Other common names: great ragweed, buffalo weed, kinghead, crown weed, wild hemp, horse weed, bitterweed, tall ambrosia, tall ragweed

Ambrosia trifida L.

Identification of Giant Ragweed

Family: Aster family, Asteraceae

Habit: Tall, branched, summer annual herb

Description: Seedling cotyledons are round to paddle shaped, 0.75–1.75 inch long by 0.25–0.5 inch wide. The stem is shiny and green with purple spots. The first two true leaves are lanceolate to oval shaped with widely spaced, shallow teeth, and they may have two basal lobes. Subsequent young leaves are opposite, roughly hairy and divided into three deep lobes. Mature plants are typically 3–12 feet tall, though they may reach up to 20 feet in height. Stems are single or branching and are covered in short, rough hairs. Leaves are opposite, up to 12 inches long and 8 inches wide, usually divided into three to five lobes, toothed along the edges and hairy on all surfaces. Leaf stalks are 0.4–2.75 inches long and sometimes winged. Uppermost leaves are lanceolate. Roots are fibrous, occasionally with a short taproot. Flowers are green, 0.125 inch in diameter and arranged in separate clusters of male and female flowers on a single plant. Male clusters are arranged in dense, 3–8 inch-long spikes at the ends of stems and branches. Female flowers are located in clusters of leafy bracts below the male spikes and in the upper leaf axils. Each female flower produces one thick-walled fruit encasing a single seed. Fruits are 0.2–0.6 inch long, ridged, brown to grey/black, with a blunt beak at the apex. Ridges terminate in five to eight blunt spines that form a crown around the beak. Seeds are brown and oval to egg shaped. 

Similar species: Common ragweed (Ambrosia artemisiifolia L.) leaves are much more finely dissected than those of giant ragweed. Young giant ragweed plants may resemble sunflowers (Helianthus spp.). Sunflower leaves are not lobed and become alternate as the plant grows, while giant ragweed leaves are lobed and opposite. 

Management of Giant Ragweed

Giant ragweed is a rapid growing, competitive species that can cause substantial yield losses even at low densities. It is increasingly a problem in field crops and is associated most strongly with soybean production and minimum tillage. Efforts should be made to control it in fencerows and field borders since problematic populations in fields are often associated with its presence in border habitats. 

Rotation with hay or pasture will give the relatively short-lived seed bank a chance to decline in density. Inclusion of crops or cover crops that provide early spring leaf canopy or residue cover that lowers soil temperature delays initiation of giant ragweed emergence. Rotation with cereal grains provides an opportunity after harvest to kill giant ragweed before it can go to seed. Also, since the majority of seeds are retained on plants after typical grain harvest dates, seeds can be captured or destroyed during combine harvesting. On vegetable farms, it can be eliminated by tillage after short season spring crops, which prevents reproduction, or by tillage before summer planted crops, which kills seedlings after the bulk of emergence has already occurred.

Because seeds on the soil surface are subject to high rates of seed predation, if possible, avoid fall tillage after harvest of corn, soybeans and other late harvested crops. If a cover crop is required, interseed it during the last cultivation, or broadcast it on the soil surface before or after harvest. Refuges for rodents and large invertebrate seed predators (e.g., grassed drainage ways, hedgerows) can potentially increase predation of giant ragweed seeds. However, every effort should be made to prevent reproduction when earthworm populations are high, because they can facilitate seed survival by burying seeds in their burrows within a few days. 

Giant ragweed tends to emerge early in the spring, but seeds on the soil surface may not get sufficient moisture for germination. Shallow tillage early in spring to cover the seeds and promote emergence followed by a later tillage before planting soybeans or dry beans can further eliminate a substantial proportion of the previous year’s seed production.

Due to its large seed size, giant ragweed seeds can emerge from below the planting depth of crops and grow very quickly, which makes rotary hoeing largely ineffective. The window for tine weeding is very narrow and corresponds to the stage when the seed leaves are just unfolding. A stiff tined implement can break and bury a substantial portion of the seedlings at this stage. Use a belly mounted cultivator or a three-point hitch mounted cultivator with a good guidance system to cultivate as close as possible to the crop row. Low pitch sweeps run very shallow are most effective for cutting off the giant ragweed without damaging the crop. Hilling up in the crop row is often ineffective since the weed usually grows as fast as the crop. If you need to hill up to control other species, you may need to make two passes with different machines or modify your cultivator. One possible configuration is to run shallow sweeps in front and a large sweep with a hilling attachment on the center shank in the rear. Because giant ragweed quickly emerges above the canopy of soybeans and dry beans, a front mounted mower can greatly reduce seed production and competition with the crop.

Ecology of Giant Ragweed

Origin and distribution: Giant ragweed is native to stream banks and floodplains of North America. It currently occurs throughout most of the United States, southern Canada and into Mexico, although its range has probably increased since European settlement. It is most common along tributaries of the Mississippi River north of the Ohio River. It has been introduced into Europe and Asia. 

Seed weight: The large seeds of giant ragweed vary substantially in size and shape within plants, between plants and between locations and years. Average seed weights from 17–45 mg (including the fruit coat) have been reported.

Dormancy and germination: Less than 5% of freshly produced giant ragweed seeds will germinate. Stratification at 39°F is required for germination. A minimum of six weeks of stratification was required to alleviate dormancy. Excising embryos released them from dormancy, suggesting that dormancy is partially imposed by the seed coat and associated structures. Nitrate can reduce but not eliminate the stratification requirement. Light has little influence on germination. Seeds that had been exposed to natural winter conditions in Illinois germinated at temperatures ranging from 46–97°F, but germination was greatest at 50–75°F. Optimum germination occurs at soil moisture content near field capacity.

Seed longevity: Giant ragweed seed banks deplete rapidly. In the Duvel long-term experiment, most seeds were lost in the first year, but a few seeds survived 21 years. In the absence of seed production, 96% of the seeds in the soil were depleted in two years. In Illinois, one study showed that 7% of seeds produced in the fall survived to the spring, while another study determined that 5–14% survived for one year. The large seeds of giant ragweed suffer very high rates of seed predation. A study in Ohio showed about 40% overwinter loss of seeds due to rodents. Viable seeds declined to 8–34% after one year, and few seeds lasted more than four years unless they remain deeply buried. However, earthworms can facilitate burial and persistence of giant ragweed seeds in no-till fields.

Season of emergence: In the absence of soil disturbance, most seedlings emerge in early spring. Giant ragweed is typically one of the first annual species to emerge. It also produces flushes of additional emergence following tillage or cultivation during spring and summer. Emergence began in late March in Ohio and lasted only a month for natural populations but lasted throughout the spring months for agricultural populations. Emergence similarly occurred throughout all spring months in Minnesota agricultural fields and was enhanced by colder overwinter temperatures, which presumably facilitated loss of dormancy. The trend toward longer emergence periods is positively associated with the increasing presence and difficulty of controlling this species in agricultural production.

Emergence depth: This species emerges best from the top 0.5–2 inches, but a substantial percentage of seedlings can emerge from 4 inches. None emerge, however, from 8 inches. Plant survival and vigor following emergence declines with increasing burial depth of the seed. Shallowly buried seeds (0.2 inch) and seeds on the soil surface have poor germination.

Photosynthetic pathway: C3

Sensitivity to frost: Giant ragweed is damaged but not killed by moderate frost. However, giant ragweed often matures and begins senescing before the first frost. 

Drought tolerance: Giant ragweed is not well adapted to drought. Well established plants can survive several weeks of dry weather, but the species is absent from non-irrigated land in regions with long summer droughts. Presence of this weed was associated with high rainfall and moderate temperatures in October, which probably facilitates production of viable seeds.

Mycorrhiza: Although there are no reports of mycorrhizal associations with giant ragweed, it is likely to be mycorrhizal because the closely related species, common ragweed, has been described as a strong mycorrhizal host (see the Common ragweed chapter).

Response to fertility: The species is usually found on highly fertile soils but shows only a moderate response to additional fertilization. Giant ragweed can accumulate up to 100 pounds per acre of nitrogen and could be highly competitive with nitrogen-requiring crops. 

Soil physical requirements: Giant ragweed will grow on a range of soil types, but most typically the species occurs on silty lowland soils.

Response to shade: In pure stands, plants that emerge 10–30 days later than the early emerging individuals are severely reduced in size, which indicates that the species can be suppressed by shade. The very rapid growth rate and tall stature of giant ragweed, however, often allows it to overtop crops before they can cast significant shade.

Sensitivity to disturbance: Plants re-grow well after cutting, even when cut close to the ground. Plants cut during combine harvesting of cereal grains send up side shoots that produce seeds. Cutting at 2–4 inches reduced giant ragweed growth more than seed production, and it took repeated cuts to have a substantial impact on seed production.

Time from emergence to reproduction: Plants typically emerge in early spring, flower in mid-summer and mature seeds in late summer. Giant ragweed flowers in response to shortening day length and usually flowers two to three weeks earlier than common ragweed. First viable seeds were produced approximately three weeks after pollination and fertilization. 

Pollination: Giant ragweed is wind pollinated. It will self-pollinate, but the female flowers are receptive before the male flowers release pollen, so cross pollination is normal. In high-density environments, male flower production declines in favor of female flower production. In greenhouse tests, plants produced by self-pollination were less vigorous than plants produced by cross-pollination. 

Reproduction: Individual plants in an Ohio corn field produced an average of 150 and 240 seeds in successive years, but less than half of these were viable. At a low density, individual plants produced from 1,300–3,600 seeds, with viability ranging from 59–77%. 

Dispersal: Giant ragweed is a common inhabitant of non-crop areas and field edges, and its presence in these areas is positively associated with its presence in crop fields. The difficulty of managing this weed is most strongly associated with its occurrence near waterways. Seeds were identified on the surface of irrigation water in Nebraska. Giant ragweed seeds float for several hours to a few days, which probably allows them to disperse along stream bottom lands during flood events. The species probably also occasionally moves with soil clinging to tires, machinery and animals, but the low rate of seed production and consequent low seed density in the soil probably makes such events rare. Occasional movement in combine harvesters seems likely. Low ability to disperse out of valleys may be the reason giant ragweed is less common on upland farms. 

Common natural enemies: The host specific fungus Puccinia xanthii f. sp. Ambrosia-trifidae attacks giant ragweed leaves and reduces seed production and seed size. Ten to 25% of seeds are killed by insects (fruit fly, beetles, a moth) while still on the plant, and taller plants appeared to be most susceptible. Rodents consume a large proportion of seeds on the soil surface during fall and winter, and insects kill many seeds that are on the soil surface during the summer. Several families of stalk boring insects can be prevalent in giant ragweed and may interfere with translocated herbicide activity.

Palatability: The seeds were gathered and eaten by Native Americans in the Mississippi Valley. Dehulled seeds contain 47% protein and 38% fat. The foliage is high quality forage for livestock. However, it was found to be unpalatable to sheep despite its nutritional value.

Note: The pollen of giant ragweed causes severe hay fever symptoms in sensitive individuals.

Summary Table of Giant Ragweed Characteristics

Giant Ragweed
Growth habitSeed weight (mg)Seed dormancy at sheddingFactors breaking dormancyOptimum temperature for germination (F)Seed mortality in untilled soil (%/year)Seed mortality in tilled soil (%/year)Typical emergence seasonOptimum emergence depth (inches)
tall, branched17–45Yescms, at, ni50–7566–95naearly to late spring0.5–2
Photosynthesis typeFrost toleranceDrought toleranceMycorrhizaResponse to nutrientsEmergence to flowering (weeks)Flowering to viable seed (weeks)Pollination Typical & high seed production (seeds per plant)
C3moderatelowprobablymoderate8–183cross200 & 2,000

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

Abul-Fatih, H.A. and F.A. Bazzaz. 1979. The biology of Ambrosia trifida L. II. Germination, emergence, growth and survival. New Phytologist 83: 817–827.

Bassett, I.J. and C.W. Crompton. 1982. The biology of Canadian weeds. 55. Ambrosia trifida L. Canadian Journal of Plant Science 62: 1003–1010.

Harrison, S.K., E.E. Regnier and J.T. Schmoll. 2003. Postdispersal predation of giant ragweed (Ambrosia trifida) seed in no-tillage corn. Weed Science 51: 955–964.

Regnier, E.E., S.K. Harrison, M.M. Loux, C. Holloman, R. Venkatesh, F. Diekmann, R. Taylor, R.A. Ford, D.E. Stoltenberg, R.G. Hartzler, A.S. Davis, B.J. Schutte, J. Cardina, K.J. Mahoney and W.G. Johnson. 2016. Certified crop advisors' perceptions of giant ragweed (Ambrosia trifida) distribution, herbicide resistance, and management in the corn belt. Weed Science 64: 361–377.