Mist flower

Ageratina riparia syn. Eupatorium riparium
Herb
Opposite
Simple
Cream
Green

Sprawling, low-growing perennial herb, 40-60cm tall.
Stems have roots at joints that touch ground.
Leaves are opposite, generally 7.5cm long and 2.5cm wide, toothed along edges, tapered at each end.
Flowers are small, white.
Branches end with dense heads.
Seeds are black, slender, angular, 2mm long, with fine white hairs at tip.

Common names 
Also known as: cat's paw, catspaw, creeping crofton weed, mistflower, river eupatorium, small crofton weed, white weed,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Late winter to early spring
Native/Exotic 
Exotic
Origin 
Native to Mexico and the Caribbean.
Notifiable 
No
State declaration 
Nil
Council declaration 
Class R – Reduce populations
Known distribution 

Widely naturalised in eastern Australia (i.e. in the coastal districts of New South Wales and eastern Queensland). It is most abundant in the forested coastal regions near the Queensland/New South Wales border, but is also relatively common in the coastal areas of central New South Wales. It has also been recorded in central and northern Queensland and as far south as Nowra on the New South Wales south coast. Also naturalised on Norfolk Island and possibly sparingly naturalised in south-western Western Australia.

Habitat 

Mistflower (Ageratina riparia) is generally found growing in wetter (i.e. humid) sub-tropical and tropical regions. Within such regions it is a weed of damp areas such as gullies, creek banks, waterways, roadsides, disturbed sites, pastures, native bushland, rainforest clearings and the margins of closed forests.

Habit 

A long-lived (i.e. perennial) spreading herbaceous plant or low-growing shrub. It usually only grows 40-60 cm tall, but can occasionally reach up to 1 m in height.

Impact and control methods 

Mistflower (Ageratina riparia) is a significant environmental weed in New South Wales and Queensland. It is also currently listed as a priority environmental weed in at least one Natural Resource Management region in Australia. This species can quickly invade disturbed bushland on frost-free slopes and rainforest gaps, displacing the native vegetation. However, it is especially invasive in gullies and riparian habitats and is often reported to dominate the groundcover vegetation along waterways. It is shade tolerant and can almost totally exclude native plants in these areas, an may even displace many of the animals which were reliant upon those plants.

Mistflower is a very aggressive weed in south-eastern Queensland, and is listed among the top 25 most invasive weed species in this region. It prefers wetter habitats in coastal districts. It is thought to have a negative impact on the habitats occupied by several species of rare stream frogs in south-eastern Queensland. It is thought to have reduced the number of suitable egg-laying sites available to Fleay's barred-frog (Mixophyes fleayi). It also threatens the habitat of the  southern gastric-brooding frog (Rheobatrachus silus), which is now thought to be extinct. This species, which is restricted to the Blackall and Conondale Ranges in south-eastern Queensland, has not been sighted since 1981.

Misflower (Ageratina riparia) can spread into wetter pastures, significantly reducing their carrying capacity. It may also restrict the movement of livestock and farm machinery.

Stem and leaves 

The numerous stems are relatively weak and the lower branches readily produce roots at their joints (i.e. nodes) if they come into contact with the soil. Branches tend to grow obliquely at first, then upwards (i.e. they are decumbent or ascending). These stems are often reddish or purplish in colour and have a sparse covering of fine hairs (i.e. they are sparsely pubescent).

The leaves are oppositely arranged along the stems and borne on short stalks (i.e. petioles) 5-15 mm long. They are relatively narrow (i.e. narrowly-ovate or lanceolate) and prominently veined. These leaves (3-11 cm long and 8-30 mm wide) are mostly hairless (i.e. glabrous) and taper to a point at their tips (i.e. they have acute apices). Their margins are sharply toothed (i.e. serrated), especially toward their tips.

Flowers and fruits 

The small white flower-heads (i.e. capitula) do not have any obvious 'petals' (i.e. ray florets) and consist of several tiny flowers (i.e. tubular florets) surrounded by two rows of greenish bracts (i.e. an involucre) about 4-5 mm long. The tiny tubular florets are white in colour and also about 5 mm long. Several of these flower-heads are clustered together at the tips of the branches (i.e. into terminal corymbose inflorescences). Flowering occurs from late winter through to late spring, but is most abundant during mid-spring.

The small 'seeds' (i.e. achenes) are slender, blackish-brown in colour and slightly curved. These 'seeds' (1-2 mm long) have four or five hairy ridges which run lengthwise (i.e. longitudinally) and they are topped with a ring (i.e. pappus) of several larger whitish coloured hairs (3-4 mm long).

Reproduction and dispersal 

This species reproduces by seeds, which are easily dispersed by wind and readily float on water. They may also become attached to animals, clothing, vehicles or machinery and can be spread in contaminated agricultural produce.

Similar species 

Mistflower (Ageratina riparia) is very similar to crofton weed (Ageratina adenophora) and relatively similar to Siam weed (Chromolaena odorata) and Senegal tea plant (Gymnocoronis spilanthoides). These species can be distinguished by the following differences:

mistflower (Ageratina riparia) is a creeping (i.e. decumbent) or scrambling plant 0.4-0.6 m tall with relatively narrow or elongated (i.e. lanceolate) leaves and young stems that are sparsely covered in fine hairs. Its 'seeds' (i.e. achenes) are tiny (1-2 mm long), have roughly hairy edges, and are topped with a ring (pappus) of whitish hairs (3-4 mm long).

crofton weed (Ageratina adenophora) is an upright (i.e. erect) plant 1-2 m tall with relatively broad, diamond-shaped (i.e. rhomboid) or almost triangular, leaves and young stems that are densely covered in sticky (i.e. glandular) hairs. Its 'seeds' (i.e. achenes) are tiny (1-2 mm long), have hairless edges, and are topped with a ring (i.e. pappus) of whitish hairs (3-4 mm long).

Siam weed (Chromolaena odorata) is a large upright (i.e. erect) shrubby plant 1.5-5 m tall with relatively broad, egg-shaped (i.e. ovate) or triangular, leaves and young stems that are sparsely covered in fine hairs. Its 'seeds' (i.e. achenes) are relatively large (4-5 mm long), and are topped with a ring (i.e. pappus) of whitish or brownish hairs (about 5 mm long).

Senegal tea plant (Gymnocoronis spilanthoides) is a semi-aquatic plant less than 1 m tall with somewhat hollow stems and relatively narrow, narrowly egg-shaped (i.e. ovate) or lance-shaped (i.e. lanceolate), leaves. Its 'seeds' (i.e. achenes) are relatively large (about 5 mm long), and are not topped with a ring (i.e. pappus) of hairs.
Blue billygoat weed (Ageratum houstonianum), billygoat weed (Ageratum conyzoides subsp. conyzoides) and praxelis (Praxelis clematidea) are slightly similar when in the vegetative stages of growth, but usually have hairy leaves. They can be easily distinguished by their bluish or pinkish-coloured flower-heads.