Common Ragweed is a native erect annual, growing to 40 inches high with many upper branches. The stems are hairy and green to pinkish red.
The leaves are opposite lower, up to 4 inches long and alternate upper. They are 1-2 pinnatifid, that is once divided and the second division not fully cut to the central rib - feathery or fern-like in appearance, triangular in shape overall, fine hair on new leaves which dissipates with age. Leaf margins are usually smooth but they may have small teeth.
The floral array is a raceme like spike at the top of the stem containing numerous small green flower heads that look like green beads. The spike can be from 1 to 6 inches long and main stems many develop side shorter side spikes along side the main spike.
Flowers: The 'green' beads' are the male flowers and they turn yellow to brown with maturity. Male and female flowers are separate. Each male flower has a cup-shaped head and is only 2 to 3 mm in diamter, 1/8 inch long with a short stalk no more than 1.5 mm long, and the flowers produce a yellow pollen that is wind dispersed. The male flower head has 12 to 20 florets and the outside of the head usually does not have black nerves visible like the Giant Ragweed does. Female flowers are whitish-green, have one floret with a branching style protruding. They are clustered in the leaf axils, obscured by leafy bracts, near and below the male flowers. The outside of the flower head of both sexes is enclosed in a series of green phyllaries (bracts) in several series.
Seeds: Fertile flowers produce small ovoid bur, that contains a 1/8 inch long cypsela (the seed) that has 3 to 5 spines and a short central beak. The bur is dispersed by clinging to fur or clothing. Cypselae are viable for several years.
Habitat: Common Ragweed grows in disturbed sites in wet to dry soils of many types. Full to partial sun is required. The root system is fibrous with deep branches.
Names: The genus, Ambrosia, is Greek for 'food of the gods' but explanations of this appellation to plants of this genus are lacking. The species, artemisiifolia, is more appropriate, meaning having leaves that resemble those of the Artemisia plant, which these do.
Comparisons: The closest ragweed in appearance to this species is the Western or Perennial ragweed, A. psilostachya, where the leaves are not as fern-like and per the name, it is a perennial. Giant Ragweed, A. trifida, is the other ragweed native to Minnesota, but there the plant is much taller and leaves are not fern-like.
Above: 1st photo - Common Ragweed branches profusely. 2nd photo - Flower spikes develop from the upper leaf axils. These spikes contain the staminate (male) flowers (3rd photo)
Below - Male and Female flowers are separate. 1st photo - The male flowers (1st photo) are in small bead-like heads on the elongated spike. 2nd photo - The female flowers are clustered in a group at the base of the spike. They are obscured by leafy green bracts but the long styles are visible. Note the hair on all parts.
Below: 1st photo - The leaf is 1 or 2-pinnatifid, smaller and alternate toward the top of the stem. 2nd photo - a female flower cluster.
Below - Seeds: Inside the bur that develops from the female flower is a 1/8 inch long cypsela that has 3 to 5 spines and a short central beak. These somewhat resemble the cypselae of Giant Ragweed, but shorter.
Below: The green stem can have reddish tones and is hairy.
Notes: Common Ragweed is considered to be indigenous to the area of the Garden. The plant is found throughout North America except Alaska, Yukon, Nunavut and Labrador. Within Minnesota there are some widely scattered counties where it has not been found. There are three ragweed species native to Minnesota as described above, only this species is found in the Garden.
Pollen: The male flowers of Ragweed are said to produce up to a billion grains of pollen per large plant, which is carried far and wide by the wind, making the pollen of this plant the bane of hay fever sufferers. Wetter years produce more pollen. Yet, the plant is beneficial to many birds and animals as the seeds are rich in oil.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"