U.S. as well as southern Canada.
Seeds of it apparently traveled in hay for horses during World War I to spread into Europe. It’s also now entrenched in Asia and South America.
Ragweed is a tough and versatile customer, growing well in poor soil and tolerating drought, occasional wetness, and a fair amount of soil saltiness.
You’ll usually find it in sunny open areas, such as farm fields, orchards, landscapes, meadows, and roadsides.
Infertile, compacted soil may stunt a ragweed plant’s height to just a foot or so, but given full sun, good soil, and ample moisture, plants can grow up to four feet tall in a single season.
At its best, one ragweed plant is capable of producing as many as 50,000 to 60,000 seeds. Those seeds are long-lived in the soil, too. One study found that 85 percent of them were still viable after 20 years lightly buried in the ground. Some can survive 40 years.
Ragweed seeds sprout from spring into early summer, but it takes plants two to four months to flower. That’s why ragweed pollen doesn’t become a sneezy problem until late in the season (August through October in most of the U.S.), even though ragweed plants are some of the earliest summer annuals to germinate (primarily in May and June).
Seedlings grow quickly into bushy plants that have two- to four-inch-long green leaves. The leaves are deeply divided into lobes, giving plants an almost fern-like appearance.
Curiously, the early leaves are arranged opposite one another on the stems, while the ensuing leaves are arranged alternately.
As an annual, ragweed plants die with frost. New colonies form when seeds drop or are transported by birds, compost, shoes, garden equipment, and in the manure of grazing animals, such as cows, sheep, and especially horses, which are fond of ragweed.