New Zealand Pygmy Weed, Tillaea recurva, Crassula recurva, Tillaea helmsii

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.

C. helmsii grows in three forms:

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 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). C. helmsii is documented to occur growing as an understory layer associated with P. australis and may benefit from the expansion of P. australis australis in the Great Lakes region (Smith and Buckley, 2020).

Potential pathway(s) of introduction: Aquarium or pond escape

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. UK specimens are believed to come from a single introduction (Dawson 1994).  It has also been reported that this species can be easily dispersed by waterfowl and by attaching itself to equipment (Mumby et al., 2018)

C. helmsii is documented to occur growing as an understory layer associated with P. australis and may benefit from the expansion of P. australis australis in the Great Lakes region (Smith and Buckley, 2020).

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 similar climates in its European range  (United Kingdom (Bridge 2005), Belgium, Denmark, France, Germany, and the Netherlands (European and Mediterranean Plant Protection Organization 2007) which suggests it would survive and reproduce in the Great Lakes. C. helmsii can overwinter in temperatures of 0°C. C. helmsii reproduces very easily and would likely spread once introduced.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

C. 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 suppresses surrounding epiphytic and planktonic algae more than the effect of shade (Brunet 2002). 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). This species has negatively affected the breeding success of the protected great crested newt Triturus cristatus potentially due to its rigid leaves which may make habitats unsuitable for egg laying (Watson 1999).  Additionally, this species can limit water circulation and change the oxygen regime Robert et al., 2013). C. helmsii can also influence macroinvertebrate community composition, increasing total overall abundance but decreasing evenness (Giordano, 2022).

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

Crassula helmsii can clog drainage ditches (EPPO 2007) and can impact recreational use (EPPO 2007). In England C. helmsii is estimated to have cost angling 170,636 pounds across a 10km affected stretch of river and costs 1,450-7,225 pounds per pond to remove (Williams et al., 2010). Eradicating C. helmsii in a 4.5 ha area of Wadden Island cost 1.5 million euro (van der Loop et al.,  2022).

Crassula helmsii has the potential for moderate beneficial impact if introduced to the Great Lakes.

Studies have indicated that Crassula helmsii can be used for the control of a few other organisms. It is also a copper hyperaccumulater giving it the potential to be used as a method to reduce copper levels in polluted waterways. There is no evidence to suggest that this species is commercially, recreationally, or medically valuable.