Ambrosia trifida_C022-07
Photo courtesy UC Davis Weeds of California

Ambrosia trifida Risk Assessment

Common names: giant ragweed

Ambrosia trifida -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
May 3, 2016
Evaluation Time (hrs): 
3 Hours
Evaluation Status: 
Completed
Plant Information
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
Regional Information
Region Name: 
Climate Matching Map
These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.
Climate Matching Maps PDF: 
Invasive History and Climate Matching
1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Naturalized in California (and not native here) (Calflora), and Arizona (Yavapai County Native & Naturalized Plants). Ambrosia trifida L. is the second alien species of the genus Ambrosia L., which is found to be naturalized in Bulgaria (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Naturalized in: Asia-Temperate Caucasus: Georgia China: China Eastern Asia: Japan Europe East Europe: Lithuania Middle Europe: Germany; Netherlands; Slovakia Northern Europe: Denmark; United Kingdom Southeastern Europe: Italy Southwestern Europe: France; Spain Adventive: Asia-Temperate Western Asia: Israel Europe East Europe: Belarus; Estonia; Latvia; Moldova; Russian Federation-European part - European part; Ukraine Middle Europe: Austria; Belgium; Czech Republic; Poland; Switzerland Northern Europe: Ireland Southeastern Europe: Serbia; Slovenia (ARS GRIN). Many of these regions overlap in USDA hardiness zones with California (Cal-IPC).
Reference(s): 
2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
Naturalized in California (and not native here) (Calflora), and Arizona (Yavapai County Native & Naturalized Plants). Ambrosia trifida L. is the second alien species of the genus Ambrosia L., which is found to be naturalized in Bulgaria (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Naturalized in: Asia-Temperate Caucasus: Georgia China: China Eastern Asia: Japan Europe East Europe: Lithuania Middle Europe: Germany; Netherlands; Slovakia Northern Europe: Denmark; United Kingdom Southeastern Europe: Italy Southwestern Europe: France; Spain Adventive: Asia-Temperate Western Asia: Israel Europe East Europe: Belarus; Estonia; Latvia; Moldova; Russian Federation-European part - European part; Ukraine Middle Europe: Austria; Belgium; Czech Republic; Poland; Switzerland Northern Europe: Ireland Southeastern Europe: Serbia; Slovenia (ARS GRIN). Many of these regions overlap in USDA hardiness zones with California (Cal-IPC).
Reference(s): 
3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
Native to North America and considered invasive both in its native area and in other parts of the world, including several countries in Europe, e.g. France, Italy, Germany, Switzerland, Austria, Hungary, the Czech Republic, Slovakia and Poland. North American publications report significant yield losses in maize and soya bean caused by A. artemisiifolia and A. trifida, with weed density and time of emergence being key factors. (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). A. trifida is a declared noxious weed in California, Delaware, Illinois and New Jersey, USA (USDA-ARS, 2003) and a quarantine weed in Poland and Russia (CABI). Invasive in Bulgaria (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Ambrosia trifida L. (Asteraceae) has been identified by growers in the central and southern regions of the U.S. as one of the most problematic weeds of corn and soybean production (Jordan 1985; Loux and Berry 1991; Webster et al. 2000; Webster et al. 2001). Currently, A. trifida ranks as the most troublesome crop production used in the eastern U.S. Corn Belt (Gibson et al. 2005) (Biology and ecology of Ambrosia trifida L. seedling emergence). Weed of disturbed sites, wastelands, damp soils; below 1600 m. Hebei, Heilongjiang, Hunan, Jiangxi, Jilin, Liaoning, Shandong, Sichuan, Zhejiang (eFloras). Noxious weed in California, Illinois, and Delaware (USDA PLANTS). As a weed, it has been successful on all the temperate continents. It is noted as a major invasive weed of northeastern China (Giant Ragweed Ambrosia trifida).
Reference(s): 
4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
3
Confidence Level: 
Very High
Answer / Justification: 
Native to North America and considered invasive both in its native area and in other parts of the world, including several countries in Europe, e.g. France, Italy, Germany, Switzerland, Austria, Hungary, the Czech Republic, Slovakia and Poland. North American publications report significant yield losses in maize and soya bean caused by A. artemisiifolia and A. trifida, with weed density and time of emergence being key factors. (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). A. trifida is a declared noxious weed in California, Delaware, Illinois and New Jersey, USA (USDA-ARS, 2003) and a quarantine weed in Poland and Russia (CABI). Invasive in Bulgaria (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Ambrosia trifida L. (Asteraceae) has been identified by growers in the central and southern regions of the U.S. as one of the most problematic weeds of corn and soybean production (Jordan 1985; Loux and Berry 1991; Webster et al. 2000; Webster et al. 2001). Currently, A. trifida ranks as the most troublesome crop production used in the eastern U.S. Corn Belt (Gibson et al. 2005) (Biology and ecology of Ambrosia trifida L. seedling emergence). Weed of disturbed sites, wastelands, damp soils; below 1600 m. Hebei, Heilongjiang, Hunan, Jiangxi, Jilin, Liaoning, Shandong, Sichuan, Zhejiang (eFloras). Noxious weed in California, Illinois, and Delaware (USDA PLANTS). As a weed, it has been successful on all the temperate continents. It is noted as a major invasive weed of northeastern China (Giant Ragweed Ambrosia trifida). Many of these areas overlap in USDA hardiness zones with California
Reference(s): 
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Western ragweed [Ambrosia psilostachya D.C.][AMBPS] is a widespread erect, native perennial that is sometimes weedy in orchards and vineyards. Its foliage is similar to that of annual bursage [Ambrosia acanthicarpa], but its burs are shaped like those of giant ragweed. Unlike annual bursage and giant ragweed, western ragweed can reproduce vegetatively from creeping roots and leaves are mostly 1-pinnatem divided, with irregularly toothed margins. Western ragweed commonly inhabits roadsides and dry fields throughout California (except some regions in the Mojave Desert and Great Basin) and most of the Western U.S. to 1000 m (3280 ft). Flowers July-November. Common ragweed [Ambrosia artemisiifolia L.][AMBEL] is an annual introduced from the Eastern U.S. that closely resembles annual bursage. It is not as common in California as it is in the Mid-western and Eastern U.S. Common ragweed is most easily distinguished by having staminate flower heads with green phyllaries that lack a black midvein and burs similar to those of giant ragweed, but considerably smaller (2-4 mm long). It inhabits disturbed sites in Northwestern California, eastern Sacramento Valley, South Coast, and low regions of the Eastern Sierra Nevada, to 650 m (2100 ft) (Encycloweedia). As a weed, it has been successful on all the temperate continents. It is noted as a major invasive weed of northeastern China (Giant Ragweed Ambrosia trifida). Locations where these species occur match the climate of California based on the Cal-IPC climate map.
Reference(s): 
California Department of Food and Agriculture (CDFA) (0).  Encycloweedia.
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
A. trifida is found primarily in temperate areas around the world and is native to eastern North America. It is now found throughout much of the USA and southern Canada. In Canada, this weed ranges from Nova Scotia westward to the Northwest Territory. Within the USA, it is most common in the mid-Atlantic states and the Ohio and Mississippi River valleys (CABI). As a weed, it has been successful on all the temperate continents. It is noted as a major invasive weed of northeastern China (Giant Ragweed Ambrosia trifida). Most areas overlap substantially with California USDA hardiness zones (Cal-IPC).
Reference(s): 
Impact on Native Plants and Animals
7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
When present in disturbed habitats, it is the dominant species throughout the entire growing season. A. trifida competes with natural vegetation as well as crop plants, reducing more desirable plants in natural areas (CABI). Drastically reduces the kinds, numbers, and growth of other annuals when present; its seedlings emerge before any other annual in the early spring. Drastically reduces species diversity, contributes the most biomass, and controls community properties. Seedlings emerge early and quickly overtop other species (The biology of Ambrosia trifida II. Germination, emergence, growth and survival). Seedlings quickly grow taller than coexisting species. By late May, plants can be three times taller than neighboring plants (Abul-Fatih and Bazzaz 1979a) and by season’s end, plants can extend 6 m (Bassett and Crompton 1982) (Biology and ecology of Ambrosia trifida L. seedling emergence). Effectively compete for light; Ambrosia trifida, is the tallest of the ragweeds (genus Ambrosia) reaching some 12 to 14 feet (3.6 to 4.3 meters) in height (Giant Ragweed Ambrosia trifida). It forms huge colonies when it is left undisturbed (Tree of Life).
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
This species is adapted to a high frequency fire regime (Characterizing Fire Regimes from Understory Vegetation Composition in Ponderosa Pine Dominated Forests of the Northern Colorado Front Range). An exhaustive Google/Google Scholar search did not reveal any aditional evidence and this defaults to a “no” answer at this time.
9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Ambrosia spp. are of particular public concern due to the allergenic properties of their pollen. Ambrosia spp. produce allergenic pollen, which can induce allergic diseases: rhinitis, conjunctivitis, asthma and also contact dermatitis or urticaria (Allard 1943; Bassett and Crompton 1975; Gadermaier et al., 2004; Taramarcaz et al., 2005; Wopfner et al., 2005). Cross-reactions to certain kinds of food (e.g. melon and banana) may also occur in Ambrosia-sensitised persons upon consumption of such food (Egger et al., 2006). Ambrosia pollen is known to be a particularly strong allergen (Lewis et al., 2000; Smith, 1984). During the pollen season, pollen allergic individuals have reduced quality of life. Productivity and learning are reduced and considerable levels of absence from jobs and schools are observed (Burton et al., 2001; Wilken et al., 2002). The costs of Ambrosia pollen-related allergic rhinoconjunctivitis and asthma are considered significant (Bachert et al., 2007). (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Allergies caused by A. trifida pollen can affect livestock although any reduction in production is not known. Pollen from this species is a principal cause of hay fever, which produces symptoms of allergic rhinitis and bronchial asthma. Large amounts of money are spent each year treating hay fever symptoms, both in doctor visits and for medications. These health hazards result in tremendous discomfort for millions of people worldwide (CABI). The representatives of the genus Ambrosia have a serious impact on human health as well. They produce copious amounts of wind-dispersed pollen in male flower-heads, which is the main cause of various allergies in sensitive people in the late summer (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). The pollen of this plant is a major cause of hayfever in N. America. Ingesting or touching the plant can cause allergic reactions in some people (PFAF). This huge plant is probably an allergy sufferer's worst nightmare (
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
While these plants clearly grow very tall and block light, an exhaustive Google/Google Scholar search did not reveal any additional evidence of thicket formation and this defaults to a “no” answer at this time.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
Giant ragweed plants produce new stems and inflorescences when cut in late July and August during the combining of grain fields, but propagation is by seed (CABI). An exhaustive Google/Google Scholar search did not reveal any additional evidence of vegetative reproduction and this defaults to a “no” answer at this time.
Reference(s): 
12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Giant ragweed plants produce new stems and inflorescences when cut in late July and August during the combining of grain fields, but propagation is by seed (CABI). An exhaustive Google/Google Scholar search did not reveal any evidence of production of new plants by fragments and this defaults to a “no” answer at this time.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Propagation is by seed (CABI).
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
An average size A. trifida plant produces roughly 275 seeds, which is not considered highly prolific in terms of seed numbers, but the seeds retain their vitality for long periods of time and are readily carried from place to place (CABI). Despite low fecundity, A. trifida is a severe weed in row-crop agriculture (Biology and ecology of Ambrosia trifida L. seedling emergence).
Reference(s): 
15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Most germination in the field occurs early to mid-spring and at soil depths to 16 cm, optimal 2 cm. Germination rarely occurs on the soil surface. Seedlings emerge in mid to late spring. Seedlings emerging from shallow depths are most likely to survive. Newly matured seed is usually dormant and requires a cold, moist period to germinate. Seeds can germinate in a wide range of temperatures from 8°C to 41°C, but with an optimum range of 10-24°C (Abul Fatih and Bazzaz, 1979) (CABI). Its seedlings emerge before any other annual in the early spring. Peak germination occurred 13 days after planting, and was optimum between 10-24°C, following stratification. A large number of seeds did not germinate at all and were lost to predation or decay, or were inviable. However, germination was above 25%. Dormancy mechanisms in some seeds ensure seeds are still available to germinate when disturbance occurs, which favors germination of this species (The biology of Ambrosia trifida II. Germination, emergence, growth and survival). Seedling emergence continues sporadically throughout the growing season due to embryo dormancy and interactions with soil temperature and seed size. Giant ragweed seeds are dormant at dispersal (Davis 1930). Dormancy is alleviated naturally in seeds buried in soil over the winter and artificially by cold, moist storage (stratification). Following stratification, for populations from ruderal environments, seedling emergence reached greater than 95% before May 1. For populations from agricultural environments, seedling emergence occurred more gradually from April to July(Biology and ecology of Ambrosia trifida L. seedling emergence). Most germination in the field occurs early to mid-spring and at soil depths to 16 cm, optimal 2 cm. Germination rarely occurs on the soil surface. Seedlings emerging from shallow depths are most likely to survive (Encycloweedia). While seed dormancy factors clearly allow for germination over long periods and maintenance of a soil seedbank, evidence is compelling that substantial germination occurs early on in spring.
Reference(s): 
California Department of Food and Agriculture (CDFA) (0).  Encycloweedia.
16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Annual plant (Jepson).
Reference(s): 
17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Flowers June - September (Encycloweedia; Jepson).
Reference(s): 
California Department of Food and Agriculture (CDFA) (0).  Encycloweedia.
Dispersal
18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Most seeds fall near the parent plant, but some can disperse long distances with water, animals, and human activities. Seeds are rarely consumed by animals. Birds and other animals do not commonly transport seeds of A. trifida, apparently because they are not an attractive food source, partially because of the large size of the seeds. One of the primary mechanisms of spread in crop areas is by cultivation and harvesting equipment which move from field to field and farm to farm. The most common means of introduction of A. trifida to countries or regions appears to be by seeds as a contaminant in other seeds. Once introduced to a region, this weed can flourish, especially in moist, heavy soils. Intentional introduction is considered highly unlikely (CABI). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence). No evidence was found to demonstrate that seeds are regularly dispersed long distance by animals.
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
For A. trifida the likelihood of seed dispersal is considered to be higher since the seeds float better than A. artemisiifolia. The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Most seeds fall near the parent plant, but some can disperse long distances with water, animals, and human activities. A. trifida produces seeds, and though the plants can be destroyed in early growth stages, the seeds retain their vitality and are scattered far and wide by flood waters and with blowing and drifting snow (CABI). A. trifida grows in its natural habitat in North America where it initially inhabited mainly river valleys, floodplains, drainage channels, moist disturbed open sites and others (AMBROSIA TRIFIDA (ASTERACEAE), A NEW NON-NATIVE SPECIES FOR THE BULGARIAN FLORA). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence). Large populations growing along waterways and evidence of water-aided dispersal and seeds that float mean this is a "yes".
Reference(s): 
20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Likely: North American publications report significant yield losses in maize and soya bean caused by A. artemisiifolia and A. trifida, with weed density and time of emergence being key factors. For example, according to OMAFRA10, in maize A. artemisiifolia in densities of 1 plant/m2 caused yield loss of 5 % while 25 plants /m2 decreased the yield by 45 %. The figures for A. trifida were 13 and 56 % respectively. The data were obtained in conditions of known weed populations, assuming that all weeds emerged with the crop. Harrison et al. (2001) reported on losses of maize yield between 13.6 to 90 %, for weed densities varying from 1.7 to 13.8 plants/m2, when the weeds emerged concurrently with the crop. In a study by Baysinger and Sims (1991), A. trifida was shown to reduce the yield of the soya seeds dramatically: a density of two A. trifida plants for every 9 metres of soya beans in each row caused a reduction of 46 % and 50 % in the two consecutive years 1988 and 1989. The document states that the seeds of Ambrosia spp. can spread naturally by water (including heavy rains, floods, rivers, canals), wind and animals. The import of plant products (seeds and grain) originating in areas where Ambrosia spp. occur is considered to be the major source of infestation and data are given on the amounts of such grain imported into Poland from other European countries, Canada and the USA in 1998. Examples of interceptions of A. artemisiifolia and A. trifida seeds are given from the Russian Plant Quarantine Inspection and Finnish Plant Quarantine Service suggesting that the main sources of the infestation are seeds and grain of wheat, maize, soya bean, grasses and rice and soya meal from different countries (North America, Argentina, western Europe, Japan). Interceptions of the weeds in Poland, in grain of maize and soya bean originating from the USA, grain of soya bean from Canada, cereals, soya bean and sunflower from Hungary and other European countries (Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.). Spread will be primarily as a seed contaminant. Further spread of A. trifida is highly likely, due to the risks of both accidental movement in agricultural equipment (locally) and as a contaminant of crop seeds. Humans move seed in or on cultivation and harvesting equipment (CABI). Seeds fall from maternal plants typically in autumn, but a portion remains on the maternal plant into winter. Incorporation of seeds into the soil occurs by rainfall, cryoturbation, and earthworm activity (Regnier et al. 2001) (Biology and ecology of Ambrosia trifida L. seedling emergence).
Evaluation Notes

The following webpages were consulted for this screen:

GBIF: http://www.gbif.org/species/3110588;

Tropicos: http://www.tropicos.org/Name/2700037?tab=synonyms;

USDA PLANTS: http://plants.usda.gov/java/nameSearch;

The Plant List: http://www.theplantlist.org/tpl/record/gcc-7636;

ARS GRIN: https://npgsweb.ars-grin.gov/gringlobal/taxonomydetail.aspx?id=103827;

Wikipedia: https://en.wikipedia.org/wiki/Ambrosia_trifida;

Opinion of the Scientific Panel on Plant Health on the pest risk assessment made by Poland on Ambrosia spp.: http://www.efsa.europa.eu/sites/default/files/scientific_output/files/main_documents/528.pdf;

CABI: http://www.cabi.org/isc/datasheet/4693;

Thesis: https://etd.ohiolink.edu/!etd.send_file?accession=osu1181937971&disposition=inline;

Encycloweedia: https://www.cdfa.ca.gov/plant/ipc/encycloweedia/weedinfo/ambrosia.htm;

Jepson: http://ucjeps.berkeley.edu/eflora/eflora_display.php?tid=835;

eFloras: http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=200023073;

PFAF: http://www.pfaf.org/user/Plant.aspx?LatinName=Ambrosia+trifida;

USDA PLANTS: http://plants.usda.gov/core/profile?symbol=AMTR;

Beal Botanical Garden: http://www.cpa.msu.edu/beal/plantofweek/plants/ambrosia_trifida_20070924.pdf;

Illinois Wildflowers: http://www.illinoiswildflowers.info/weeds/plants/giant_ragweed.htm;

Tree of Life: http://tolweb.org/treehouses/?treehouse_id=4605;

Dave’s Garden: http://davesgarden.com/guides/pf/go/32069/;

Calflora: http://www.calflora.org/cgi-bin/species_query.cgi?where-calrecnum=300;

Yavapai County Native & Naturalized Plants: http://cals.arizona.edu/yavapaiplants/SpeciesDetailForb.php?genus=Ambrosia&species=trifida

Reviewed by Barb Castro and Elizabeth Brusati.

Total PRE Score

  • < 13 : accept (low risk of invasiveness)
  • 13 - 15 : evaluate further
  • > 15 : reject (high risk of invasiveness)

PRE Score: 
19
Number of questions answered: 
20
Screener Confidence (%): 
90.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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