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The identity of the large-flowered Ludwigia grown in British gardens and occasionally naturalised in the wild requires resolution.
This will consist of a combination of morphological, chromosomal and molecular analysis.
Ludwigia is a genus still under revision, and environmental workers across the world have used a diverse range of classifications in dealing with the species in section Oligospermum. Zardini, Gu & Raven (1991) have recognised most of the relevant variants at specific level, partly based on chromosome counts, and it is recommended that this treatment should be used as a basis from which to begin the project.
To protect the natural environment from invasive alien plants, it is essential to have a precise understanding of the organisms involved. The aim of this project is to establish the identity of the species (singular or plural) of non-native Ludwigia growing in British waterways and gardens.
Knowledge of the identity of these species is essential if we are to understand the potential for invasiveness of these plants and to take appropriate control measures. This project will allow advice to be offered to environmental bodies and gardeners from a sound basis of taxonomic knowledge.
Members of the genus Ludwigia (Onagraceae) belonging to the section Oligospermum are attractive aquatic plants native to South America, which are sometimes grown as ornamentals. In several parts of the world they have become established outside their native range and are a particular problem in areas of France. No account of the plants is provided in Stace’s New Flora of the British Isles and no records are given in the New Atlas of the British Flora. However, a paper commissioned by Defra and compiled by the Centre for Ecology and Hydrology (CEH) gives six sites in which non-native Ludwigia is growing wild in Britain.
At least four species – L. grandiflora, L. uruguayensis, L. peploides and L. hexapetala – are implicated in the British records.
The CEH report treats L. grandiflora as synonymous with L. uruguayensis but states that only specimens of L. peploides have been found in Britain and that plants in horticulture have been misnamed. No account is made of L. hexapetala. A press release from Defra in 2007 concerning attempts to eradicate the problem plant in Britain refers to it as L. grandiflora.
Clement (2000) gives the British plants as L. grandiflora (syn. L. uruguayensis) and mentions that L. peploides is naturalised in SW France and New Zealand. L. hexapetala is reported to be probably referable to L. grandiflora. L. adscendens is given as a casual in the Netherlands.
Only Ludwigia uruguayensis is offered in the RHS Plant Finder, but the identity of plants in cultivation is unclear. Plants grown in ponds on the Rock Garden at RHS Garden Wisley since 1994 under the name L. hexapetala were received from Wychwood Waterlily & Carp Farm, who found the plant in their ditch, with source and name unknown.
Similar species from other sections of Ludwigia are invasive weeds in some parts of the world and it is possible that these may have representatives in the British Isles.
Centre for Ecology and Hydrology (2007). Development of eradication strategies for Ludwigia species. Defra Research Project Final Report.
Clement E J (2000). Ludwigia x kentiana E.J. Clement: a new hybrid aquatic. Watsonia 23: 167-172
Nesom G L, Kartesz J T (2000). Observations on the Ludwigia uruguayensis complex (Onagraceae) in the United States. Castanea 65: 123-125.
Raven P M (1964). The Old World species of Ludwigia (including Jussiacea) with a synopsis of the genus (Onagraceae). Reinwardtia 6; 327-427.
Zardini E M, Gu H and Raven P M (1991). On the Separation of Two Species within the Ludwigia uruguayensis Complex (Onagraceae). Systematic Botany 16: 242-244.
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