Ludwigia grandiflora
(water primrose)

L. grandiflora is a productive emergent perennial native to South and Central America and parts of the USA. It was introduced to France in 1830 and has become one of the most damaging invasive plants in that country (Dandelot et al., 2008). It was more recently introduced beyond its native range in the USA, where it also causes severe problems (IPAMS, 2009). In its adventive range, L. grandiflora can transform ecosystems both physically and chemically. It can sometimes be found growing in impenetrable mats; under these conditions, L. grandiflora can displace native flora and interfere with flood control and drainage systems, clog waterways and impact navigation and recreation (IPAMS, 2009). The plant also has allelopathic activity that can lead to dissolved oxygen crashes and the accumulation of sulphide and phosphate in the water. These not insubstantial and year-round effects on water quality can cause ‘dystrophic crises’ and intoxicated ecosystems (Dandelot et al., 2005).

L. grandiflora is native to the Americas, ranging from the Rio La Plata in Argentina north to the south/southeastern USA. In the USA, its range is primarily along the Atlantic coast and through the Gulf Coastal Plain (southeastern New York through Florida, westward to Texas) (McGregor et al., 1996). Recent reports of the plant from California and Washington indicate that its range may be expanding (Okada et al., 2009). Reports exist placing this species as present in Great Britain, although it is likely that these reports resulted from a misidentification of the species Ludwigia peploides (CEH, 2007). L. peploides is considered to be native to California, but Ludwigia peploides subsp. montevidensis is considered to be introduced.

L. grandiflora is often used as an ornamental. Escape from cultivation likely explains its introduction beyond its native range. Its ability to spread rapidly through vegetative means coupled with a high degree of phenotypic plasticity (Ruaux et al., 2009) means the likelihood of establishment once the plant is released is high. L. grandiflora ramets collected from five populations in California, USA were genotypically identical, indicating that clonal reproduction explains the spread of the species in that region (Okada et al., 2009). As vegetative propagules are the most likely means of growth and expansion of the species, education and preventative measures should be targeted at pathways that might spread plant material.

L. grandiflora is often found growing in freshwater wetlands, in slow-moving rivers and streams, on lake and reservoir shorelines and in shallow canals and on floodplains in California, USA (Okada et al., 2009). The plant displays a high degree of adaptability and flexibility in its habitat requirements due to its phenotypic plasticity (Ruaux et al., 2009). It is said to be highly tolerant of fluctuating water levels. In the interior portions of its range in the USA, the plant is found in three kinds of habitats: 1) emergent marshes and swamps in permanently pooled bottomland depressions that experience periodic flooding; 2) along shorelines and extending out into shallow bays; and 3) on sandy banks and gravel bars of shallow streams (Chester and Holt, 1990).

Impacts on the local environment by L. grandiflora can be severe. The species possesses an allelopathic activity that changes water quality throughout the year and can lead to impoverished flora by decreasing seedling survival of vulnerable native taxa (Dandelot et al., 2008). L. grandiflora has been shown to cause severe hypoxia or even anoxia during summer months as well as leading to reduced sulphate and nitrate levels and increased sulphide and phosphate concentrations, leading to what Dandelot et al. (2005) refer to as ‘a dystrophic crisis’ and an intoxicated ecosystem.

Genetics

Large-flowered variants of sect. Oligospermum have both hexaploid (2n = 48) and decaploid (2n = 80) forms, and this distinction forms the basis of their separation into the hexaploid L. grandiflora and the decaploid Ludwigia hexapetala. A polyploidy form (n = 48) has also been observed and was interpreted as a variant of L. grandiflora. Natural vigorous hybrids often form where the two species co-occur (2n = 8x = 64) (Zardini et al., 1991b).

Reproductive Biology

Clonal expansion is the primary means of reproduction for the species; stem fragments are spread by animals, humans, and water currents (Okada et al., 2009). Most populations flower, but sexual reproduction is likely of lesser importance than vegetative reproduction. Fruit formation in L. grandiflora is variable in French populations; the species is self-incompatible, but the species has a very high potential seed output (around 10,000 seeds per square metre) (Ruaux et al., 2009). In a study of locally-collected seed material from five populations in the middle Loire River in France, fruits had a buoyancy duration of approximately 11 weeks and fruits were frequently viable, indicating that although clonal expansion is the species’ primary means of reproduction, sexual reproduction may be an important means of survival and spread (Ruaux et al., 2009).

Physiology and Phenology

High levels of polymorphism and phenotypic plasticity have been reported for this species in France, which allows the species to grow in a wide range of environments (Ruaux et al., 2009). Its allelopathic properties mean it is an ecosystem engineer, and by making habitats unsuitable for native flora, it increases its competitive potential (Dandelot et al., 2008). It can reproduce vegetatively quite rapidly, but can also repopulate disturbed areas from seed banks. L. grandiflora is quite tolerant of fluctuations in water level and flooding.

The water primrose beetle Lysathia ludoviciana is relatively host-specific, and natural hosts include parrotfeather (Myriophyllum aquaticum), Ludwigia peploides and L. grandiflora (McGregor et al., 1996). The beetle is native to the southern USA and Caribbean region, and its USA distribution is reported to include Texas, Georgia, South Carolina, Ohio and Alabama (Habeck and Wilkerson, 1980). The chrysomelid Altica palustris has also been found in large numbers on L. grandiflora (Dauphin, 1997). Several genus-specific chrysomelid and curculionid beetles native to France may also target Ludwigia species (Gassmann et al., 2006). However, it is usually reported that the plant is generally avoided by herbivores and pathogens (Dandelot et al., 2008).

General

• Botanical garden/zoo
• Pet/aquarium trade
• Sociocultural value

L. grandiflora is very likely to be confused with other Ludwigia species. Zardini et al. (1991b) report that taxa of sect. Oligospermum are “notoriously difficult taxonomically; morphological distinctions between them are often not sharp”. The entire sect. Oligospermum is a polyploidy complex whose members form a very closely related group. L. grandiflora and Ludwigia hexapetala are known to hybridize where their ranges overlap, producing plants with intermediate characteristics (Nesom and Kartesz, 2000). Therefore, distinctions between them are slight, and often overlapping. Nesom and Kartesz (2000) report that plants with larger leaves and flowers and that are sparsely villous are L. hexapetala, whereas plants with smaller leaves and flowers and that are densely villous are L. grandiflora.

L. peploides stems grow more horizontally and their petals are usually 1.0-1.5 cm long, and anthers are 1.0-1.7 mm, whereas L. grandiflora and L. hexapetala stems grow vertically and have larger petals and anthers. Additionally, the small leaves at the base of the flower are triangular to egg-shaped in L. peploides, whereas those of L. hexapetala are ovate (EPPO, 2004).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

L. grandiflora is valued as an ornamental, therefore educational programmes must be directed to educate the public about the dangers this plant poses outside its native range. Teaching users how to clean equipment in a way that decreases the chance of transmission is one way to lessen the impact of human-mediated transport.

Rapid Response

It is much easier and more effective to attempt to control this plant early in its introduction timeline. Small populations are effectively controlled by hand pulling, prior to significant clonal expansion.

Public Awareness

Numerous educational campaigns have been directed at informing the public about the danger of aquatic invasive species in countries in Europe to which L. grandiflora poses a threat. Governmental organizations disseminate educational materials about the identification and control of this species.

Control

Cultural Control and Sanitary Measures

As stem fragments are easily transportable, it is extremely important to decrease the instances of accidental introduction by addressing humans as a vector. By establishing guidelines on how to properly clean equipment, dispose of aquarium water, and identify target plants, it is likely that instances of accidental transportation and release will decrease. This species is widely available in the horticultural trade, therefore education and legislation should be targeted at addressing the threat of intentional introduction.

Physical/Mechanical Control

A number of physical control measures have been used to control L. grandiflora, including mechanical harvesters, rotovators and hand removal, but most of the results have been poor (IPAMS, 2009).

Movement Control

Plants can spread locally as seeds and fragments drift in water currents or are carried by wildlife, but most attention should be given to addressing forms of human-mediated transport.

Biological Control

Sterile grass carp, Ctenopharyngodon idella,has been used to control L. grandiflora (Manuel, 1989). However, grass carp are non-selective herbivores that will almost certainly harm native species. Some study of native biological control measures has revealed promise in using Lysathia beetles to control populations of the plant (McGregor et al., 1996), although no programmes are currently underway (IPAMS, 2009).

Chemical control

Control of L. grandiflora is possible with both spot and broadcast application of several herbicides. Broadleaf-selective auxinic 2,4-D and triclopyr can be used to minimize damage to monocots. Broad-spectrum herbicides glyphosate and imazapyr can also be used, as well as diquat for spot treatment (IPAMS, 2009).

14/12/09 Original text by:

Alison Mikulyuk, Wisconsin Dept of Natural Resources, Science Operations Center, 2801 Progress Rd, Madison, WI 53716, USA