Ambrosia trifida L.
Images above: Upper Left: Young plant of giant ragweed (Antonio DiTommaso, Cornell University). Upper Right: Giant ragweed male flower (Antonio DiTommaso, Cornell University). Bottom: Giant ragweed seedling (Antonio DiTommaso, Cornell University).
Other common names: great ragweed, buffalo-weed, kinghead, crown-weed, wild hemp, horse-weed, bitterweed, tall ambrosia, tall ragweed
Family: aster family, Asteraceae
Habit: Tall, branched, summer annual herb.
Description: Seedling cotyledons are round to paddle shaped, 0.75 -1.75” (1.9-4.4 cm) long by 0.25-0.5” (0.6-1.3 cm) wide. The stem is shiny, green with purple spots. First two true leaves are lance to oval shaped with widely spaced, shallow teeth and may have two basal lobes. Subsequent young leaves are opposite, roughly hairy, and divided into three deep lobes. Mature plants are typically 3-12 ft (0.9-3.7 m) tall though, they may reach up to 20 ft (6.1 m) in height. Stems are single or branching and are covered in short, rough hairs. Leaves are opposite, up to 12” (30 cm) long and 8” (20 cm) wide, usually divided into 3 to 5 lobes, toothed along the edges, and hairy on all surfaces. Leaf stalks are 0.4-2.75” (1-7 cm) long and sometimes winged. Uppermost leaves are lance shaped. Roots are fibrous, occasionally with a short taproot. Flowers are green, 0.125” (0.3 cm) in diameter, and arranged in separate clusters of male and female flowers on a single plant. Male clusters are arranged in dense, 3-8” (8-20 cm) 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 incasing a single seed. Fruits are 0.2-0.6” (0.5-1.5 cm) long, ridged, brown to grey/black, with a blunt beak at the apex. Ridges terminate in 5-8 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.
Giant ragweed is a rapid growing, competitive species that can cause substantial yield losses even at low densities (Harrison et al. 2001, Regnier et al. 2016). It is increasingly a problem in field crops and is associated most strongly with soybean production and minimum tillage (Regnier et al. 2016). 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 (Regnier et al. 2016).
Rotation with hay or pasture will give the relatively short-lived seedbank a chance to decline in density (Goplen et al. 2017). Inclusion of crops or cover crops that provide early spring leaf canopy or residue cover that lowers soil temperature delays giant ragweed emergence (Goplen et al. 2018). 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 may be retained on plants after typical grain harvest dates, they could be captured or destroyed during combine harvesting (Goplen et al. 2016). 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, soybean 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 (Harrison et al. 2007). 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 (Regnier et al. 2008).
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 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 3-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 soybean and dry bean, a front mounted mower can greatly reduce seed production and competition with the crop.
Because giant ragweed quickly emerges above the canopy of soybean and dry bean, a front mounted mower can greatly reduce seed production and competition with the crop. Electrocution with a Weed ZapperTM controlled most plants that were reproductive and growing above a crop canopy, while it reduced viability of the majority of seeds present during treatment (Schreier et al. 2022).
Origin and distribution: Giant ragweed is native to stream banks and floodplains of North America. It currently occurs throughout most of the U.S.A., southern Canada and into Mexico (USDA Plants), although its range has probably increased since European settlement. It is most common along tributaries of the Mississippi River north of the Ohio River (Regnier et al. 2016). It has been introduced into Europe and Asia. (Bassett and Compton 1982, Qin et al. 2014)
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 to 45 mg (including the fruit coat) have been reported: 17-33 mg (Stevens 1932), 21-28 mg (Goplen et al. 2016), 27-45 mg (Abul-Fatih and Bazzaz 1979), 34-41 mg (Stoller and Wax 1973), 38-41 mg (Hartnett et al. 1987), 45 mg (Shirgill et al. 2020).
Dormancy and germination: Less than 5% of freshly produced giant ragweed seeds will germinate (Harre et al. 2019, Schutte et al. 2012, Stoller and Wax 1973). Stratification at 39 °F (4 °C) is required for germination (Bassett and Compton 1982, Harre et al. 2019, Schutte et al. 2012). A minimum of 6 weeks of stratification was required to alleviate dormancy (Page and Nurse 2015). Excising embryos released them from dormancy suggesting that dormancy is partially imposed by the seed coat and associated structures (Harre et al. 2019, Page and Nurse 2015). Nitrate can reduce, but not eliminate the stratification requirement (Page and Nurse 2015). Light has little influence on germination (Stoller and Wax 1974). Seeds that had been exposed to natural winter conditions in Illinois germinated at temperatures ranging from 46-97° F (8-36° C), but germination was greatest at 50-75 °F (10-24 °C) (Abul-Fatih and Bazzaz 1979). Optimum germination occurs at soil moisture content near field capacity (Abul-Fatih and Bazzaz 1979).
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 (Toole and Brown 1946). In the absence of seed production, 96% of the seeds in the soil were depleted in 2 years (Goplen et al. 2017). In Illinois, one study showed that 7% of seeds produced in the fall survived to the spring (Abul-Fatih and Bazzaz 1979), while another study determined that 5 to 14% survived for one year (Davis et al. 2005). The large seeds of giant ragweed suffer very high rates of seed predation. A study in Ohio showed that 40% over-winter loss of seeds was attributed to rodents (Harrison et al. 2003). Viable seeds declined to 8 to 34% after one year, and few seeds lasted more than 4 years unless they remain deeply buried (Harrison et al. 2007). Earthworms can facilitate burial and persistence of giant ragweed seeds in no-till fields (Regnier et al. 2008).
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 (Abul-Fatih and Bazzaz 1979, Doll 2002, Stoller and Wax 1973, Werle et al. 2014). 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 (Schutte et al. 2012). Emergence similarly occurred throughout all spring months in Minnesota agricultural fields (Goplen et al. 2017, 2018) and was enhanced by colder overwinter temperatures which presumably facilitated loss of dormancy (Goplen et al. 2018). The trend toward longer emergence periods is positively associated with the increasing presence and difficulty of controlling this species in agricultural production (Regnier et al. 2016).
Emergence depth: This species emerges best from the top 0.5-2” (1.3-5 cm), but a substantial percentage of seedlings can emerge from 4” (10 cm) (Stoller and Wax 1973, Harrison et al. 2007). None emerge, however, from 8” (20 cm) (Harrison et al. 2007). Plant survival and vigor following emergence declines with increasing burial depth of the seed (Abul-Fatih and Bazzaz 1979). Shallowly buried seeds (0.2” or 0.5 cm) and seeds on the soil surface, have poor germination (Abul-Fatih and Bazzaz 1979, Harrison 2007).
Photosynthetic pathway: C3.
Sensitivity to frost: Giant ragweed is damaged but not killed by moderate frost (Stevens 1924). However, giant ragweed often matures and begins senescing before the first frost (Goplen et al. 2016).
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 (Allard 1945). Presence of this weed was associated with high rainfall and moderate temperatures in October (Qin et al. 2014), 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.
Response to fertility: The species is usually found on highly fertile soils, but shows only a moderate response to additional fertilization (Hunt and Bazzaz 1980). Giant ragweed can accumulate up to 100 lb/acre of nitrogen and could be highly competitive with nitrogen-requiring crops (Johnson et al. 2007).
Soil physical requirements: Giant ragweed will grow on a range of soil types, but most typically, the species occurs on silty lowland soils (Bassett and Compton 1982).
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 (Abul-Fatih and Bazzaz 1979, Harrison et al. 2001). The very rapid growth rate and tall stature of giant ragweed, however, often allows it to overtop crops before they can cast significant shade (Bassett and Compton 1982).
Sensitivity to disturbance: Plants re-grow well after cutting, even when cut close to the ground (Bassett and Compton 1982, Mager et al. 2006). Plants cut during combine harvesting of cereal grains send up side shoots that produce seeds. Cutting at 2-4” (5-10 cm) reduced giant ragweed growth more than seed production and it took repeated cuts to have a substantial impact on seed production (Butler et al. 2013).
Time from emergence to reproduction: Plants typically emerge in early spring, flower in mid-summer and mature seeds in late summer (Bassett and Compton 1982, Doll 2002, Goplen et al. 2016). Giant ragweed flowers in response to shortening daylength and usually flowers two to three weeks earlier than common ragweed (Allard 1945, Bassett and Compton 1982, Doll 2002). First viable seeds were produced approximately 3 weeks after pollination and fertilization (Goplen et al. 2016).
Pollination: Giant ragweed is wind pollinated. It will self-pollinate, but cross pollination is normal. Female flowers are receptive before the male flowers release pollen, thus facilitating cross pollination. In high-density environments, male flower production declines in favor of female flower production (Abul-Fatih et al. 1979, Jurik 1991). In greenhouse tests, plants produced by self-pollination were less vigorous than plants produced by cross-pollination. (Bassett and Compton 1982). High pollen production by this species (one million pollen grains per day) contributes to pollen-mediated gene flow of traits that can render plants more resistant to herbicides (Jhala et al. 2021) and, potentially, to consistently applied cultural management practices as well.
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 (Harrison et al. 2001). At a low density, individual plants produced from 1,300 to 3,600 seeds (Abul-Fatih et al. 1979, Goplen et al. 2016, Stevens 1932) with viability ranging from 59 to 77%. An average of 25% of giant ragweed seeds were shattered by soybean harvest with a high degree of variation in one report (Goplen et al. 2016), whereas 66% of seeds were shattered with little variability in another report (Schwartz-Lazaro et al. 2021).
Dispersal: Giant ragweed is a common inhabitant of non-crop areas and field edges (Sosnoskie et al. 2007), and its presence in these areas is positively associated with its presence in crop fields (Regnier et al. 2016). The difficulty of managing this weed is most strongly associated with its occurrence in waterways (Regnier et al. 2016). Seeds were identified on the surface of irrigation water in Nebraska (Wilson 1980). Giant ragweed seeds float for several hours to a few days (Parker and Leck 1985), which probably allows them to disperse into 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 (Batra 1981). Ten to 25% of seeds are killed by insects (fruit fly, beetles, a moth) while still on the plant (Abul-Fatih et al. 1979, Harrison et al. 2001), and taller plants appear to be most susceptible (Abul-Fatih et al. 1979). 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 (Harrison et al. 2003). Several families of stalk boring insects can be prevalent in giant ragweed and may interfere with translocated herbicide activity (Ott et al. 2007).
Palatability: The seeds were gathered and eaten by Native Americans in the Mississippi valley. Dehulled seeds contain 47% protein and 38% fat (Harrison et al. 2003). The foliage is high quality forage for livestock (Bassett and Compton 1982). However, it was found to be unpalatable to sheep despite its nutritional value (Martens and Andersen 1975).
Notes: The pollen of giant ragweed causes severe hay fever symptoms in sensitive individuals (Bassett and Compton 1982).
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