Identification: Crassula helmsii is an aquatic or semiterrestrial succulent perennial herb that ranges 10–130 cm in length. Roots: Can be floating or creeping depending on the growth form of the plant.
Stem: Round with roots forming at the nodes.
Leaves: Opposite, sessile, and succulent. Leaves are 4–20 mm long, 0.7–1.6 mm wide, linear-lanceolate to ovate-lanceolate, and acute.
Flowers: White or pinkish and borne singly in the axils of leaves. Inflorescences have a diameter of 3–3.5 mm and are 4-merous. Petals are slightly longer than the sepals.
Fruits: Fruits are follicles containing 2–5 elliptical and smooth seeds about 0.5 mm long.
Crassula helmsii grows in three forms:
| Form | Characteristics: (EPPO 2007; DAISIE 2008). |
| Emergent | Grows as stands of short, densely packed stems in water of 0.6 m or less in depth |
| Submerged | Grows from a basal rosette, well rooted at the base, with long sparsely leaved stems that may reach the water surface |
| Terrestrial | Creeping or erect stems and aerial leaves that are yellowish-green in color and succulent in appearance |
Ecology: Crassula helmsii inhabits the margins and shallow waters of freshwater lakes and ponds (Bridge 2005) and is found in ponds, lakes, gravel pits, and other static water bodies, and; also, streams with low flow (Brunet 2002). This species can grow near streams but usually establishes in lentic or very slow flowing regions (water velocity up to 0.3 m/s) Smith and Buckley 2020). Crassula helmsii inhabits freshwaters areas in a range of depths: from drying soils to submerged depths of 3 m (Dawson 1996). It is found in waters with pH from 4.29-9.57, and alkalinity from 0-0.92 m.eq/L. It can tolerate drying for extended periods (Kirby 1965, Dawson and Warman 1987, Smith and Buckley 2020), and can survive in poor or rich phosphorus and potassium (Brunet 2002). Additionally, C. helmsii can survive CO2 concentrations as low as 3 mmol/m3, prolonged droughts up to 4 months, and high levels of metals (Smith and Buckley 2020). This species appears to do better in high light, but has also been found despite shading (Dawson and Warman 1987). It has a temperature range of 0 to 25°C, including extended periods under snow (Kirby 1965, European and Mediterranean Plant Protection Organization 2007) and can withstand temperatures up to -6°C (Leach and Dawson 1999). Propogation occurs via vegetative fragmentation as small as 1 cm and is very tolerant of herbicides (Brunet 2002, Dawson 1996). Crassula helmsii can withstand warm water temperatures, so it may benefit from increased temperatures due to climate change. Invasiveness may result, in part, from Crassulacean acid metabolism, which confers a competitive advantage in relation to growth through carbon conservation (Klavsen and Maberly 2009). Crassula helmsii is capable of outcompeting native species, increasing its ability to establish itself in a new region (Brower et al. 2017). Establishment is more likely in areas that are eutrophic (van der Loop et al. 2020). Crassula helmsii is documented to occur growing as an understory layer associated with Phragmites australis and may benefit from the expansion of P. australis australis in the Great Lakes region (Smith and Buckley 2020).
References: (click for full references)
Bridge, T. 2005. Controlling New Zealand pygmyweed
Crassula helmsi using hot foam, herbicide and by burying at Old Moor RSPB Reserve, South Yorkshire, England. Conservation Evidence 2:33-34. http://www.conservationevidence.com/individual-study/2146.
Brunet, J. 2002. Effect of chemical and physical environment on Crassula helmsii spread. Centre for Ecology & Hydrology, Natural Environment Research Council, Dorset, UK. http://nora.nerc.ac.uk/18652/1/N018652RE.pdf.
Cockerill, D. 1979. Crassula helmsii. Botanical Society of the British Isles 21:18.
Dawson, F.H., and E.A. Warman. 1987. Crassula helmsii (T. Kirk) Cockayne: Is it an aggressive alien aquatic plant in Britain? Biological Conservation 340(1):247-272. http://link.springer.com/article/10.1007%2FBF00012762.
Dawson, F. 1996. Crassula helmsii: attempts at elimination using herbicides. Hydrobiologia 340(1):241. dx.doi.org/10.1007/BF00012762.
European and Mediterranean Plant Protection Organization (EPPO). 2007. Crassula helmsii. European and Mediterranean Plant Protection Organization Bulletin 37:225-229. https://www.eppo.int/QUARANTINE/data_sheets/plants/Crassula_helmsii_DS.pdf.
European and Mediterranean Plant Protection Organization (EPPO). 2014. PM 9/19 (1) Invasive alien aquatic plants . European and Mediterranean Plant Protection Organization Bulletin 44(3):457-471.http://doi.wiley.com/10.1111/epp.12165.
Gassmann, A., M.J.W. Cock, R. Shaw, and H.C. Evans. 2006. The potential for biological control of invasive alien aquatic weeds in Europe: a review. Hydrobiologia 570(1):217-222. http://link.springer.com/10.1007/s10750-006-0182-4.
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Kirby, J.E. 1965. Notes on Crassula helmsii. The Cactus and Succulent Journal of Great Britain 27(1):9-10. https://www.cactuspro.com/biblio/en:csjgb.
Klavsen, S.K., and S.C. Maberly. 2009. Crassulacean acid metabolism contributes significantly to the in situ carbon budget in a population of the invasive aquatic macrophyte Crassula helmsii. Freshwater Biology 54(1):105-118. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2427.2008.02095.x/full.
Langdon, S.J., R.H. Marrs, C.A. Hosie, H.A. McAllister, K.M. Norris, and J.A. Potter. 2004. Crassula helmsii in U.K. Ponds: Effects on plant biodiversity and implications for newt conservation. Weed Technology 18:1349-1352. http://www.jstor.org.proxy.lib.umich.edu/stable/3989649?pq-origsite=summon&seq=1#page_scan_tab_contents.
Leach, J., and H. Dawson. 1999. Crassula helmsii in the British Isles- an unwelcome invader. British Wildlife 234-239.
Minchin, D. 2008. Species factsheet: Crassula helmsii. Delivering Alien Invasive Species inventories for Europe (DAISIE). http://www.europe-aliens.org/speciesFactsheet.do?speciesId=8738. Accessed on 01/19/2017.
Robert, H., R. Lafontaine, R.C. Beudels-Jamar, and T. Delsinne. 2013. Risk analysis of the Australian swamp stonecrop Crassula helmsii (Kirk) Cockayne. Royal Belgian Institute of Natural Sciences. https://www.researchgate.net/publication/266617673_Risk_analysis_of_the_Australian_swamp_stonecrop_Crassula_helmsii_Kirk_Cockayne_-_Risk_analysis_report_of_non-native_organisms_in_Belgium_from_the_Royal_Belgian_Institute_of_Natural_Sciences_for_the_Fe.
Smith, C., and E. Harris. 2008. Lound lakes - Botanical survey. Norfolk Wildlife Services. http://www.norfolkbiodiversity.org/pdf/biodiversitysurveys/Lound%20lakes%20aquatic%20plant%20survey%20summary.pdf.
Swale, E., and H. Belcher. 1982. Crassula helmsii, the swamp stonecrop, near Cambridge. Nature in Cambridgeshire 25:59-62. http://www.natureincambridgeshire.org.uk/covers/cover-25.htm.
Washington State Department of Ecology. 2017. Aquatic plant management - aquatic herbicides. http://www.ecy.wa.gov/Programs/wq/plants/management/aqua028.html. Accessed on 01/13/2017.
Watson, W. 1999. Amphibians and Crassula helmsii. Froglog Newsletter of the Declining Amphibian Populations Task Force. 31:2. http://www.amphibians.org/froglog/fl031/.
This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.
