Watsonia (Watsonia meriana)

(c) Forest and Kim Starr

Family: Iridaceae

Growth Habit: Long lived herbaceous plant with very large strap like leaves, up to 110 cm long and 1-5 cm wide, with entire margins and pointed tips. Most leaves arise from the base of the plant, with a few smaller leaves alternatively arranged along the stems. The tall flower stems are upright, founded, often reddish in colour and only rarely branched near their tips. 

Type of Plant: Vigorous herb to 2 m.

(c) Forest and Kim Starr

Flowers: Flowers are numerous (10-15 on each spike), orange, red or salmon pink tubular, (5-8 cm long and 3-4 cm across), slightly curved, and have six partially fused ‘petals’. They are stalkless (i.e. sessile), subtended by small leafy bracts, and are quite widely spaced along the spike (about 3-4 cm apart). Each flower also has three stamens, with purplish anthers (about 10 mm long). Flowering occurs mainly during spring and early summer.

Fruits/Seeds: Fruit (i.e. capsules) and seeds are generally not produced in Australia. Instead, clusters of small reproductive structures (known as cormils or bulbils) are produced at the upper stem joints (i.e. nodes). These ‘bulbils’ are reddish-brown or brown in colour (10-25 mm long and 5-7 mm across), shiny in appearance, and have a short curved beak. Four to sixteen of these ‘bulbils’ are produced in each cluster.

Dispersal: This species reproduces vegetatively via underground corms and smaller ‘bulbs’ (i.e. bulbils or cormils) on the stems. The underground corms (4-8 cm across) sprout 1-3 new smaller ‘bulbs’ during each season, which readily become detached from the parent corm. Seeds are not produced in Australia.

Corms may be dispersed during soil moving activities (e.g. road grading), by water, and in dumped garden waste. The bulbils are also spread in dumped garden waste, by water, and by slashers and other vehicles.

Distribution: Watsonia is a highly invasive species found in a variety of habitats including coastal and grassy woodlands, heathlands, forests and riparian habitats. Prefers wet areas and is tolerant of waterlogging. Weed of roadsides, railways, drains, streams, waterways, ungrazed pastures, recreational and industrial areas, wetlands and disturbed areas.

Status: Undeclared in Tasmania but is a significant Environmental Weed.

Weed Impact:

  • Mature plants are relatively unpalatable but young grown may be grazed.
  • Produces pure stands in suitable areas, excluding almost all other vegetation, reducing habitat and diversity.
  • Can take over garden areas.

Control Methods:

  • Do not plant Watsonia, and replace those in your garden with other species such as Dianella or Diplarreana species. 
  • Do not dump garden waste or soil in bushland areas.
  • Manual Removal. Dig out in moist soil, ensuring all the corms are removed. Bag all material and dispose of appropriately.
  • Mowing and slashing are ineffective unless repeated very regularly. Slashing at 100 mm or closer to the ground when stems emerge and well before flowering can reduce stem and cormel formation and weaken the corm.
  • Grazing provides effective control but will not eradicate the weed.
  • Cultivation to 100 mm provides good control if done after the old corm is exhausted and before the new corms form or before the flower stem emerges. Establishment of suitable pasture species will prevent reinfestation.
  • Germinated corms and cormels can be controlled with herbicides. 
  • Herbicide Control. Use herbicide when leaves are green (spring/summer). Corms often resprout after spraying, follow-up is likely. Wipe individual leaves with glyphosate 10% or spray dense infestations 2,2-DPA 10 g/L + Pulse®. Apply just as flower spikes emerge at corm exhaustion. 2,2-DPA at 5 g/L+ Pulse®

N.B. Always check the herbicide label before use.