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Scientific Name
Synonyms
Family
Common Names
Origin
Cultivation
Naturalised Distribution
Habitat
Habit
Distinguishing Features
Stems and Leaves
Flowers and Fruit
Reproduction and Dispersal
Environmental Impact
Legislation
Management
Similar Species
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Click on images to enlarge

infestation (Photo: Sheldon Navie)

infestation (Photo: Sheldon Navie)

habit (Photo: Sheldon Navie)

habit (Photo: Sheldon Navie)

paired leaves with toothed margins (Photo: Sheldon Navie)

elongated flower clusters (Photo: Sheldon Navie)

flowers (Photo: Sheldon Navie)

close-up of flowers (Photo: Sheldon Navie)

young seedlings (Photo: Sheldon Navie)

seedling (Photo: Sheldon Navie)

young plants (Photo: Sheldon Navie)

Stachytarpheta cayennensis

Scientific Name

Stachytarpheta cayennensis (Rich.) Vahl

Synonyms

Verbena cayennensis Rich.
Stachytarpheta dichotoma (Ruiz Lopez & Pavon) Vahl (misapplied)
Stachytarpheta urticaefolia (Salisb.) Sims
Stachytarpheta urticifolia (Salisb.) Sims

Family

Verbenaceae

Common Names

blue porter weed, blue porterweed, blue rat's tail, blue snake-weed, blue snakeweed, bluetop, Cayenne snakeweed, dark blue snake weed, dark blue snake-weed, dark blue snakeweed, false verbena, nettleleaf velvetberry, nettleleaf vervain, rattail, rough leaved false vervain, rough-leaf false vervain, rough-leaved false vervain, snake weed, snake-weed, snakeweed

Origin

Native to Mexico, Central America (i.e. Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama), the Caribbean, tropical South America (i.e. French Guiana, Guyana, Surinam, Venezuela, Brazil, Colombia, Ecuador, Peru and Argentina) and the Galapagos Islands.

Cultivation

Dark blue snakeweed (Stachytarpheta cayennensis) is often cultivated as a garden ornamental, particularly in the warmer parts of Australia.

Naturalised Distribution

This species is becoming widely naturalised in the northern and eastern parts of Australia. It is most common in the coastal districts of the Northern Territory and northern Queensland. Also present in the coastal districts of central and southern Queensland, and in the northern parts of Western Australia.

Also naturalised in tropical Africa, Madagascar, tropical Asia (i.e. the Philippines, Malaysia, Taiwan and Papua New Guinea), the Mascarenes (i.e. the Seychelles and La Réunion), south-eastern USA (i.e. Florida) and Oceania (i.e. the Cook Islands, Fiji, French Polynesia, Guam, Kiribati, the Marshall Islands, Nauru, Niue, New Caledonia, Palau, the Solomon Islands, Western Samoa, Tonga and Hawaii).

Habitat

A weed of forests and forest margins, native bushland, roadsides, disturbed sites, waste areas, waterways, gardens, plantation crops and pastures in tropical and sub-tropical regions.

Habit

A small, long-lived (i.e. perennial), shrub usually growing 1-1.5 m tall, but occasionally reaching up to 2.5 m in height.

Distinguishing Features

Stems and Leaves

Younger stems are greenish or purple-tinged, mostly hairless (i.e. glabrous) except for a few hairs near the joints (i.e. nodes), and somewhat square in cross-section (i.e. quadrangular). These branched stems (up to 6 mm thick) tend to become rounded, light brown in colour, and somewhat woody as they mature.

The oppositely arranged bright green leaves (1.5-10 cm long and 1-5 cm wide) are borne on stalks (i.e. petioles) 5-30 mm long. They are somewhat glossy in appearance and either egg-shaped in outline (i.e. ovate) or oval (i.e. elliptic) in shape with pointed tips (i.e. acute apices). These leaves are hairless (i.e. glabrous) or have a few hairs along the veins on their undersides (i.e. sparsely strigose). Because they have sharply toothed (i.e. serrate) margins and a quite wrinkled (i.e. rugose) surface texture, they resemble "nettle" leaves.

Flowers and Fruit

Numerous small flowers are arranged on long, curved, slender spikes (10-45 cm long and 1-3 mm thick) at the top of the branches (i.e. in terminal spikes). These flowers are stalkless (i.e. sessile) and are usually dark blue, purple, or violet in colour. They are tubular in shape (6-8 mm long), with a slender tube and five broad petal lobes (about 5 mm across). Each flower also have five sepals (4-7 mm long), that are fused together for most of their length (i.e. into a calyx tube), and two fully developed stamens. Only a small number of these flowers are open at any one time, and each is subtended by a persistent, small, green bract (3-7 mm long and 1-2 mm wide) with a pointed tip (i.e. acuminate apex). Flowering occurs throughout the year, but is most abundant during spring, summer and autumn.

The small oblong fruit (3-6 mm long and 1.5-2 mm wide) are somewhat flattened (i.e. compressed) and hairless (i.e. glabrous). These fruit (i.e. schizocarps) turn dark brown to black in colour and split into two one-seeded structures (i.e. mericarps) at maturity.

Reproduction and Dispersal

Reproduction is almost entirely by seed.

These seeds are most commonly spread in dumped garden waste, soil, and contaminated agricultural produce. They may also become attached to animals, clothing, vehicles and machinery.

Environmental Impact

Dark blue snakeweed (Stachytarpheta cayennensis) is regarded as an environmental weed in the Northern Territory, Queensland and Western Australia.

Legislation

This species is declared under legislation in the following states and territories:

Management

For information on the management of this species see the following resources:

Similar Species

Dark blue snakeweed (Stachytarpheta cayennensis) is very similar to light blue snakeweed (Stachytarpheta jamaicensis), white snakeweed (Stachytarpheta australis) and pink snakeweed (Stachytarpheta mutabilis). These species can be distinguished by the following differences:

Dark blue snakeweed (Stachytarpheta cayennensis) also hybridises with other species, and hybrids between it and both light blue snakeweed (Stachytarpheta jamaicensis) and pink snakeweed (Stachytarpheta mutabilis) have also become naturalised in Queensland. These hybrids exhibit characters that are intermediate between the two species involved.

It is also relatively similar to the common verbenas (Verbena litoralis and Verbena officinalis). However, the common verbenas (Verbena litoralis and Verbena officinalis) can be distinguished from dark blue snakeweed (Stachytarpheta cayennensis) by having four fully developed stamens (instead of two) and smaller flowers that are less than 4 mm across.