Swamp Stone-crop, New Zealand Pygmy Weed, Tillaea recurva, Crassula recurva, Tillaea helmsii

In some areas of the United Kingdom it is highly invasive and it out-competes many native plant species by forming dense smothering mats of vegetation (Bridge 2005). Crassula helmsii shows extreme competitiveness, dominates many sites, and can cover the entire water surface (Brunet 2002). Crassula helmsii is very tolerant of herbicides (Dawson 1996). Aggressive competition with other aquatic species to their near exclusion, originally observed in the 1920s with Nympizaea (Swale and Belcher 1982) but more recently with Elodea spp. (Cockerill 1979; FHD), with natural flora in three areas including the New Forest (A. Byfield 1984; pers. comm.), and with the rarer Potomogeton spp. (M. Briggs, pers. comm.; Dawson and Warman 1987).

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). C. helmsii suppresses germination of other species up to 83%, yet there was no effect on seed bank and no loss of plant species on four ponds in England. However, the experiment was over a limited time scale (Langdon et al. 2004). Crassula helmsii propagates via vegetative fragmentation of fragments as small as 1cm (Brunet 2002).

Potential pathway(s) of introduction: Unauthorized intentional release

This species has been found for sale at large outdoor stores (e.g., Lowe's Hardware mistakenly sold it under another name in Florida), as well as recommended for hobbyists online. It is unknown, however, if it is being bought and sold in the Great Lakes region.

Crassula helmsii has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Crassula helmsii has a broad physiological tolerance and this range suggest it would fair just fine in the Great Lakes. C. helmsii can overwinter in temperatures of 0°C. In the United Kingdom, it is highly invasive and outcompetes many native plant species by forming dense smothering mats of vegetation; this same occurrence would happen in the Great Lakes region if introduced. C. helmsii reproduces very easily and would spread once introduced.

Crassula helmsii suppresses surrounding epiphytic and planktonic algae more than the effect of shade (Brunet 2002). This species has negatively affected the breeding success of the protected great crested newt Triturus cristatus (Watson 1999). In some areas of the United Kingdom, C. helmsii is highly invasive and outcompetes native plant species by forming dense smothering mats of vegetation (Bridge 2005). Crassula helmsii shows extreme competitiveness, dominates many sites, and can cover the entire water surface (Brunet 2002) and is very tolerant of herbicides (Dawson 1996).

Crassula helmsii has the potential for moderate socio-economic impact if introduced to the Great Lakes.

Crassula helmsii can clog drainage ditches (European and Mediterranean Plant Protection Organization 2007) and can impact recreational use (European and Mediterranean Plant Protection Organization 2007).

There is little or no evidence to support that Crassula helmsii has the potential for significant beneficial impacts if introduced to the Great Lakes.

It has not been indicated that Crassula helmsii can be used for the control of other organisms or improving water quality. There is no evidence to suggest that this species is commercially, recreationally, or medically valuable. It does not have significant positive ecological impacts.

Wisconsin prohibits the transport, possession, or introduction of C. helmsii (Wisconsin Chapter NR 40). Minnesota also lists C. helmsii as prohibited meaning that a person may not possess, import, purchase, sell, propagate, transport, or introduce C. helmsii (Minnesota Rule 6216.0250). There are no regulations on C. helmsii in Ontario, New York, Pennsylvania, Ohio, Michigan, Indiana, or Illinois.

Control

Biological

One report has hypothesized that the presence of geese and other plant-grazing waterfowl can reduce the presence of C. helmsii, but it is also possible that waterfowl could be partly responsible for the spread of the C. helmsii by transporting fragments on their feet (Smith and Harris 2008). The use of herbivorous fish, mainly grass carp (Ctenopharyngodon idella), as a bio-control of C. helmsii has been investigated. Laboratory and field tests have shown that grass carp will eat C. helmsii but this behavior seems to be contingent on habitat-specific variables such as temperature and species composition (grass carp favor other aquatic plants over C. helmsii) (Dawson and Warman 1987). The use of grass carp as a bio-control agent for C. helmsii is not preferable for the Great Lakes since grass carp are not indigenous to the basin.

Physical

Mechanical control methods are the most common way of managing invasive plants (Hussner et al. 2016) but are difficult to implement for successful treatment of C. helmsii. Excavation of C. helsmii has been proposed as a control method but it must be regularly repeated due to the release of small plant fragments into the water, which can rapidly regrow and colonize new sites or recolonize excavated sites (Smith and Harris 2008).  Smothering C. helmsii with carpet or black polythene to prevent it from receiving light has experienced some success, but the application of this method is limited to plants found in shallow depths (less than 3 m) and the shading must remain in place between 8 weeks to 6 months (Smith and Harris 2008; Bridge 2005; Centre for Aquatic Plant Management 2004 via Robert et al. 2013). Although Bridge (2005) observed a 100% kill rate by smothering and burying C. helmsii, but this is often not a feasible option due to labor requirements and the resulting environmental disturbance.

Chemical 

Bridge (2005) measured and compared the effectiveness of spraying biodegradable Waipuna hot foam, Glyphos bioactive, and a ‘smothering and burying’ technique as methods of control for C. helmsii populations. Both of the chemical controls studied yielded an approximate kill rate of 50% (Bridge 2005). These chemical agents will likely not eradicate C. helmsii due to its ability to regrow quickly but they could be used as a means of managing the spread of C. helmsii. This is especially the case for Waipuna organic hot foam, which is not toxic to wildlife and can be applied with accuracy and without damage to other plants (Bridge 2005). A study done in southern England, UK showed that the herbicidal agent, diquat, was the most effective means of reducing low-biomass, submerged C. helmsii while glyphosate caused the greatest biomass reduction (82%) in plants above water (Dawson 1996). Both diquat and glyphosate are best applied directly to target plants to avoid damaging native species and multiple applications are often needed to remove plants missed during the first application (Washington Department of Ecology). Freezing small areas of C. helmsii with liquid nitrogen has also been successful and this method can be used on medium surfaces (20-1000 m2) as long as the area to be treated is enclosed by a fine wire mesh (5 mm) fence (Robert et al. 2013). Another study in England observed that inundating areas with saltwater killed 99% of the C. helmsii infestation (Durnell, pers. comm. in European and Mediterranean Plant Protection Organization 2014).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

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.

Hussner, A., I. Stiers, M.J.J.M. Verhofstad, E.S. Bakker, B.M.C. Grutters, J. Haury, J.L.C.H. van Valkenburg, G. Brundu, J. Newman, J.S. Clayton, L.W.J. Anderson, and D. Hofstra. 2016. Management and control methods of invasive alien freshwater aquatic plants: a review. Aquatic Botany 136:112-137. dx.doi.org/10.1016/j.aquabot.2016.08.002.

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/.