Invasive Watch List Species
Invasive Watch List
The plants below are NOT yet reported from Orange County, but are in adjacent areas. We believe these are likely to appear in the county soon. Please learn a few of these and look for them. If you believe you see any of these, post your observation to Calflora and/or iNaturalist and report them to invasives@occnps. Instructions for reporting are here.
Alternanthera philoxeroides Alligatorweed
Ammophila arenaria European Beachgrass
A perennial grass that has invaded sandy beaches, dunes and the immediate coastal areas of Central and Northern California and now as far South as Ventura County, perhaps to the Los Angeles area.
Arctotheca calendula Cape Weed
Brassica juncea India Mustard
Carduus tenuiflorus Slender Flowered Thistle
A modest sized annual thistle that is very similar to the more well known Italian thistle (Carduus pycnocephalus) and may be a subspecies or variety. This species generally contains flowerheads of 5-20 flowers each, while Italian thistle generally has heads of 5 or less flowers. A plant of woodland understories and disturbed grassland sites.
Carrichtera annua Ward’s Weed
An annual member of the mustard family that has recently invaded the north-coastal area of San Diego County, as for north as USMC Camp Joseph Pendleton.
Centaurea calcitrapa Purple Star Thistle
An annual to perennial spiny-flowered weed of fields, roadsides, waste areas and other disturbed sites. Has become rather common in Northern CA and down the coast. Also now in San Diego County.
Centaurea iberica Iberian Knapweed
An annual that grows 1-4 feet and closely resembles purple star thistle (above). The leaves are divided into narrow linear segments. Flowering in late spring through early summer. Present at scattered sites throughout much of California.
Also sometimes referred to as C. maculosa. A species that is well established through much of California, including Los Angeles, Riverside and San Diego Counties (and incorrectly in East Orange, OC). This modest-sized perennial knapweed is very similar to Centaurea diluta – North African knapweed and reminiscent of the more well-distributed, but smaller, Acroptilon repens – Russian knapweed.
An distinctive invasive annual to short-term perennial clumping grass that has become well established just over the border in western Riverside County. Distinguished by its very large whirled flower heads, arranged like the ribs of an umbrella. The leaves are also quite long.
An annual to short-lived perennial of up to three feet that is well established in Northern California and is now moving along the southern California coast. The plant seeds readily and invades rangelands, fields, grasslands and disturbed sites.
Now found through most of California, but not yet in Orange County. A perennial, it forms dense colonies, which can spread vegetatively as well as from seed. Can be difficult to control, since very small root fragments can generate new plants. Be cautious not to confuse this with our native California thistle (Cirsium occidentale).
One of many brooms in the Cytisus and related genera that are problematic in wildlands. It is a medium to large shrubby perennial that can invade many different habitats, change fire regimes and overwhelm native vegetation. It is present at several sites in Los Angeles County and was recently discovered in San Diego County.
Like Dipsacus sativus (below), this large upright biennial has the ability to invade natural areas, form dense stands and crowd out native plants, especially in moist fields and riparian areas. Long thin bracts below the flower are flexible.The distinctive flower heads are used ornamentally and occasionally in floral arrangements. Present in all our surrounding counties.
Like Dipsacus fullonum (above), this large upright biennial has the ability to invade natural areas, form dense stands and crowd out native plants, especially in fields and riparian areas. The distinctive flower heads are used ornamentally and occasionally in floral arrangements. The long thin bracts below the flower are stiff.Present in surrounding counties.
Elaeagnus angustifolius Russian Olive
An annual invasive grass that can overwhelm montane grasslands, similar to cheatgrass (Bromus tectorum). The long awns are obvious on bending, wispy flower heads and are reminiscent of several other invasive grasses. As the awns dry, they twist and spread in all directions, similar to a multi-headed Medusa.
A modest-sized spreading irregular plant that has invaded portions of souther-coastal San Diego County and has the potential to do the same in Orange County. Prefers poorly drained alkaline or saline areas. The distinctive yellow, green, pink or red, fleshy fruits are present through much of the year. Leaves and stems are covered in wooly hairs.
Destined to appear in Orange County, this weedy annual/perennial Euphorbia is already well established as an invasive in the Santa Monica Mountains and on the Palos Verdes Peninsula. In can be confused with other native and invasive Euphorbias, especially Euphorbia peplus (non-native) and Euphorbia spathulata (native).
May be the same species as Euphorbia esula. A rhizomatous perennial species that can invade and colonize several plant communities. Present in both Los Angeles and San Diego Counties. The leaf margins are smooth on this species and toothed in Euphorbia terracina.
Galenia pubescens Coastal Galenia
Drought and salt tolerant, galenia grows over and smothers existing vegetation by forming a thick dense mat. It invades coastal dunes, pastures, disturbed areas, lawns, roadsides and rocky outcrops. Galenia produces nitrates that can be toxic to stock.
Bees that collect the nectar produce honey with such a disagreeable flavor that is unsaleable.
Genista monosperma Bridal Broom
Has established infestations as near as Fallbrook and the southern edges of USMC Camp Pendleton. Also sometimes referred to as Retama monosperma. A large brushy member of the pea family, closely related to many of the problematic brooms in the genera Cytisus and Retama, but with white flowers.
Another of the large, rank growing shrubby and invasive brooms; others are in the genera Cytisus or Retama. This species is already present in each of the counties surrounding Orange County. Very similar to Cytisus scoparius and only distinguished by small subtilties of the fruit and flower.
A horticultural plant that has shown up in a few places in San Diego County and central and northern California. We are uncertain of its invasiveness, but believe it may appear in our area, due to the plant’s popularity in gardens. This is similar, but not to be confused with the also invasive, Plecostachys serpyllifolia – petite licorice plant.
Heliotropium supinum Drawf Heliotrope
A small, prostrate invasive annual of low, exposed alkaline or saline soils. The stems and leaves are both hairy, unlike our native Heliotropium curassavicum. This plant recently appeared at the far southern end of coastal San Diego county.
Hydrilla verticillata Hydrilla
Kochia scoparia Summer Cypress
Lathyrus tingitanus Tangier Pea
An annual “sweet pea” relative that grows at a couple of roadside loactions along Hwy 74, just East of the OC line. There are old records for Orange County that appear to now be extirpated. Somewhat similar to our native San Diego Pea – Lathyrus vestitus as well as the occasionally invasive everlasting pea – Lathyrus latifolius and the common garden sweet pea – Lathyrus odoratus.
Lepidium appelianum Hairy Whitetop
Ludwigia hexapetala Creeping Water Primrose
Lythrum salicaria Purple Loosestrife
A flowery perennial that has been well established in Northern California for some time, but recently has established just inland of The San Elijo Lagoon, near Encinitas in San Diego County. It is a concern in areas of moist wetlands, streamsides, flood control areas, etc.
Myoporum acuminatum Strichnine Bush
Myriophylum aquaticum Milfoil
Pentameris airoides Annual Pentachistis
An annual to perennial small grass that was discovered recently at the southeast edge of USMC Camp Pendleton in San Diego County.
Perilla frutescens Shiso
Pentameris aroides Annual Pentameris
Phragmites australis Common Reed
Very similar to the well known giant reed – Arundo donax, but smaller in stature. This riparian invader is confirmed from all of our surrounding counties and may already occur in Orange County. Before any management begins, carefull inspection of a colony will be needed to ensure that the plants do not represent a remnant of the native population.
Phytolacca americana American Pokeweed
Senecio dolicocephalus Cotton Burnweed
Previously identified as Senecio quadridentatus. A colony of this new invasive has become established only about five miles south of San Clemente at USMC Camp Pendleton. Additional plants occur in Santa Barbara County. An bushy-upright annual to perennial herb with long, slender leaves and branched, yellow flowers on top.
A pea family relative with conspicuous flowers that grows as a large deciduous shrub. A threat to riparian edges and waterways, where it can create dense monocultures. The seed is very long-lived. First discovered in 1999, it is now well established in parts of northern California and has now colonized an area of San Diego County. A single plant found at Upper Newport Bay appears to have been a waif.
A member of the pea family that can colonize nutrient poor soils, especially in disturbed sites, pastures, roadsides, etc. it is superficially similar to many of the brooms in the genera Cytisus, Genista and Retama. Gorse is recorded in Los Angeles, San Bernardino and San Diego Counties.
Aquatic Plant Note