Bulbil watsonia

Bulbil watsonia is an aggressive bulb weed of native vegetation and invades idle pastures and roadsides.

Description

Bulbil watsonia is an erect perennial (occurring year to year), long-lived herb that produces dense stands of light green rigid, flat, sword-like leaves up to 1 m long during winter.

Its leaves are produced from a central corm (underground storage stem) in winter and die off over summer.

It has spring flowering stalks, which grow to 2 m high, usually unbranched, straight and erect red-brown. Bulbil watsonia has open spikes of 10-15 red to dull pink curved trumpet-shaped flowers which are 5 to 8 cm long.

No viable seed is produced, and instead reproduction is clonal. Cormils (bulbils) are small, shiny red-brown reproductive buds that develop in clusters in leaf axils and floral bracts. They scatter when the leaves die, and each one can form a new plant. The corms from existing plants also divide and can create 2 to 3 new plants each year.

The cormils are short lived in the soil and only a few last more than one season. Once developed though, established plants can be long lived.

Dense infestation of bulbil watsonia
Dense infestation of bulbil watsonia. Photo: Kate Blood

Impacts

Bulbil watsonia is a garden escapee which can establish in native woodlands or along watercourses and form continuous clonal stands that exclude other plants.

This prevents regeneration of native ground plants and degrades habitat for native insects, birds and animals.

Bulbil watsonia also invades unimproved idle pasture in high rainfall and waterlogged areas.

It is suspected of being toxic, but stock avoid mature plants and are apparently unaffected by young shoots. Bulbil watsonia does not tolerate cultivation.

Distribution

Bulbil watsonia is a native of South Africa and was introduced to Australia as an ornamental garden plant.

It is widely naturalised across southern Australia and found predominantly in wetter areas of bushland, roadsides, idle pastures and watercourses.

The Green Adelaide region infestations are confined to the hills areas with higher rainfall, or along watercourses.

It is spread mostly by movement of contaminated soil, however aerial cormils can also be spread by running water.

Bulbil watsonia flower head
Bulbil watsonia flower head. Photo: Monica Seiler

Control

Watsonia meriana var. Bulbillifera (bulbil watsonia) is a declared weed under the Landscape South Australia Act 2019.

In the Green Adelaide region the sale of bulbil watsonia or contaminated goods; and its movement on a public road are prohibited. Land owners must take reasonable steps to control plants on their property and prevent their spread.

Green Adelaide encourages control of plants where there is a risk to biodiversity and water assets.

Control methods

With seed being non-viable and cormils short lived, eradication from an area can usually be achieved in 2-3 years with timely and effective controls.

Hygiene

Restrict movement of any contaminated machinery or soil and decontaminate equipment before leaving a site.

Hand removal

Although control by hand is difficult, it is a suitable method for the odd plant or small patches growing in bushland without the need for herbicide use.

Carefully grub out the plant using a knife or trowel. Make sure the entire root system and all corms are extracted. These can then be burnt or buried at least 1 m deep.

Mechanical control

Slashing is ineffective and may increase spread by dispersing aerial cormils.

Cultivation of soil to 100 mm provides good control if done after the old corm is exhausted, before the new corms form and before the flower stem emerges.

Repeated cultivation is usually needed.

Chemical

Due to the plant’s seasonal growth there is only a small window of opportunity for herbicide control.

Most effective treatments should be applied from early emergence when leaf shoots first appear and well before flower stems appear, as this is the stage of underground corm exhaustion.

Select the appropriate method and herbicide for your situation. What may suit pasture control may not suit control in areas of native vegetation.

Targeted spot spraying using a knapsack sprayer suits small to medium infestations. A boom spray or wick wiper is more efficient for larger paddock infestations.

In bushland, near watercourses or other sensitive areas where there may be a risk to off-target damage to native plants or aquatic species from spraying, apply herbicide to a third or more of the leaves using a sponge, weed tongs or a wick applicator.

Cut and swab

Cut and swab can be used for small numbers of plants as well as in sensitive areas.

Cut the plant as close to the ground as possible using secateurs or loppers. Use a dabber bottle with a registered herbicide, to paint the stump immediately.

Acting quickly will ensure the plant draws the poison into its root system.

For advice on chemical options please refer to Controlling declared weeds in SA at: www.pir.sa.gov.au/biosecurity/weeds.

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