There are three species of Ragweed that can be found in North America. Ambrosia artemisiifolia L. (Common Ragweed 1), Ambrosia psilostachya (Cuman Ragweed 2) and Ambrosia trifida (Giant Ragweed 3)
Ambrosia artemisiifolia, Common Ragweed, is the most widespread plant of the genus Ambrosia in North America. It has also been called Annual Ragweed, Bitterweed, Blackweed, Carrot Weed, Hay Fever Weed, Roman Wormwood, Stammerwort, Stickweed, Tassel Weed, and American Wormwood. It is native in North America. The species name, artemisiifolia, is given because the leaves were thought to bear a resemblance to the leaves of Artemisia, the true wormwoods.
It is an herbaceous
plant in the Asteraceae family native to North America. It is a successful
pioneer species widely distributed throughout the continental United States.
Common ragweed is most often found in frequently disturbed habitats such as
cultivated fields, orchards, landscapes, nurseries, roadsides, and waste
places. it is especially abundant along ditch and canal banks,
places where its seeds are easily disseminated into cultivated fields. Common
ragweed growing in cultivated fields will compete with crops for light,
moisture, nutrients, and space and will result in significant yield losses.
Additionally, allergenic airborne pollen from common ragweed is a primary cause
of hay fever and thus a public health concern.
Biology and life cycle
Common ragweed is an erect, branching summer annual that can
rapidly grow to more than 7 ft tall. As a prolific seed producer, it
is capable of producing 32,000 to 62,000 seeds per plant when growing without
any competition. These seeds can survive and remain viable for many years in
undisturbed soil. Dormant common ragweed seeds require a period of exposure to
cold temperatures to germinate. Germination is optimal in the spring at soil
temperatures of 50 to 80° F. However, temperatures above 86° F will halt
germination and send the seeds back to dormancy until a repeat of the cold requirement
the following winter.
Measuring 0.2-0.4 inches long, the cotyledons are thick,
dark green, spatulate, and often deep purple underneath. The first true leaves
are lobed and hairy. Subsequent leaves are deeply cleft on the
margins, forming rounded or slightly pointed lobes. The youngest
leaves are initially opposite but later become alternate at the forth node. The
blades are hairy, but the underside is densely hairy compared to the upper
surface and margins.
Stems are green to purple, erect, and branching above; also,
they have long rough hairs with a shallow taproot that produces a
fibrous root system. The leaves are compound, deeply lobed, and usually much
wider at the base than the tip. Mature leaves are relatively hairless, but
small leaves often have hairs on the underside. In some biotypes, leaves will remain
hairy throughout their entire growth cycle. Petioles are present on lower
leaves and absent in upper leaves. Lower leaves are arranged oppositely while
upper leaves are often arranged alternately on the stem of older plants.
Inconspicuous small (1/8 inch long), green male and female flowers are present
on separate heads on the same plant. Male flowers, usually drooping, are at the
top of the plant, and female flowers are in the axils of the upper leaves and
branches. Individual plants produce in excess of 1 billion
wind-dispersed pollen grains. A single small seed is enclosed within each
fruit, which has several longitudinal ridges ending in short spines resembling
There are 4 stages in the growth of Ragweed; Seed Germination, Vegetative growth, Flowering & seed - fruit development , Maturity & Senescence.
A - plant beginning to flower /
B - spikes of male flower heads ready to release pollen
Common ragweed. A. Seedling, top and side views. B. Portion of stem with 2 leaves. C. End of flowering branch. D. 1-sided "raceme" with 7 heads, each with 1 female flower.
Stems & Roots: Stems erect,
15-150cm (6-60in.) high, usually much-branched, hairless or hairy throughout;
lower leaves opposite (2 per node) but becoming alternate (1 per node) higher
on the plant, bright green to slightly yellowish-green on young plants,
becoming grayish-green on older plants, compound and finely divided, the final
divisions usually coarsely toothed.
Flowers & Fruit: Flower heads not showy, individually
small, 2-5mm (1/12-1/5in.) across, green and inconspicuous but very numerous
and forming distinctive inflorescences; individual florets either male or
female, but never both; all flowers within one flower head either only male or
female, but both male flower heads and female flower heads usually present on
the same plant; heads of male (pollen-producing) flowers in raceme-like
elongated clusters at ends of branches, each male head hanging downwards on a
short stalk like a tiny inverted umbrella; female (seed-producing) flower heads
in axils of short, narrow, green bracts near the base of each long cluster of
male flower heads, each female head with only a single flower and producing a
single, hard, somewhat triangular or diamond-shaped seed with several, short,
sharp spines around the upper shoulder, the whole seed 3-5mm (1/8-1/5in.) long.
Flowers from August to October.
Habitat: Common ragweed is one of the most abundant weeds
of cultivated land throughout southern Ontario, but is rare or absent in
northern and northwestern parts of the province. It also occurs in gardens,
flower borders, poorly kept lawns, edges of sidewalks, roadsides, fencelines,
waste places, and in disturbed areas in pastures and meadows.
Similar Species: It is distinguished
by its finely divided leaves, which are opposite in the lower part and
alternate in the upper part of the plant, these being yellow-green at first,
later gray-green with age, and its very numerous, tiny, non-showy, greenish
male flower heads clustered along slender branches in the upper part of the
Seasonal Allergy (Hay Fever)
Ragweeds are weeds
that grow throughout the United States. They are most common in the Eastern
states and the Midwest. A plant lives only one season, but that plant produces
up to 1 billion pollen grains. Pollen-producing and seed-producing flowers grow
on the same plant but are separate organs. After midsummer, as nights grow
longer, ragweed flowers mature and release pollen. Warmth, humidity and breezes
after sunrise help the release. The pollen must then travel by air to another
plant to fertilize the seed for growth the coming year.
usually grow in rural areas. Near the plants, the pollen counts are highest
shortly after dawn. The amount of pollen peaks in many urban areas between 10
a.m. and 3 p.m., depending on the weather. Rain and low morning temperatures
(below 50 degrees Fahrenheit) slow pollen release. Ragweed pollen can travel
far. It has been measured in the air 400 miles out to sea and 2 miles up in the
atmosphere, but most falls out close to its source.