Crooked Run Valley
annual ragweed (Ambrosia artemisiifolia)
Ambrosia monophylla (Walter) Rydb.
Ambrosia artemisiifolia var. artemisiifolia L.
Ambrosia artemisiifolia var. elatior (L.) Descourtils
Ambrosia artemisiifolia var. paniculata (Michx.) Blank.
Ambrosia glandulosa Scheele
Ambrosia elatior L.
Ambrosia paniculata Michx.
Ambrosia maritima L.
TAXONOMY: The currently accepted scientific name for annual
ragweed is Ambrosia artemisiifolia L.
NATIVE STATUS: Native and introduced, United States and
GENERAL BOTANICAL CHARACTERISTICS:
Habit: Common ragweed is an annual broadleaved weed and grows 2 to 4 feet (60 to 120 cm) high. Its stems vary from unbranched to bushy. Stems may be hairless, but usually they are densely covered with stiff erect hairs about 1/8 inch (3 mm) long.
Leaves: Mature leaves are 6 to 12 inches ( 15 to 30 cm) long and 4 to 6 inches (10 to 15 cm) wide and are deeply indented. On the second and subsequent leaf pairs, the veins are visible as depressions on the upper surface and as ridges on the lower surface.
Flowers: Male and female flowers of common ragweed are in separate flower heads on the same plant (monoecious habit). The female flower heads are green, stemless, and inconspicuous. They are borne singly or in small clusters in the crooks (axils) of the upper leaves. The male heads are more clustered; 10 to 100 flowers are arranged in tight spikes at the tops of stems and branches.
Fruit/Seed: Common ragweed reproduces by seeds that are about 1/8 inch (3 mm) long and have several short spiny projections near one end.
Roots: Common ragweed has a shallow, fibrous root system.
REGENERATION PROCESS: Annual ragweed propogates itself by
reseeding. Although each female flower produces only one seed, a plant that emerges in mid-May can easily have 30,000 to 62,000 seeds. Seeds are dispersed by water (through rain-wash channels or gulleys), birds, burrowing animals, and humans. During summer and fall, wind plays a minor role in dispersal, but in winter it may roll the seeds for long dis-
tances over the surface of crusted snow.
HABITAT TYPES: Common ragweed is widespread on arable land and is found in cultivated fields, gardens, vacant lots, and waste places, and along roadsides and fence rows. Other native habitats include hill prairies, gravel prairies, meadows in woodland areas, and the edges of gravelly seeps. It can be found in disturbed areas of mesic to dry black soil prairies, particularly along the margins near developed areas.
Ragweed is a typical after-harvest cover in grain and hay fields. It is also abundant in cereal crops and cultivated row crops. It appears as a weed in gardens and lawns.
SITE CHARACTERISTICS: Annual ragweed prefers full sun and average to slightly dry conditions. This plant is rather indifferent to soil type, and will thrive in soil containing high amounts of clay, gravel, or sand – in fact, it prefers sterile soil because of the reduced competition from other plants. Resistance to drought is very good, although some of the lower leaves may wither away. It can grow in clay, silt, and sand mixtures but prefers heavier, moist soils with a pH between 6.0 and 7.0. Common ragweed is extremely competitive, partly because it can accumulate large quantities of trace elements. Corn studies show that common ragweed generally absorbs much more boron, copper, magnesium, zinc, tin, galium, vanadium, bismuth, nickel, chromium, potassium, and calcium than do com leaves harvested at tassel stage. Ragweed grows well on soils containing enough zinc to be toxic to other plants. When com and ragweed were grown in soil with a heavy concentration of zinc, ragweed absorbed about seven times more zinc than com did. A severe ragweed problem causes extreme nutritional deficiencies in crops.
SEASONAL DEVELOPMENT: Common ragweed flowers in August and early September, producing huge amounts of dry dusty pollen that is dispersed mainly by wind.
GENERAL DISTRIBUTION: Common ragweed is found throughout the United States and Canada except for the northernmost Great Lakes region and northern Maine.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: Common ragweed is very valuable to many kinds of wildlife. Honeybees have been observed collecting pollen from the male flowers, otherwise flower-visiting insects are not attracted to this plant. The caterpillars of several moths eat the foliage, flowers, or seeds, including Schinia rivulosa (ragweed flower moth), Synchlora aerata (wavy-lined emerald), Tarachidia erastrioides (small bird-dropping moth), Tarachidia candefacta (olive-shaded bird-dropping moth), and others. In my experience, some species of grasshoppers are quite abundant around colonies of common ragweed, probably because they eat the foliage and prefer the disturbed, open habitats where this plant occurs. Many upland gamebirds and granivorous songbirds are attracted to the oil-rich seeds. Because the spikes of seeds often remain above snow cover, they are especially valuable to some of these birds during winter. The seeds are also eaten to some extent by the thirteen-lined ground squirrel, meadow vole, and prairie vole. The seeds are probably semi-digestible, thus some of them are likely distributed far and wide by these animals. On the other hand, the foliage is quite bitter, therefore mammalian herbivores do not often consume it.
While annual ragweed has ecological benefits, it also negative ecological consequences. It invades and suppresses weak and overgrazed pastures, reducing productivity and infestations can become particularly dense in pastures which are overgrazed.
Annual ragweed pollen contains highly potent allergens that can cause respiratory allergies such as hay fever and aggravate asthma.