Ruellia

Ruellia tweediana syn. Ruellia simplex, Ruellia caerulea
Herb
Opposite
Simple
Purple
Blue
Green
Common names 
Also known as: Britton's wild petunia, creeping ruellia, Mexican bluebell, Mexican petunia, wild petunia,
Family 
Acanthaceae
Deciduous 
No
Flowering time 
Warmer months
Native/Exotic 
Exotic
Origin 
Native to Mexico and South America
Notifiable 
No
State declaration 
Nil
Council declaration 
Class R – Reduce populations
Known distribution 

This species has become widely naturalised in the warmer parts of eastern Australia. It is widespread in the coastal districts of Queensland and is also becoming naturalised in the coastal districts of northern New South Wales.

Habitat 

A weed of waterways, riparian vegetation, dams, ponds, wetlands and drainage ditches in sub-tropical and tropical regions.

Impact and control methods 

Mexican petunia (Ruellia simplex) is regarded as an environmental weed in Queensland and is of particular concern in the south-eastern parts of this state, where it was recently ranked among the 200 most invasive plant species. For example, it is listed as a significant non-declared pest plant in Maroochy Shire, an undesirable plant in Caboolture Shire, and a weed of natural bushland and waterways in Gold Coast City.

In the last 20 years, Mexican petunia (Ruellia simplex) has gone from being relatively uncommon to being one of the most common and widespread species in Brisbane's waterways. It often forms dense monocultures in wetter sites, particularly along creekbeds and on creekbanks. Such infestations prevent the natural growth and regeneration of native riparian plants (e.g.  Backhousia myrtifolia, Glochidion ferdinandi, Waterhousea floribunda, Carex spp. and Callistemon spp.), and unlike some other riparian weeds it will grow in shady creeks.

 

Stem and leaves 

The older stems are often slightly woody, while younger stems are green and four-angled (i.e. quadrangular). These youngers stems are hairless (i.e. glabrous) or sparsely hairy (i.e. puberulent), but often have more prominent tufts of hairs at their jointed (i.e. nodes).

The oppositely arranged leaves are borne on short stalks (i.e. petioles) 1-2 cm long. These leaves (6-20 cm long and 4-20 mm wide) are long and narrow in shape (i.e. linear) with entire margins and pointed tips (i.e. acute or acuminate apices). They are mostly hairless (i.e. glabrous) and usually dark green or slightly purplish-tinged.

Flowers and fruits 

The flowers are borne in few-flowered clusters in the upper leaf forks (i.e. in axillary cymes). These flowers are tubular in shape and lavender, blue or purplish in colour. They have five narrow green sepals (5-10 mm long) at the base and five petals (2.5-4 cm long) that are fused for most of their length into a tube (i.e. corolla tube). The petals separate into five spreading lobes with rounded tips (i.e. obtuse apices). Each flower also has four stamens and an ovary topped with a style and stigma.

The club-shaped (i.e. clavate) fruit are capsules that are green or purplish-tinged when young. These fruit (2-2.5 cm long) turn brown in colour and explosively release their seeds when fully mature.

Reproduction and dispersal 

This species seeds profusely and also reproduces vegetatively via creeping underground stems (i.e. rhizomes) and stem segments.

Its seeds are spread short distances when they are explosively released from the mature fruit. They may also be spread in water, externally on animals and in dumped garden waste. Colonies spread laterally via their creeping underground stems (i.e. rhizomes) and stem segments may be dispersed by water during floods and in dumped garden waste.

Similar species 

Mexican petunia (Ruellia simplex) has also become invasive in south-eastern USA, where it has spread rapidly and displaced native flora. It is proving especially problematic in wet areas and can out-compete closely related native species in such environments (e.g. Ruellia caroliniensis). Because of this, it has been listed as a Category 1 invasive pest plant by the Florida Exotic Pest Plant Council.

Mexican petunia (Ruellia simplex) is very similar to minnieroot (Ruellia tuberosa) and relatively similar to creeping ruellia (Ruellia squarrosa). These species can be distinguished by the following differences:

Mexican petunia (Ruellia simplex ) is an upright plant (growing up to 1 m tall) with long and narrow (i.e. linear) leaves that are mostly hairless (i.e. glabrous). Its tubular flowers have five small narrow sepals (5-10 mm long).

minnieroot (Ruellia tuberosa ) is an upright or creeping plant (growing up to 0.5 m tall) with relatively broad (i.e. elliptic to ovate) leaves that are mostly hairless (i.e. glabrous). Its tubular flowers have five large narrow sepals (18-30 mm long).

creeping ruellia (Ruellia squarrosa ) is a creeping plant (less than 0.5 m tall) with slightly elongated (i.e. ovate to narrowly-ovate) leaves that are hairy (i.e. pubescent). Its tubular flowers have five small narrow sepals (10-16 mm long).