Baccharis halimifolia - Sea-myrtle

Sea myrtle is a halophilic species native to the east coast of North America and the northeastern part of the Gulf of Mexico ^[2]. In Dutch, sea myrtle is known as ‘breedbladig roerkruid’ ^[3] and ‘kruisstruik’ ^[4].

First observation in Belgium

The first observation of sea myrtle in Belgium dates back to 1924. It concerned a plantation in the dunes of Raversijde, west of Ostend. The first observation of non-planted sea myrtle dates from 1948 in the port of Ostend ^[5].

Spreading in Belgium

Since 1924, there has been a gradual increase in sea myrtle. Since the second half of the 1990s, the shrub got observed more regularly along the Belgian coast ^[5]. The species is present in Knokke (in the nature reserve at the Bay of Heist), the inner port of Zeebrugge, the old fishing port of Blankenberge, Bredene, Koksijde, De Panne and more inland in Veurne ^{[5, 6]}. Sea myrtles in a natural environment are often feral forms of nearby plantations ^[2]. There were rapidly expanding populations in the Bay of Heist and De Panne ^[5], but these were actively combated and controlled ^[7].  This alien species occurs anywhere along the coast, but is rare. Sea myrtle was recently observed in other parts of Flanders, though the numbers remain limited ^{[6, 7]}.

Spreading in neighbouring countries

In Europe, sea myrtle was introduced to France as an ornamental plant in 1683 ^{[8, 9]}. First, plantations occurred along the South Atlantic coast. Then, plantations started to occur in the north, along the coast of Brittany and Normandy ^[10]. Here, sea myrtle is also used as a hedge to break the wind ^[5]. Even though sea myrtle is a rare species in Belgium, it belongs to the top 20 most harmful invasive species in France and Spain. This is not surprising when looking at the distribution of the plant: sea myrtle grows and displaces the native plant species in almost all estuaries around the Bay of Biscay ^{[11, 12]}.

In Great Britain, the first wild plant was reported in 1924 along the coast of Hamworthy (Dorset). This individual was an escaped ornamental plant ^[13]. Sea myrtle was also planted in Great Britain for its wind resistance ^[14].

The first plantings of sea myrtle in the Netherlands took place in the early 19^th century ^[3]. After 2014, the plant was observed only twice (one individual each time) in South Holland ^[15].

Sea myrtle got introduced in 1683 to France as an ornamental plant ^{[8, 9, 14]}. The species is appreciated by ornamental growers in coastal areas for its high tolerance to salt concentrations and strong winds. Today, the species is usually introduced as an ornamental plant to gardens and parks or along roadsides because of its striking and abundant flowering ^[5]. Once planted, nature promotes the local spread. The wind disperses large quantities of seeds produced by the plant across long distances ^[16].

Sea myrtle produces a large number of seeds: up to half a million per plant. These seeds are light and have fluffy appendages. The wind can carry the seeds away over hundreds of meters. Sometimes, dispersion takes place via water ^[16]. The plant has rapid growth and can resist the mild frost in our regions ^[16].

Sea myrtle feels at home in various habitats, such as periodically submerged saline grasslands (high salt meadows), high water marks, brackish pools, dunes and beaches. The species can tolerate human impact. Therefore, it can be found in the ruderal dunes (dunes strongly enriched by humans, e.g. with organic matter), raised lands and uncultivated clay fields along our coast ^{[2, 5]}. Furthermore, the plant can recover quickly after a fire or other damage ^{[11, 16]}.

In France, wild sea myrtle is often observed where the fresh and salt environment meet. In brackish environments, this alien species is often more successful than the native species, which may lead to monotonous vegetation consisting almost exclusively of sea myrtle ^[17]

The distribution of sea myrtle is mainly determined by the ambient temperature. Like in North America, this plant has a northern distribution limit in our regions. This is due to their toleration of only mild periods of frost. During the 20^th century, this limit shifted to the north as a result of climate change ^[5].

Sea myrtle can thrive in both salt and fresh environments (from 1-30 PSU), although the survival of this plant decreases with increasing salinity due to the stress caused by a salty environment ^[18]. Sea myrtle grows on a large variety of different soils, including sand, clay, gravel and peat, but it is not found in heavy clay soils ^[19].

Feral sea myrtle is often found in the vicinity of plantations. An increase in the number of plantations (popular with ornamental growers) can lead to a broader distribution of the species in the wild ^[5].

For the time being, sea myrtle is less prominent in Belgium than in the coastal areas to the south of our country, despite the similar ecological (e.g. availability of substrate and little competition with other species) and climatological characteristics (e.g. temperature). Currently, the species is actively controlled (e.g. in the Bay of Heist and Zwin Nature Park), which explain its low abundance in nature. Nonetheless, a future invasion and, therefore, threat to local biodiversity cannot be ruled out ^[5].

In areas where the invasiveness of the species has already been demonstrated (e.g. in western France ^{[11, 13]} and Australia ^[21]), it is often seen as an aggressive pest species or persistent weed. Despite this fact, the species remains a popular ornamental garden plant in coastal areas ^[5]. In southern Europe (Spain), sea myrtle is also seen as an invasive species that replaces native plant communities in brackish coastal environments and tidal marshes ^[18]. Particularly the nature reserve of Urdaibai (Basque Country) has been strongly affected. Between 2010 and 2013, a European LIFE project removed sea myrtle to create space for native vegetation. Young shoots were pulled out manually and older shrubs were cut down. Afterwards, herbicide was applied to the remaining stump to prevent the formation of new shoots. This method proved to be very efficient, but to keep the results in the long-term young shoots should be removed again ^{[21, 22]}.

Sea myrtle is included on the list of Invasive Alien Species of Union concern (Regulation (EU) No. 1143/2014), which prohibits the merchandising, possession and release of the species. The EU Member States should detect and remove the species, or in the case of established populations, take measures to prevent further spread. Raising awareness to ornamental plant growers about the possible effects of imported ornamental plants on local biodiversity ^[5] and proposing non-invasive alternative plants are important too ^[23].

The pollen can cause allergic reactions in humans, and its leaves are toxic to cattle ^{[17, 18]}

Sea myrtle is a moderately extending shrub that grows 1 to 2 m in height but can grow to 6 m. In our regions, the plant usually loses its leaves during winter. These leaves are obovate to rhombic, silvery-green in colour and have a leathery texture. Sea myrtle produces clusters of white-yellow tubular flowers on the end of its branches (terminal flower heads). There are plants with only male flowers and plants with only female flowers. After blooming, the flower head has a brush-like appearance. Pollination of flowers and spreading of the seeds occurs via the wind ^{[5, 18, 24, 25]}.

The leaves of sea myrtle contain a toxin that can be fatal to sheep (that graze on the plant) when they eat more than one per cent of their body weight of these leaves ^[2]. In our regions, sea myrtle loses its leaves in winter. However, this is not the case everywhere. For example, in northern Australia, the plant retains its leaves all year round. In its area of origin, sea myrtle loses its leaves during winter because it is generally colder there ^[5]

[1]          World Register of Marine Species (WoRMS) (2020). Baccharis halimifolia L. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=578001] (2020-11-17).

[2]          Van Landuyt, W.; Hoste, I.; Vanhecke, L.; Van Den Bremt, P.; Vercruysse, W.; de Beer, D. (Ed.) (2006). Atlas van de flora van Vlaanderen en het Brussels Gewest. Nationale Plantentuin van België/Instituut voor Natuur- en Bosonderzoek/Flo.Wer: Brussel. ISBN 90-726-1968-4. 1007 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=170947]

[3]          Krauss, J.C. (1802). Afbeeldingen der fraaiste, meest uitheemsche boomen en heesters, die tot versiering van Engelsche bosschen en tuinen, op onzen grond, kunnen geplant en gekweekt worden, benevens de beschrijving van derzelver kenmerken, voortkweeking, nuttigheden en andere bijzonderheden, ingericht om aan de liefhebbers van zodanige bosschen of tuinen de kennis van dezelve zo aangenaam als nuttig te maken. Johannes Allart: Amsterdam. 126 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=206887]

[4]          Boom, B.K. (1949). Nederlandse dendrologie: Geïllustreerde handleiding bij het bepalen van de in Nederland voorkomende soorten en variëteiten der gekweekte houtgewassen. Derde druk. H. Veenman & Zonen/De Sikkel: Wageningen en Antwerpen. 444 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=206956]

[5]          Rappé, G.; Verloove, F.; Van Landuyt, W.; Vercruysse, W. (2004). Baccharis halimifolia (Asteraceae) aan de Belgische kust. Dumortiera 82: 18-26. [http://www.vliz.be/nl/catalogus?module=ref&refid=59040]

[6]          Waarnemingen afkomstig van Waarnemingen.be: een initiatief van Natuurpunt Studie vzw en de Stichting Natuurinformatie (2011). Struikaster - Baccharis halimifolia L. [https://waarnemingen.be/soort/view/18441?from=2000-08-02&to=2011-08-02] (2011-08-02).

[7]          Van Landuyt, W. (2019). Persoonlijke mededeling

[8]          Fournier, P. (1977). Les quatre flores de la France, Corse comprise (générale, alpine, méditerranéenne, littorale). Editions Lechevalier: Paris. SBN 2-720-50493-9. 1105 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=207301]

[9]          Loudon, J.C. (1842). An Encyclopaedia of Trees and Shrubs; Being the Arboretum et Fruticetum Britannicum abridged: containing the Hardy Trees and Shrubs of Britain, Native and Foreign, scientifically and popularly described; with their Propagation, Culture, and Uses in the Arts; and with Engravings of nearly all the Species. Abridged from the large edition in eight volumes, and adapted for the use of nurserymen, gardeners, and foresters. Longman, Brown, Green and Longmans: London. 1237 pp. [http://www.vliz.be/en/imis?module=ref&refid=207299]

[10]        Gèze, M. (1999). Le Baccharis: un envahisseur indésirable. Bull. Soc. Sci. Nat. Ouest Fr. (1983) 21(1): 39-40. [http://www.vliz.be/en/imis?module=ref&refid=206894]

[11]        Caño, L.; Campos, J.A.; García-Magro, D.; Herrera, M. (2013). Replacement of estuarine communities by an exotic shrub: distribution and invasion history of Baccharis halimifolia in Europe. Biological Invasions 15(6): 1183-1188. [http://www.vliz.be/nl/catalogus?module=ref&refid=303191]

[12]        Fried, G.; Caño, L.; Brunel, S.; Beteta, E.; Charpentier, A.; Herrera, M.; Starfinger, U.; Dane Panetta, F. (2016). Monographs on invasive plants in Europe: Baccharis halimifolia L. Botany Letters 163(2): 127-153. [http://www.vliz.be/nl/catalogus?module=ref&refid=312512]

[13]        Clement, E.J.; Foster, M.C.; Kent, D.H. (1994). Alien plants of the British Isles: a provisional catalogue of vascular plants (excluding grasses). Botanical Society of the British Isles: London. ISBN 0-901158-23-2. 590 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=163106]

[14]        Webster, A.D. (1918). Seaside planting for shelter, ornament & profit. T. Fisher Unwin: London. 156 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=206724]

[15]        van Valkenburg, J.; Meerman, H.; Bollen, G.; Zwart, A. (2017). Baccharis halimifolia L. in de herkansing. Gorteria 39. 1-4 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=312511]

[16]        Westman, W.E.; Panetta, F.D.; Stanley, T.D. (1975). Ecological studies on reproduction and establishment of the woody weed, groundsel bush (Baccharis halimifolia L.: Asteraceae). Australian Journal of Agricultural Research 26: 855-870. [http://www.vliz.be/nl/catalogus?module=ref&refid=206613]

[17]        Vanden Berghen, C. (1967). Notes sur la végétation du sud-ouest de la France: V. Les peuplements de Scirpus americanus Pers. dans le département des Landes. Bull. Jardin Bot. Nat. Belg 37(3): 335-355. [http://www.vliz.be/nl/catalogus?module=ref&refid=206835]

[18]        Cao, L.; Tens, M.; Fuertes-Mendizábal, T.; Gonzlez-Moro, M.B.; Herrera, M. (2011). The role of plasticity, genetic variation and maternal effects in the tolerance to salinity in the invasive plant Baccharis halimifolia, 96th ESA Annual Convention 2011, 7-12 August, 2011, Austin, Texas. ESA: Washington, DC.: pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=303194]

[19]        USDA NRCS National Plant Data Center & Biota of North America Program (2018). Plant guide. Groundsel tree, Baccharis halimifolia L. [https://plants.usda.gov/plantguide/pdf/pg_baha.pdf] (2018-10-12).

[20]        Provoost, S. (2020). Persoonlijke mededeling

[21]        Beteta, E.; Oreja, L.; Prieto, A.; Rozas, M. (2012). Life+ Project Estuaries of the Basque Country: control and elimination of Baccharis halimifolia L. in Urdaibai, NEOBIOTA 2012 – 7th European Conference on Biological Invasions. Halting biological invasions in Europe: from Data to Decisions, Pontevedra (Spain), 12-14 September 2012: Abstracts. Life+: Pontevedra: pp. 237-238. [http://www.vliz.be/nl/catalogus?module=ref&refid=303189]

[22]        European Commission (2018). Estuarios del Pais Vasco - Restoration of habitats of Community interest in the Basque Country's estuaries. LIFE08 NAT/E/000055. [http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=search.dspPage&n_proj_id=3578#RM] (2018-07-20).

[23]        AlterIAS (2018). Alternatieve planten. [http://www.alterias.be/nl/lijst-van-invasieve-en-alternatieve-planten/alternatieve-planten] (2018-07-20).

[24]        Pacific Island Ecosystems at Risk (PIER) (2018). Baccharis halimifolia L., Asteraceae. [http://www.hear.org/pier/species/baccharis_halimifolia.htm] (2018-08-02).

[25]        USDA NRCS National Plant Data Center & Biota of North America Program (2018). Plant fact sheet. Eastern Baccharis, Baccharis halimifolia L. [https://plants.usda.gov/factsheet/pdf/fs_baha.pdf ] (2018-10-12).

VLIZ Alien Species Consortium (2020). Baccharis halimifolia – Sea myrtle. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 6 pp.