Guide to Alert Species
By Jennifer Holmes

Guide to Alert Species

Alert Species: Beware, or Be Aware!

Invasive species are increasing globally and as an industry it’s wise to keep ahead. From the relatively harmless looking Water primrose or the noxious, sticky Chilean needle-grass to the deceptive and deadly Purple pitcher plant, these alert species are ones to watch!

What is an alert species?

Alert species are those considered “a threat to Great Britain but are not yet widely established” [1]. Early identification and recording of such species can prevent their widespread establishment and menace in the future, as well as save financial and ecological resources.

Threats to native species

Invasive alien species pose threats to UK wildlife, human health and the economy. Alert species have the capacity to inflict the same impacts as invasive species, but still fall within the time period for early intervention and eradication.

Current UK alert species

The UK is currently host to 19 alert species, six of which are plants. This blog provides a run-down of their characteristics, preferred habitat, invasive traits and threats. Some species have ID guides, which have been linked for easy access.

Water primrose (Ludwigia grandiflora)

An established pest costing millions of Euro to the French economy, this species was introduced as an ornamental plant in the 1800s and has since spread into 9 member states.  Aquaria trade and pond owners emptying their contents into the wild is the most likely pathway for invasion. Water primrose is easily mistaken as L. peploides and Water forget-me-not which has similar floating leaves with dissimilar flowers, and is labelled in garden centres as Jussiaea. Listed under Schedule 9 to the Wildlife and Countryside Act 1981 for England and Wales, it is an offence to grow this plant in the wild [2].

  • Characteristics – Perennial herb with stems growing horizontally from water bodies across 4-5m. Leaf shape varies from slender to round with a lighter central vein and branching veins, and stems are either smooth or hairy. Distinct vibrant yellow flower, can grow vertically to 1m above the water surface. Flowers from July-August and spreads by plant fragmentation and seeds. In winter months, seen as brown floating stems. Click here for an ID guide [3].
  • Habitat – Native to South America and some US states. Found in still or slow flowing freshwater [2].
  • Invasive traits – Rapid growth rate, efficient reproductive capacity, high plasticity in growth response, broad ecological tolerance, allelopathic compounds [4].
  • Threat – Forms dense impenetrable mats leading to oxygen depletion in the water, competes with native species for space and resources, releases chemicals that suppress other organisms (allelopathy) leading to poisoning of entire water ecosystems. Linked to significant biodiversity losses in France, with year-round effects on water quality causing ‘dystrophic crises’. Clogs waterways, drainage systems, bathing waters, impedes boating and increases flood risk [5].

Carolina fanwort (Cabomba caroliniana)

Established as a pest in the Netherlands with a few established populations in Southeast England, this species is not yet invasive here due to imperfect climate conditions but with rapid climate change it’s one on the horizon to look out for.

A popular aquarium plant, its likely paths of invasion are export from commercial growers in Asia and dumping of aquarium water into rivers and streams. Carolina fanwort is listed as an “invasive alien species of union concern” in the EU thus cannot be traded or commercialised. Identification issues are exacerbated by this species’ many names; Carolina water shield, Green cabomba, Fanwort and Fish grass [6].

  • Characteristics – An aquatic perennial herbaceous plant often found submerged, but also floats. Has short fragile rhizomes and often roots. Shoots appear grass-green to olive green, sometimes appearing red. Submerged leaves are finely divided and paired around the stem, whilst floating leaves are alternating and linear. The leaf blade is long and narrow, attaching to the centre. Small, solitary white or pale-yellow flowers (<2cm) appear on stalks rising up from the water. Click here for an ID guide [7].
  • Habitat – Native to South America and southern areas of North America. Prefers to root in the mud of slow-flowing or stagnant water [6].
  • Invasive traits – Highly competitive and spreads from rhizome fragments and reproduces by seed in native habitats, but vegetatively outside of native habitats. No known herbivores outside its native range. High growth rates of up to 50mm per day reported in Australia where it is an established pest. During summer stems become brittle and break apart, facilitating new invasions [8].
  • Threat – Forms dense mats which eclipse other aquatic plants from sunlight, prevent germination of seed and obstruct movement of both animals and humans within water bodies. Decaying mats cause odour pollution and dissolved oxygen depletion. Inhibits germination and growth in aquatic species via allelopathic qualities [9].

Chilean needle-grass (Nassella neesiana)

This noxious weed is likely spread by sticking to clothes, livestock, vehicles and farm machinery. It can also contaminate fodder and is moved downstream by floodwater. A symptom of prolonged grazing, this species benefits from the elimination of palatable plants by herbivores. This leaves room for unrestricted multiplication of unpalatable species, such as Chilean needle-grass. A category 3 restricted invasive plant under the Biosecurity Act 2014, this plant should not be given away, sold or released into the environment [10].

  • Characteristics – Perennial tussock grass in dense clumps up to 1m tall. Leaves are 1-5mm wide, flat and strongly ribbed with rough edges. New flowers are dark red and occur at the end of the seeds. Dry, straw-coloured flower heads can remain on the plant even after seeds drop. Germinates in autumn and spring [10].
  • Habitat – Observed in roadsides, drainage lines and pastures [10].
  • Invasive traits – Reproduces from seed, with hidden seeds also produced in nodes and the base of flowering stems. High survival rate among seedlings, producing >20,000 seeds per m². Seedlings flower and produce seeds within their first season [11].
  • Threat – Reduces natural biodiversity by replacing native species, decreases pasture productivity by 50% and can injure animals with its long, sharp seeds. Impacts the sheep industry by downgrading the quality of meat, wool, skins and hides [12].

Purple pitcher plant (Sarracenia purpurea)

“Deceptive and deadly”, this carnivorous plant traps and drowns insects in its pouch-like leaves [13]. It is localised within the UK, with 20 sites in England and two in Scotland. First introduced to a bog in Wareham Forest in the ‘80s, it is the hardiest and most widespread of the eight pitcher plants. Deliberate planting by carnivorous plant enthusiasts for personal, commercial and scientific reasons exacerbates the spread of this species [14].

  • Characteristics – Cluster of up to 100 ‘pitchers’ which are age-dependent, varying from green to purple with open lids. These fill with rainwater and host insects, algae and bacteria which assist in prey capture. Solitary cup-shaped purple flowers form on the end of long stalks [15]
  • Habitat – Native to North America, preferring nutrient-poor acid bog habitats. Usually found in SSSIs, SACs and NNRs. Favours raised bogs, bog pools, blanket bog, step and seepage mires [14].
  • Invasive traits – Carnivorous, steals the food supply of native bog vegetation such as bog mosses and liver-worts [16]. Evolved carnivory to supplement a life in nutrient poor environments [16].
  • Threat – Outcompetes native bog vegetation, stealing food supply from an already nutrient poor environment. Impacts invertebrate communities, disrupts trophic interactions and nutrient cycling [17].

Sea myrtle (Baccharis halimifolia)

Also called ‘tree groundsel’, this hardy coastal species persists at one site in South Hampshire where it is not yet spreading. With scented flowers, hardiness and autumn flowering, Sea myrtle is a popular ornamental and hedging species. It is invasive in Australia, locally established in New Zealand, and scattered across Europe as a known invasive pest [18].

  • Characteristics – Branching shrub or tree with slender young stems which mature to deeply ridged, thick trunks. Stems are densely branched with pale green, alternating leaves. Bright white, small flowers in clusters of one to five [18, 19].
  • Habitat – Native to North American tidal wetlands, upper saltmarshes and disturbed communities inland including open woods, abandoned fields and deserts [18].
  • Invasive traits – Forms a dense understorey, survives periodic flooding and drought and is able to rapidly regrow after dieback periods. Tolerant to shade, salt and low soil fertility. Highly competitive, producing toxic leaves and allelopathic pollen. Fast growing, regenerating almost entirely by vast quantities of seed which are small and attached to a pappus to aid wind dispersal [18, 19].
  • Threat – Outcompetes native plants leading to altered ecosystem, leaves are toxic to livestock, and irritates humans via pollen and pappus. Competes with forage species in agricultural pastures, and is toxic to livestock. Eradication is time-consuming and expensive [18, 19].

Variable-leaved watermilfoil (Myriophyllum heterophyllum)

“Among the worst invasive species in Europe”, this evergreen species is seen all year round and has been listen on several occasions as a species of concern by the EU [20]. Often misidentified as M. verticillatum, it is only distinguishable by its flowers which are rarely sighted. A popular aquarium species, it is thought to have been introduced through ornamental cultivation and has been sold in Europe for years under different aliases. Variable-leaved watermilfoil has been recorded in a canal in the UK, and several artificial ponds but is yet to be labelled an invasive in the UK. [20, 21].

We think it’s one to keep an eye on.

  • Characteristics – Submerged evergreen aquatic perennial herb producing flowering spikes (5-35cm) consisting of flowers in whorls of 4-6. Leaves are whorled with 5-12 filamentous segments. Flowering in May-October, flowers are tiny and red with 4 sepals but rarely observed in native or invasive ranges [21].
  • Habitat – Native to North America and Mexico, preferring lakes, ponds and slow-moving rivers with alkaline, conductive conditions. Grows in open water, but can also survive as a semi-terrestrial plant in muddy margins of water bodies [21].
  • Invasive traits – grows rapidly, smothering native species and competing for resources. Survives all year round, forming overwintering organs called ‘turions’ to survive challenging environmental conditions [21].
  • Threat – Forms dense aquatic mats which disperse native species, restrict water flow and reduce availability of sunlight and oxygen. This interferes with navigation and recreational activities in waterways. Potentially increases flood risk by increasing suspended organic matter and sedimentation. Reduction in wildlife resulting from overcrowding can detriment recreational revenue in touristic areas [21, 22].

We can help

Our aim at Environment Controls is to keep you informed and up to date with invasive species and how to control them. Our identification tool is ideal if you suspect you have an invasion, send us photos and we will identify the plant for you. Or book a survey with us using the form below.


Thanks to Eloise M. K. Holmes Msc. Bsc. for providing the above insights and references.


[1] Invasive species: the silent threat to our ecosystems – APHA Science Blog

[2] Uruguay Water-primrose » NNSS (

[3] ID_Ludwigia_grandiflora_Water_Primrose-2.pdf (

[4] Grewell BJ, Netherland MD, Skaer Thomason MJ. Establishing research and management priorities for invasive water primroses (Ludwigia spp.).

[5] Thouvenot L, Haury J, Thiebaut G. A success story: water primroses, aquatic plant pests. Aquatic Conservation: Marine and Freshwater Ecosystems. 2013 Oct;23(5):790-803.

[6] Carolina Water-shield » NNSS (

[7] ID_Cabomba_caroliniana_(Carolina_Watershield_or_Fanwort) (

[8] Huang X, Ke F, Lu J, Xie H, Zhao Y, Yin C, Guan B, Li K, Jeppesen E. Underwater light attenuation inhibits native submerged plants and facilitates the invasive co‐occurring plant Cabomba caroliniana. Diversity and Distributions. 2023 Apr;29(4):543-55.

[9] Roberts J, Florentine S. A global review of the invasive aquatic weed Cabomba caroliniana [A. Gray](Carolina fanwort): Current and future management challenges, and research gaps. Weed Research. 2022 Feb;62(1):75-84.

[10] American Needle-Grass » NNSS (

[11] Weller, S., 2016. Detection and Prevention of the Dispersal of the Seeds of Chilean Needle Grass (Nassella Neesiana Trin. & Rupr.(Barkworth)) in Hay Bales (Doctoral dissertation, Federation University Australia).

[12] Fox JC, Buckley YM, Panetta FD, Bourgoin J, Pullar D. Surveillance protocols for management of invasive plants: modelling Chilean needle grass (Nassella neesiana) in Australia. Diversity and Distributions. 2009 Jul;15(4):577-89.

[13] Purple pitcher plant – Sarracenia purpurea | Plants | Kew

[14] Pitcher Plant » NNSS (

[15] Adlassnig W, Mayer E, Peroutka M, Pois W, Lichtscheidl IK. Two American Sarracenia species as neophyta in central Europe. Phyton. 2010 Mar 29;49(2):279-92.

[16] Ellison AM, Parker JN. Seed dispersal and seedling establishment of Sarracenia purpurea (Sarraceniaceae). American Journal of Botany. 2002 Jun;89(6):1024-6.

[17] Walker KJ, Auld C, Austin E, Rook J. Effectiveness of methods to control the invasive non-native pitcherplant Sarracenia purpurea L. on a European mire. Journal for Nature Conservation. 2016 Jun 1;31:1-8.

[18] Tree Groundsel » NNSS (

[19] Fried G, Caño L, Brunel S, Beteta E, Charpentier A, Herrera M, Starfinger U, Panetta FD. Monographs on invasive plants in Europe: Baccharis halimifolia L. Botany Letters. 2016 Apr 2;163(2):127-53.

[20] Gross EM, Groffier H, Pestelard C, Hussner A. Ecology and environmental impact of Myriophyllum heterophyllum, an aggressive invader in European Waterways. Diversity. 2020 Mar 30;12(4):127.

[21] Twoleaf Watermilfoil » NNSS (

[22] Bailey JE, Calhoun AJ. Comparison of three physical management techniques for controlling variable-leaf milfoil in Maine lakes. Journal of Aquatic Plant Management. 2008 Jul 1;46(2):163-7.