Euphorbia hypericifolia -- California

Euphorbia hypericifolia is native to the tropical Americas (CABI). It is uncertain whether this species is native to or has naturalized in the tropical, Southern USA (efloras, CABI). It has naturalized outside its native range in the US (HI, CA, AZ, NM, eastern US), France, South Africa, the Mediterranean basin, India, Thailand, Indonesia, and Taiwan (GBIF, iNaturalist).

Euphorbia hypericifolia is native to the tropical Americas including the southern US (CABI). Although it is unclear exactly where the native boundaries are in the southeastern US, Euphorbia hypericifolia has been introduced into other parts of the US (CABI, GBIF). This plant has naturalized in areas matching California's climate including the US (CA, AZ, NM, AL, and TN), Mediterranean basin, South Africa, Saudia Arabia, southeastern China, and one record in Afghanistan (GBIF, iNaturalist). In California, Euphorbia hyperificolia occurrences are confined to more urban areas (GBIF, iNaturalist).

Euphorbia hypericifolia is an invasive weed in many of the Pacific Islands, especially Hawaii, where it is rated ‘high risk’ (CABI, PIER). It is also a weed in Singapore and Taiwan, though the situations in which it is causing problems are not well documented (CABI). E. hypericifolia is a known weed in sugar cane, tomato, soybean, cotton, lettuce, and maize farming (CABI, Ndam et al 2021). Euphorbia hypericifiolia is currently an A-rated weed, "a pest of known economic or environmental detriment," in California although this rating alone is not enough to be considered invasive in California (CDFA). Euphorbia hypericifiolia can lower nursery productivity in California; however, there are no given accounts of this occurring (CDFA). Cattle avoid foraging on spurge when possible, which affects grazing systems (CDFA). However, this species is confined to more urban areas in California and may be unlikely to affect rangeland grazing systems.

Euphorbia hypericifiolia is currently an A-rated weed, "a pest of known economic or environmental detriment," in California although this alone is not enough to be considered invasive in California (CDFA). Euphorbia hypericifiolia can lower nursery productivity in California; however, there are no given accounts of this occurring (CDFA). Cattle avoid foraging on spurge when possible, which affects grazing systems (CDFA). However, this species is confined to more urban areas in California and may be unlikely to affect rangeland grazing systems.

There are several Euphorbias that are invasive in a similar climate. Euphorbia oblongata, E. terracina, and E. virgata are listed as invasive in California (Cal-IPC). Euphorbia myrsinites is an A-rated weed in California that is also listed as invasive in matching climates including Oregon, Washington, Utah, Colorado, and New Mexico (CDFA, Invasive Plant Atlas). Euphorbia esula is also listed as invasive in the Western US in matching climates including Oregon, Washington, Nevada, Utah, Idaho, Colorado, and New Mexico (Invasive Plant Atlas).

Many more Euphorbia species are either naturalized or invasive in California including: E. characias, dendroides, graminea, helioscopia, lathyris, maculata, nutans, peplus, prostrata, serpens (Calflora and iNaturalist).

Less than 50% of Euphorbia hypericifolia occurrences are in areas that match California's climate. Most occurrences of Euphorbia hypericifolia are concentrated in its native range in areas more tropical than CA including the southeastern US, Central America, and South America (CABI, GBIF). It has also naturalized in several areas more tropical than CA including Madagascar, India, Thailand, Taiwan, several Pacific Islands, central Africa, and southern China (GBIF, iNaturalist).

Occurrences that match California's climate are more scattered and include South Africa, Chile, eastern China, the Mediterranean basin, and US (southern California, Arizona, New Mexico, Alabama, and Tennessee) (GBIF, iNaturalist).

Euphorbia hypericifolia forms disproportionately dense, virtually monospecific stands in Mount Bambouto Caldera, Cameroon, which may be attributed to its allelopathic potential (Ndam et al 2021). There is no evidence of this species dominating the plant community in areas with a similar climate to California.

There was no evidence of Euphorbia hypericifolia affecting fire regimes. Since E. hyperificolia is an herbaceous annual with latex sap, it is unlikely to promote fire (Flora of North America, NCSU Extension).

Euphorbia hypericifolia is a health risk to humans and potentially animals. Euphorbia hypericifolia contains a milky sap that is a skin and eye irritant to humans and may cause nausea in humans if eaten (NCSU Extension). E. hypericifolia may be able to poison livestock but goats and sheep are generally immune to its irritant properties (CDFA). Euphorbia hypericifolia also affects grazing systems as cattle avoid foraging on E. hypericifolia (CDFA). However, this species is confined to more urban areas in California and may be unlikely to affect rangelands. The latex in several Euphorbia species is toxic to fish (Neuwinger 2004).

No evidence that Euphorbia hypericifolia produces impenetrable thickets. This is unlikely as E. hyperificiolia is a short-lived perennial that grows up to 2 ft tall (CABI, NCSU Extension).

Euphorbia hypericifolia is an herbaceous short-lived perennial with a taproot that spreads by seeds (Flora of North America).

Euphorbia hypericifolia is an herbaceous short-lived perennial with a taproot that spreads by seeds (Flora of North America). There is no evidence of naturally detached fragments producing new plants.

Euphorbia hypericifolia is a short-lived perennial that reproduces by seed (Flora of North America).

There is not enough evidence to answer this question. There are accounts of "prolific seed production" and "numerous seeds" but without more evidence I will leave this question unanswered (Chormanski et al 2022; CDFA).

Euphorbia hypericifolia seeds have no dormancy requirement and can germinate immediately (Chormanski et al 2022). The seedbank is also moderately persistent (CDFA). However, CDFA also noted that Euphorbia hypericifolia is not likely to spread widely beyond disturbed, human-mediated landscapes, as California is too dry to favor its growth.

Euphorbia hypericifolia is a short-lived perennial that reproduces by seed (Flora of North America).

Euphorbia hypericifiolia has more reports of flowering over >3 months a year than less than 3 months a year. Flowering and fruiting early spring–late fall in North America (Flora of North America). Seeds are generally produced from late spring to the end of fall in Florida (Chormanski et al 2022). This plant is reported to flower in August and September in California, however, photos from observations show this plant flowering in July, October, and November (Calflora).

There is no evidence of Euphorbia hypericifolia spreading by mammals or birds. Euphorbia hypericifolia seeds are 0.8 mm long with poorly defined transverse ribs and a very thin whitish mucilaginous coat (Sciandrello et al 2016; Flora of North America). Although the layer of mucilage may facilitate transport by humans and animals, there is no evidence of this occurring for Euphorbia hypericifolia.

There is no evidence of Euphorbia hypericifolia spreading long distances by wind or water. The seeds do not have any features that typically contribute to long distance wind or water dispersal. Euphorbia hypericifolia seeds are 0.8 mm long with poorly defined transverse ribs and a very thin whitish mucilaginous coat (Sciandrello et al 2016; Flora of North America).

There is no evidence that Euphorbia hypericifolia is frequently dispersed via contamination. CDFA notes that Euphorbia hypericifolia seeds are able to spread via nursery stock and other means (CDFA). This species is a "VERY common weed in nursery stock," which supports Euphorbia hypericifolia frequently dispersing via nursery stock (per. comm. Ron Vanderhoff).