California Pest Rating
Mercurialis ambigua Spanish mercury
Former Pest Rating: B
CURRENT Pest Rating: B | Proposed Seed Rating: R
This plant has been rated as “B” on the CDFA Plant Pest Rating list for 3 years
History & Status:
Spanish mercury is a modest annual herb with alternate, oval leaves. It has tiny flowers in short spikes in the axils of the leaves. In Flora Europaea Spanish mercury is included as a variant of dog mercury (Mercurialis annua), a European weed that has been found in the San Francisco Bay Area. In most Spanish floras the taxa are treated as distinct. Spanish mercury differs from dog mercury in its wider leaves, shorter flower spikes and in its polyploidy (multiple chromosome sets). It has been demonstrated that these hexaploid plants arose between a tetraploid M. annua (dog mercury) and a diploid M. huetii. In addition to these differences, Spanish mercury may differ from dog mercury in its weedy behavior. In its natural habitat in Spain it occurs in open areas protected from the hottest sun with common weeds such as bristly oxtongue (Helminthostachys echioides), shining geranium (Geranium lucidum), purple false-brome (Brachypodium distachyon), ripgut brome (Bromus diandrus), hairy cress (Cardamine hirsuta), common chickweed (Stellaria media), sow-thistle (Sonchus oleraceus), mouse-ear chickweed (Cerastium glomeratum), bur-chervil (Anthriscus caucalis), common pimpernel (Anagallis arvensis), red brome (Bromus madritensis), and plumeless thistle, (Carduus tenuiflorus). Spanish mercury has shown itself to be very invasive in nursery situations. It has also been found in mulch adjacent to public buildings and on the edge of a vineyard.
Official Control: There is no known official control in California at this time.
California Distribution: Because of the taxonomic confusion with dog mercury, Spanish mercury was not recognized in California until the 2000s. Nevertheless, its incursion seems to be rather recent, as older vouchered specimens are dog mercury. It is, so far, always limited in distribution. It has been found in Alameda, Contra Costa, Napa, Monterey, and Yolo Counties.
California Interceptions: Spanish mercury has never been found coming into California.
Consequences of Introduction:
1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score: 3
– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.
Risk is high (3), as the plant is naturalized in mulched landscapes, around nurseries and in vineyards. Once its population builds up it can be expected in areas occupied by the widespread weed associates mentioned above.
2) Known Pest Host Range: Evaluate the host range of the pest. Score: 3
– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.
Risk is high (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.
3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score: 3
– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.
Risk is High (3). The plant produces numerous seeds that seem to be able to spread rather well via pathways that have to do with landscaping and nursery operations.
4) Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below. Score: 2
A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.
– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.
Risk is Medium (2). Based on the mercuries being known crop weeds on several continents, the plant is likely to interfere with crop production and can lower crop value. It shows an incipient ability to be a nuisance weed in gardens, especially in open garden beds.
5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.
A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
D. The pest could trigger additional official or private treatment programs.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.
Score the pest for Environmental Impact. Score: 3
– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.
Risk is high (3) as the plant can dominate open, disturbed areas, excluding other plants and lowering biodiversity and can exclude cultural plants from a landscape.
Consequences of Introduction to California for Spanish mercury:
Add up the total score and include it here. (13)
–Low = 5-8 points
–Medium = 9-12 points
–High = 13-15 points
6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (-2)
–Not established (0) Pest never detected in California, or known only from incursions.
–Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
–Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
–High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.
7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (11)
Annual mercuries are widespread as agricultural weeds throughout the world. Which of these is dog mercury versus Spanish mercury is unknown. Nevertheless, it seems to be actively spreading in Central California. The eventual effects are in the future and must be projected with significant uncertainty. The seeds of Spanish mercury are quite similar to those of dog mercury and identification of seed contaminants must reflect this uncertainty.
Conclusion and Rating Justification:
A potentially bad weed of horticultural areas, vineyards, and possibly open, grassy habitats. Deserves a B rating as it is present in > 5 cos.
Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).
Fernández, I. F. 2001. Claves de Flora Iberica: Plantas Vasculares de la Península Ibérica e Islas Baleares, Volume 1. CSIS Press. Madrid.
Flora of Sierra de La Parra (Badajoz). Accessed 11/15/2015:
López G.B., M. C. Romero, B. Cabezudo, C. M. Torres, & C. Salazar. 2011.
Claves de la Flora Vascular de Andalucía Oriental. Universidad Almería.
Obbard, D. J., Harris, S. A., Richard J. A. Buggs, & Pannell, J. R. 2006. Hybridization, Polyploidy, and the Evolution of Sexual Systems in Mercurialis (Euphorbiaceae). Evolution 60: 1801–1815.
Thomas, R. G. 1958. Sexuality in Diploid and Hexaploid Races of Mercurialis annua L. Annals of Botany, N.S. 22: 55-72.
Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312; plant.health[@]cdfa.ca.gov.
Comment Period: CLOSED
12/21/15 to 2/4/16