Other common names: Roman wormwood, hog weed, bitter weed, wild tansy, mayweed, hay fever weed, blackweed, Roman wormweed, annual ragweed, short ragweed, small ragweed
Ambrosia artemisiifolia L.
Identification of Ragweed
Family: Aster family, Asteraceae
Habit: Erect, branched, summer annual herb
Description: Seedling stems are purple-spotted to purple. Cotyledons are dark green, paddle shaped, thick and 0.2–0.4 inch long. Purple flecking is possible along the edges. Young leaves are green, deeply divided into lobes with slightly pointed to rounded tips; leaves are hairy above and densely hairy below. The first four pairs of true leaves are opposite, but later leaves are alternate. Mature plants are 1–3 feet tall, erect, branched and are with a shrub-like appearance. Stems are densely covered with rough, 0.1 inch hairs. Leaves are 2–4 inches long, with or without hairs on all surfaces. Leaves are fern like, deeply divided into many lobes or composed of many smaller, strongly divided and lobed leaflets. Leaf stalks are longer on the lower part of the plant. Roots are fibrous and shallow, often close to the soil surface. Flowers are green, 0.1 inch wide and arranged in clusters of male and female flowers on each plant. Male flowers are produced in spikes at branch tips of upper branches, whereas female flowers occur singly or in clusters in lower leaf axils. Plants can flower even when 2 inches tall if cut or mowed. Fruits are top shaped, woody, brown with spots or streaks, 0.14 inch long by 0.1 inch wide. Ridges on the side extend over the top, resulting in a crown-like appearance. A longer beak rises out from the center of the crown. Seeds are brown, 0.1 inch long by 0.08 inch wide and disperse in the hard fruits.
Similar species: Seedlings of giant ragweed (Ambrosia trifida L.) and corn chamomile (Anthemis arvensis L.) are similar to common ragweed. Giant ragweed cotyledons are larger, measuring 0.75–1.75 inches long by 0.5 inches wide, and young leaves are either unlobed or, more commonly, are divided into three lobes. The first leaves of the chamomile species are more finely lobed or divided, have longer stalks and lack dense hairs on the underside. Western ragweed (Ambrosia psilostachya DC.) is similar to common ragweed, however western ragweed is a perennial with a taproot. The leaves of western ragweed are lanceolate and less finely divided than those of common ragweed. Mugwort (Artemisia vulgaris L.) has similar foliage to that of common ragweed but has silvery-white hairs on the undersides of the leaves. Also, mugwort is a perennial with branching rhizomes.
Management of Ragweed
Common ragweed seeds germinate primarily in the spring. If the seeds are near the soil surface, delaying tillage as long as possible allows more to germinate before planting and decreases density in the crop. Similarly, if the seeds are mixed through the soil profile, a lag between tillage and seedbed preparation allows many to be eliminated before planting. Thus, rotations that include late planted crops tend to decrease the species, provided weed control in the crop is adequate. Since common ragweed does not set seed until late in the summer, early harvested crops like winter grains or short cycle, spring planted vegetables provide an opportunity to prevent seed production and break the life cycle. Because the seeds are highly persistent in the soil, however, you may need several years of good management before the population is well controlled.
Tine weeders and other in-row cultivators will kill many seedlings before they emerge but fewer after they emerge because the seedlings grow quickly. After emergence, aim to bury the fast growing seedlings before they begin to elongate. In a year when prolonged rains prevented tine weeding of soybeans until four weeks after planting, we controlled a dense stand of common ragweed by aggressive tine weeding followed immediately by inter-row cultivation that threw about 1 inch of soil into the row. At that time, the second leaves with three leaflets were starting to emerge on the soybeans.
Straw mulch is less effective for suppressing common ragweed than for other annual weeds but can still be useful if applied in especially heavy layers or if a paper barrier is used under the straw. An early-planted winter annual cover crop that establishes a complete leaf canopy by early spring will suppress ragweed establishment, whereas a late-planted cover crop with an incomplete canopy will permit establishment of a competitive ragweed population in no-till planted crops. Also, interseeded red clover in wheat can moderately suppress common ragweed. We have found that mowing the clover at a height of about 5 inches after common ragweed has flowered but before seeds form effectively prevents seed set because the clover rebounds from mowing more rapidly than the ragweed.
Because the seeds are so persistent in the soil, prompt cleanup of fields after harvest is recommended. Rogue large individuals out of vegetable crops. Capture the seeds when combine harvesting soybeans. These can be ground and fed to livestock. If seeds are produced, leave them on the soil surface until spring to maximize seed predation.
Ecology of Ragweed
Origin and distribution: Common ragweed is native to central and eastern United States and southern Canada, but in forested regions it was probably rare and restricted to disturbed, open areas prior to the advent of Native American agriculture. The species has been introduced to locations in Europe, Asia and South America.
Seed weight: Population mean seed weights with the fruit coat ranged from 1.7–4.6 mg, with most means greater than 4.4 mg. One population had a mean without the fruit coat of 2.4 mg. Seeds harvested in France ranged from 1.2–7.7 mg and had similar germination rates regardless of seed weight.
Dormancy and germination: Common ragweed seeds are dormant when shed in fall. The seeds require several weeks of chilling at 41°F for germination. Light and alternating temperatures in the range of 50–86°F increase germination of seeds after chilling. Optimum alternating day/night temperatures required for germination were 77/68°F to 95/86°F. Seeds that do not germinate under dark, low-temperature, early spring conditions (as would occur when buried in soil) and are subsequently exposed to increasingly warm late spring soil temperatures become dormant and again require a chilling period for germination. Seeds chilled in the absence of oxygen remain dormant, suggesting that seeds buried at soil depths that restrict oxygen would remain dormant despite cold winter temperatures. A range of concentrations of N-P-K nutrients had no effect on germination. The presence of established plants can reduce germination and emergence of common ragweed, presumably through impact on the temperature and light environment of seeds.
Seed longevity: Seeds can persist for up to 40 years in the soil, but most die earlier. Viability was 85% after 20 years of burial. In shorter experiments covering two to three years, the annual seed mortality rate was 7% and 12%.
Season of emergence: Emergence occurs mainly in spring with declining emergence rates in early summer. If a seedbed is prepared in early spring, most emergence will have occurred by late spring. It is typically one of the earliest emerging weeds and has a relatively short duration of emergence.
Emergence depth: Seedlings emerge best from seeds at or very near the soil surface. Nearly all seedlings emerge from the top inch of soil, though a few can establish from larger seeds placed as deeply as 3.1 inches, and no seedlings can emerge from 4 inches.
Photosynthetic pathway: C3
Sensitivity to frost: Common ragweed is killed by frost.
Drought tolerance: The species is drought tolerant. Extreme drought limits above and belowground growth, but the ratio of roots to shoots is not affected. Photosynthesis declines with high water stress, but remarkably rapid recovery is observed when water stress is relieved. Despite tolerance to short-term drought conditions, common ragweed generally thrives best in environments with high annual rainfall, and specifically with high October rainfall that presumably favors seed development and maturation.
Mycorrhiza: Common ragweed is considered a strong mycorrhizal host.
Response to fertility: Common ragweed is highly tolerant of infertile soils, but it also responds to fertility. On infertile sites, plants less than 4 inches tall may mature, whereas on highly fertile sites, plants may reach 6 feet. The species is known to be a strong accumulator of N, P and K and many micronutrients, particularly zinc.
Soil physical requirements: Common ragweed tolerates a wide range of soil textures and drainage conditions. It is highly tolerant of extreme soil compaction. Common ragweed tolerates moderate soil salinity by decreasing allocation to root growth relative to shoot growth. Salt tolerant populations occur on roadsides that receive deicing salt during the winter.
Response to shade: Common ragweed tolerates moderate shade, but photosynthesis progressively declines and growth is stunted as shade increases. Light reduction of 62% only reduced its growth rate under cool conditions (63/45°F) but not under warm conditions of 73/55°F to 84/66°F. It is one of the few annual weeds that regularly mature (always at a small size) in species-diverse hay meadows and goldenrod dominated fields.
Sensitivity to disturbance: Seedlings are fragile and easily broken, and they quickly dry when uprooted. However, because common ragweed rapidly produces a long taproot, it is more difficult to kill in the seedling stage than are many other small seeded annuals. Medium sized to large plants will reroot readily in moist soil. Common ragweed has greater capacity for regrowth following repeated cuttings than other upright annual broadleaf weeds. Repeated mowing of the plants at 8, 16 or 32 inches causes some mortality and reduces size at the end of the season but does not prevent reproduction. One mowing in mid-summer may reduce biomass substantially, but plants generally have a high capacity for regrowth by lateral stems that can produce viable seeds. No-till management of cover crops with a roller-crimper does not kill established common ragweed plants.
Time from emergence to reproduction: Common ragweed flowers two to five months after emergence, generally from August to October in response to decreasing day length. Flowering is initiated when day length decreases to 14 hours per day or less. Genotypes from northerly locations flower earlier than those from southerly locations.
Pollination: Common ragweed has male and female flowers on the same plant and can be either self- or cross pollinated by wind.
Reproduction: Common ragweed seeds mature as the plant senesces. Typical sized plants produce 3,000–60,000 seeds, but small, highly stressed individuals may produce as few as one or two seeds. Plants in Quebec produced up to 18,000 seeds without competition and 3,000 seeds when growing with crops.
Dispersal: Common ragweed seeds are each tightly encased in a woody fruit that is the dispersal unit. These have no adaptations for dispersal. However, because they often reach high densities in soil, they are easily spread from one site to another in soil clinging to boots, tires, tillage equipment, etc. Common ragweed is one of the most prevalent species along field margins; from there it can migrate into fields. Seeds seem to be heat tolerant and may be spread in compost. The seeds survive well in the digestive tracts of cows, sheep, horses, etc., and manure is commonly contaminated with common ragweed seeds. Horses are particularly fond of common ragweed, and horse manure is especially likely to be contaminated. Birds also sometimes disperse common ragweed. Long distance dispersal can take place in contaminated crop seed, birdseed or hay. Hay transported from North America to feed horses during World War I is speculated to have contributed to the spread of this species in France.
Common natural enemies: Two native chrysomelid beetles, Ophraella communa and Zygogramma suturalis, feed on the foliage. Ophraella communa is most adapted for use as a biological control agent in subtropical climates. White rust (Albugo tragopogi) was isolated from local populations in Quebec and severely reduced growth, pollination and reproduction of 14% of common ragweed plants tested.
Palatability: The seeds are edible but so small and difficult to extract from the hard fruit coat that their use as a grain is impractical. Palatability to sheep was lower than that of many annual weeds despite having similar protein and nutrient levels.
Note: The pollen is extremely allergenic and causes hay fever symptoms in 10% of people in the United States. Common ragweed plants can produce over a billion pollen grains per plant when growing with crops. Some people also develop allergic skin reactions on contact with the foliage and pollen.
Summary Table of Ragweed Characteristics
| Common ragweed | ||||||||
|---|---|---|---|---|---|---|---|---|
| Growth habit | Seed weight (mg) | Seed dormancy at shedding | Factors breaking dormancy | Optimum temperature for germination (F) | Seed mortality in untilled soil (%/year) | Seed mortality in tilled soil (%/year) | Typical emergence season | Optimum emergence depth (inches) |
| medium, branched | 1.2–7.7 | Yes | cms, li, at | 77/68 to 95/86 | 7–12 | – | early to late spring | 0–1 |
| Photosynthesis type | Frost tolerance | Drought tolerance | Mycorrhiza | Response to nutrients | Emergence to flowering (weeks) | Flowering to viable seed (weeks) | Pollination | Typical & high seed production (seeds per plant) |
| C3 | low | high | yes | high | 10–20 | – | both | 3,000 & 60,000 |
Table Key
General: The designation “–” signifies that data is not available or the category is not applicable.
Growth habit: A two-word description; the first word indicates relative height (tall, medium, short, prostrate) and second word indicates degree of branching (erect, branching, vining).
Seed weight: Range of reported values in units of “mg per seed.”
Seed dormancy at shedding: “Yes” if most seeds are dormant when shed, “Variable” if dormancy is highly variable, “No” if most seeds are not dormant.
Factors breaking dormancy: The principle factors that are reported to break dormancy and facilitate germination. The order of listing does not imply order of importance. Abbreviations are:
scd = seed coat deterioration
cms = a period subjected to cold, moist soil conditions
wst = warm soil temperatures
li = light
at = alternating day-night temperatures
ni = nitrates
Optimum temperature range for germination: Temperature (Fahrenheit) range that provides for optimum germination of non-dormant seeds. Germination at lower percentages can occur outside of this range. The dash refers to temperature range, and the slash refers to alternating day/night temperature amplitudes.
Seed mortality in untilled soil: Range of mortality estimates (percentage of seed mortality in one year) for buried seeds in untilled soil. Values were chosen where possible for seeds placed at depths below the emergence depth for the species and left undisturbed until assessment. Mortality primarily represents seed deterioration in soil.
Seed mortality in tilled soil: Range of mortality estimates (percentage of seed mortality in one year) for seeds in tilled soil. Values were chosen for seeds placed within the tillage depth and subjected to at least annual tillage events. Seed losses are the result of dormancy-breaking cues induced by tillage, germination and deterioration of un-germinated seeds.
Typical emergence season: Time of year when most emergence occurs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.
Optimum emergence depth: Soil depths (in inches below the soil surface) from which most seedlings emerge. Lower rates of emergence usually will occur at depths just above or just below this range.
Photosynthesis type: Codes “C3” or “C4” refer to the metabolic pathway for fixing carbon dioxide during photosynthesis. Generally, C3 plants function better in cooler seasons or environments and C4 plants function better in warmer seasons or environments.
Frost tolerance: Relative tolerance of plants to freezing temperatures (high, moderate, low).
Drought tolerance: Relative tolerance of plants to drought (high, moderate, low).
Mycorrhiza: Presence of mycorrhizal fungi. “Yes” if present; “no” if documented not to be present, “unclear” if there are reports of both presence and absence; “variable” if the weed can function either with or without, depending on the soil environment.
Response to nutrients: Relative plant growth response to the nutrient content of soil, primarily N, P, K (high, moderate, low).
Emergence to flowering: Length of time (weeks) after emergence for plants to begin flowering given typical emergence in the region of occurrence. For species emerging in fall, “emergence to flowering” means time from resumption of growth in spring to first flowering.
Flowering to viable seed: Length of time (weeks) after flowering for seeds to become viable.
Pollination: “Self” refers to species that exclusively self-pollinate, “cross” refers to species that exclusively cross-pollinate, “self, can cross” refer to species that primarily self-pollinate, but also cross-pollinate at a low rate, and “both” refers to species that both self-pollinate and cross-pollinate at relatively similar rates.
Typical and high seed production potential: The first value is seed production (seeds per plant) under typical conditions with crop and weed competition. The second value, high seed production, refers to conditions of low density without crop competition. Numbers are rounded off to a magnitude that is representative of often highly variable reported values.
Further Reading
Bassett, I.J. and C.W. Crompton. 1975. The biology of Canadian weeds. 11. Ambrosia artemisiifolia L. and A. psilostachya DC. Canadian Journal of Plant Science 55: 463–476.
Bazzaz, F.A. 1974. Ecophysiology of Ambrosia artemisiifolia: A successional dominant. Ecology 55: 112–119.
Buttenschøn, R.M., S. Waldispühl and C. Bohren. 2010. Guidelines for management of common ragweed, Ambrosia artemisiifolia. http://euphresco.org.
Mutch, D.R., T.E. Martin and K.R. Kosola. 2003. Red clover (Trifolium pratense) suppression of common ragweed (Ambrosia artemisiifolia) in winter wheat (Triticum aestivum). Weed Technology 17: 181–185.