Identify a Weed - Identifying Weeds


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Australian Swamp Stonecrop Buddleia Creeping Water Primrose Dandelion Devils Apple Dwarf Knotweed

Floating Pennywort Giant Hogweed Giant Knotweed Goldenrod Himalayan Balsam Horse Tail Hybrid Knotweed

Ragwort Rhododendron Rosebay Willowherb Tree of Heaven Water Fern Japanese Knotweed Parrots Feather

Creeping Water Primrose - Weed Killer

Creeping Water Primrose:

Species
Ludwigia hexapetala (Closely similar species, Ludwigia grandiflora) Either species possibly formerly called Ludwigia uruguayensis

Family
Onagraceae

Common Names
Creeping Water Primrose, Water Primrose, Hairy Water Primrose, False Loosestrife, Uruguayan Primrose-Willow

Introductory History
Creeping Water Primrose is the most common name for the species Ludwigia hexapctala. True Ludwigia hexapetala is chromosomally diploid. It is often stated that an earlier name was Ludwigia uruguayensis, though uruguayensis may in fact be a closely related and virtually indistinguishable species, Ludwigia grandiflora. They differ, however, in the fact that grandiflora is hexaploid. The sporadic use, by the nursery trade, of the genus jussiaea to describe these species, coupled with any of the above epithets, further confuses the taxonomy.

It is of importance to correctly identify UK outbreaks, so that factors such as likely hardiness, and potential areas suitable for invasion, can be assessed. True Ludwigia hexapetala has much more frost tolerance than Ludwigia grandiflora.

Native to South and Central America, Water Primrose was introduced to the USA. Europe and the UK as an ornamental and water garden plant, because of the showy yellow flowers it abundantly produces. The exact date of introduction to the UK is uncertain, though records of South American representatives of the genus being grown here pre-date 1895. It is a perennial plant which can exist as a floating aquatic, forming dense rafts of vegetation. From stems of indefinite length, which root at every node into the water, emergent stems rise to 1.5 metres in height. These stems bear axillary flowers which eventually give rise to seed pods bearing large numbers of seed. Reproduction is by seed and, as so often, by fragmentation.

Plants may also root in marginal mud, and frequently become terrestrial, should water levels fall. Terrestrial colonies are dense, woody and persistent.

The affected region in the USA is from New York to Florida in the east; spreading to Texas westwards and along the west coast. Here it causes the familiar damages of crowding out and shading native plants, degrading water-quality, altering the food web and habitat for animals, blocking public water and irrigation inlets and being a hazard to navigation. In malarial mosquito areas, it supplies ideal breeding habitat for the larvae.

Problems are also widespread in the south of France, and it is known to be naturalised at two sites in the UK. One is in the New Forest and the other in a Greater London wetland location.*

*Personal communication with Dr Jonathan Newman, Head of the Aquatic Plant Management Group, Centre for Ecology and Hydrology.

Description
A perennial herb, sometimes subshrub, rooting decumbently in sediment or soil, or floating on water. Where plants form particularly dense colonies or are rooted in mud, white spongy aerenchymous roots or pneumatophores are formed. Roots, wiry and much branched, giving a feathery appearance where dangling in water, or white and spongy as described above. Stems, simple or branched, floating or rooted in sediment; often becoming densely hairy where emergent. Leaves, alternate along the stems, in rosette like clusters where floating. On emergent stems becoming narrowly elliptic to lanceolate. Each leaf suborbicular to spathulate, 3-13 x 0.3-2.5cm. Flowers, bright yellow; sepals 5 or 6. 6-20 x 1.5-3mm, hairy. Petals, 5 or 6, obovate, yellow, 12-30 x 9-15mm. Stamens, 10, in unequal sets; anthers l.5-4mm. Fruit, a light brown, hairy, terete capsule, 13-25mm with 10 conspicuous darker ribs; re flexed with a l0-60mm pedicel. Seed, yellowish 1.2-1.5mm, embedded in the inner capsule walls.

Biology
Early in its growing season, Water Primrose produces light green floating stems furnished with rosettes of somewhat shiny, rounded leaves. As stems emerge from the water, to produce the flowering growths, they become considerably hairy, very woody and reddish brown. A tendency to longitudinal splitting of the stems has been observed. Flowers, which have hairy pedicels and sepals, appear in the upper leaf axils of the emergent stems and are then produced for a long period. When the bright yellow petals have dropped, the large sepals persist. Small yellowish seeds ripen in the seed capsules, which split and release seeds for much of the summer.

Scope
Creeping Water Primrose has a serious potential as an invasive weed in the United Kingdom, every step should be taken to eradicate naturalised colonies at this early stage. Confusions over taxonomy should be sorted out, and the plant, amongst others which become harmful and invasive when naturalised, removed from commerce.

Control Methods
Mechanical methods such as cutting, dredging or hand pulling have been attempted, but are often difficult or costly. They also carry the risk of fragmentation of the plants which can then float away to establish new infestations. The application of shading materials has also been found effective.

Spraying emerged or terrestrial stems during the growing season with formulations containing Glyphosate seems to offer the greatest promise at the moment.

No insect pests or pathogens offering promise as biological control agents have been identified, and the weed is unpalatable to Grass Carp. back to top

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