Great Ragweed is a hugh, coarse, native, erect, annual plant growing from 3 to 12 feet high, with occasional branching below the floral array. Stems are green with bristly white hair.
The leaves are opposite with 3 main veins and the larger leaves (up to 12 inches long and 8 inches wide) are palmately divided into 3 to 5 lobes. The margins are usually toothed and the leaf has a long winged stalk. Upper leaves are smaller and more lance shaped, usually with 3 lobes and also with a wing. The leaves have a rough surface.
The floral array is a cylindrical spike, up to 10 inches long, of many small, yellowish-green male flower heads shaped like beads. The main stems many develop shorter side spikes along side the main spike.
Flowers: Male and female flowers are composite and separate on the same plant. The male florets (staminate) have a saucer-shaped head, 2 -4 mm in diameter, only 1/4 inch long and have a short stalk no more than 1 to 3+mm long with the florets facing downward. The five stamens of each floret produce a yellow pollen that is wind dispersed. Each male flower head has 3 to 25 florets surrounded on the outside by a series of green phyllaries that are rough on the outside; the head circumference often has 1 to 3 black nerves visible. Male heads turn yellow to brown with maturity. Female (pistillate) flower heads are whitish-green, each containing one fertile floret and clustered in small groups in the leaf axils near and below the male flowers. At the base of the floral array are several bract-like leaves.
Seeds: Fertile flowers produce small bur containing a pyramidal shaped, tough, 1/4 inch long (3 - 5 mm) cypsela that has 4 to 5 (sometimes 5 to 7) rounded spines (tubercles) and a short central beak; from the side the cypsela looks like a small crown, hence one of the older common names of 'Crown Weed'. The bur is dispersed by clinging to fur or clothing. Cypselae are viable for several years. The tubercles contain air spaces allowing the seed to float on water and to be carried by the wind.
Habitat: Great Ragweed grows in disturbed sites in wet to dry loamy soils. Full sun to partial shade is tolerated. The root system is fibrous.
Names: The genus, Ambrosia, is Greek for 'food of the gods' but explanations of this appellation are lacking in context other than the genus is applied to a group of herbs that have high levels of wind borne pollen - some of which could be aromatic or sweet. The species, trifida, is appropriate, meaning having leaves that are three-cleft like most leaves of this plant are except the very upper ones. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Varieties: Older literature lists several varieties for this species, var. trifida for Minnesota as an example, but current botanical thinking places these as regional types of the same basic species, not different enough to separate. Both Flora of North America (Ref. #W7) and the Minnesota authorities at the U of M Herbarium (Ref. #28C) take this approach.
Comparisons: None of the other ragweeds approach in appearance this species. The others are smaller and with more fern-like foliage such as the Common Ragweed, A. artemisiifolia.
Above: Great Ragweed are large plants, this one is 7 feet tall. Illustration by Norman Criddle.
Below: 1st photo - Male flowers, note the black nerves on the phyllaries. 2nd photo - The crown shaped cypselae.
Below: The flowers compared. 1st photo - female flowers at the base of the flower raceme with the small leafy bracts subtending them. 2nd photo - Male flowers composed of multiple florets on the raceme above the female flowers. Note the black nerves on both.
Below: 1st & 2nd photos - Leaves of Giant Ragweed have 3 to 5 lobes and a winged stalk. 3rd photo - Spike of the male flowers.
Below: A comparison of the large lower leaf with an upper stem leaf that frequently lack lobes. Both have winged stalks.
Below: 1st photo - Note the hair on the leaf stalk and the veins. 2nd photo - Seed capsules from the fertile female flowers. 3rd photo - Note the bristly white hair on the stem.
Notes: Great Ragweed is found throughout the U.S. except Alaska and Nevada and it appears in most of the lower Canadian Provinces. Within Minnesota there are some widely scattered counties where it has not been found, principally in the central part of the state. There are three ragweed species native to Minnesota: Western or Perennial ragweed, A. psilostachya; Common Ragweed, A. artemisiifolia, and this species. Several other species are of historical note only. Only Common Ragweed is found today in the Eloise Butler Wildflower Garden. Giant Ragweed was in the Garden at the time of Martha Crone's 1951 census.
Pollen: The male florets 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. If allowed to grow in fields it starves out plants near to it.
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"