Weeds and biosecurity

Weed statistics

In the formal lists of plants that occur in Victoria, plants originating elsewhere, but established and reproducing without human assistance, are called naturalised plants. The term 'naturalised plant' is more or less synonymous with 'weed', although the latter carries the implication of some detrimental effect.

Naturalised plants make up 15 per cent of the total Australian flora and 30 per cent of the Victorian flora. More than 65 per cent of Victorian land space is covered by exotic vegetation. The area occupied by naturalised species in Victoria increases annually, and records of newly established species continually accumulate. About a dozen new weeds establish in Australia each year.

Horticulture as a source of weeds

The vast majority of Australia's environmental and economic weeds are plants that were deliberately introduced for ornamental horticulture that have escaped from cultivation into the natural environment and agricultural land. Some 60–70 per cent of the weeds in Australia have escaped from gardens. Among Australia's 20 most damaging Weeds of National Significance, 14 are garden escapes. About 40 per cent of Australia's current declared weeds are invasive garden plants. It seems inevitable that many weeds of the future are already present in Australia, growing in our gardens.

Documenting and identifying weeds in Victoria

The documentation of new weeds is dependent on a thorough understanding of both the native and naturalised flora that occur in the state. We can't know if a plant is new to the state if we don't know what plants are already present.

An important resource produced by Royal Botanic Gardens (RBG) Melbourne is A Census of the vascular plants of Victoria. The Census, now in its eighth edition, lists the scientific names of all vascular plants known to occur in the state. All names listed in the Census are substantiated by herbarium voucher specimens. Most vouchers are lodged in the National Herbarium of Victoria (MEL) at RBG Melbourne, although some are lodged in other recognised herbaria. Documenting the occurrence of new weeds in publications such as the Census plays a pivotal role in early intervention strategies and weed alert procedures.

It is important that the occurrence of weeds is supported by voucher specimens lodged in herbaria. Many people do not submit specimens to a herbarium for identification because they assume that they know the identity of the plant they are dealing with (Hosking et al. 1996). Failing to correctly identify a specimen can result in the inadvertent dispersal and establishment of weedy species, and delay their eradication or control.

Detecting new weeds

There are formal processes for early detection and notification of new weed naturalisations, such as the Department of Primary Industries (DPI) Victoria's Weed Alert program. It is important to appreciate that in such programs, no control or eradication procedures can be initiated before an authoritative identification has been made by a taxonomic botanist. Identification is one of the pivotal roles that RBG Melbourne botanists play in early detection and notification protocols.

One of the chief means by which botanists at RBG Melbourne detect new weeds is by targeting areas that have proven likely to contain previously unrecorded weeds. Hosking et al. (2004) argue that an increased detection effort should initially target areas around population centres because most plants first naturalise in urban bushland and areas with a high concentration of gardens. Many new incursions originate from discarded garden refuse, or from propagules dispersed from gardens to native bushland by birds. In Victoria, two such areas that have yielded many new weeds over the last ten years are the Dandenong Ranges and the Mount Macedon area.

Revegetation areas, particularly roadside plantations and soil conservation schemes, are another rich source of new weeds. Many of the taxa that naturalise at these sites are Australian natives. At Anglesea in Victoria 36 of a total of 45 species planted in two revegetation areas were present as escapes in adjacent heathland and heathy woodland.

Several new weed taxa recently recorded for the first time in Australia were found at alpine ski resorts in Victoria. These include Hieracium praealtum subsp. bauhinii at Falls Creek, and Juncus ensifolius, which is common in parts of Baw Baw Village.

Regional areas of long-established agriculture are also being targeted in the search for new weeds. Such sites are subject to significant movement of stock, stock feed and agricultural machinery from around the state and throughout Australia, which may act as vectors for new weeds.

Royal Botanic Gardens Melbourne botanists also work with DPI staff in conducting targeted searches for potentially invasive species. Although a look-out is kept for weeds in various high risk areas, there are currently no systematic and regular surveys for weeds in Victoria. Unless we collect, document and positively identify new naturalisations, those responsible for managing the impact of weeds will be unaware of potential new problems in their jurisdictions.

Australian Botanic Gardens Weed Risk Assessment Procedure

In 2004 the Council of Heads of Australian Botanic Gardens supported a proposal for the formation of an Australian Botanic Gardens Weed Network. This collaborative team, with the assistance of the Australian Weed Management Cooperative Research Centre, developed common policies and procedures, and a weed risk assessment methodology, for Australian botanic and regional gardens. One major outcome of the network was the completion of the Weed Risk Assessment Procedure and Management Software (WRAPM). This software enables a scored weed risk analysis that could be used to guide weed management decisions at any site in Australia. The WRAPM software can be downloaded from the Botanic Gardens Australia and New Zealand (BGANZ) website.

Royal Botanic Gardens Melbourne weed resources and strategic plan

Royal Botanic Gardens Melbourne has prepared a Weed Resource and Information Pack (MS Word - 1.16 MB). This document outlines the effects of garden plants on natural and cultivated landscapes, with a focus on Victoria. The various types of weeds, such as garden weeds, urban weeds, sleeper weeds and declared weeds, are discussed. Methods used for weed control and management are detailed, across different sectors, such as the nursery industry, botanic gardens, local government and community groups.

Weeds are an issue for Botanic Gardens in relation to the choice of plants under cultivation, especially given that 70 per cent of weeds were originally imported for ornamental horticulture. The 12 major Botanic Gardens in Australia together grow about 40,500 different kinds of plants. This figure is similar to the numbers of commercially available plants, with about 45,000 kinds being available for ornamental horticulture. In comparison, the total of native plants recorded for Australia is about 19,000.

There is a clear need for Botanic Gardens to consider weed management strategies and the risks of plants 'escaping' to damage the natural environment and primary production. Royal Botanic Gardens Melbourne has developed a Weed Strategic Plan to link education opportunities, management requirements and current scientific knowledge to minimise infestations and risk of weed spread both within and outside its managed lands.

Weed Alert program

Since 2002, RBG Melbourne has collaborated with the DPI in the Weed Alert program. The chief aims of the plan are to detect new incursions of new weeds before they become naturalised, and to instigate rapid notification of DPI staff in order to enable early remedial or eradication strategies. One of the potential benefits of this partnership for RBG Melbourne is the receipt of herbarium voucher specimens to substantiate new weed records. Royal Botanic Gardens Melbourne botanists also provide expert advice and warning of potential new weeds by helping compile 'target lists' of taxa that are either known to be serious environmental and economic weeds, but have not yet been recorded in Australia, or that have been recorded as weeds in other States, but, in Victoria, are currently known only from limited populations. Target lists are used to promote awareness of potential threats, and to focus survey efforts (Waterhouse 2003).

References and further information

Carr, G.W. (1993). Exotic flora of Victoria and its impact on indigenous biota. In D.B. Foreman and N.G. Walsh (eds), Flora of Victoria, vol. 1, Introduction, pp. 256–297. Inkata Press, Melbourne.

Carr, G.W., Yugovic, J.V. and Robinson, K.E. (1992). Environmental weed invasions in Victoria: conservation and management implications. Department of Conservation and Environment Ecological Horticulture, East Melbourne.

Chapman, A.D. (2009). Numbers of living species in Australia and the world, 2nd edn. A report for the Australian Biological Resources Study. 

Groves, R.H., Boden, R. and Lonsdale, W.M. (2005). Jumping the garden fence. Invasive garden plants in Australia and their environmental and agricultural impacts. CSIRO report for WWF- Australia.

Hosking, J., Smith, J. and Sheppard, A.W. (1996). Biology of Australian weeds: Cytisus scoparius L., Scotch Broom. Plant Protection Quarterly 11, 102–108.

Hosking, J.R., Waterhouse, B.M. and Williams, P.A. (2004). Are we doing enough about early detection of weed species naturalising in Australia? In B.M. Sindel and S.B. Johnson (eds), 14th Australian Weeds Conference Proceedings: weed management – balancing people, planet, profit, 6–10 Sept 2004, Charles Sturt University, Wagga Wagga, New South Wales, pp. 68-72. Weed Society of NSW, Sydney.

Sinden, J., Jones, R., Hester, S., Odom, D., Kalisch, C., James, R. and Cacho, O. (2004). The economic impact of weeds in Australia. CRC for Australian Weed management Technical Series 8, 1–55. See also http://www.weeds.crc.org.au/

Virtue, J.G., Spencer, R.D., Weiss, J.E. and Reichard, S.E. (2008). Australia's Botanic Gardens weed risk assessment procedure. Plant Protection Quarterly 23, 166–178.

Waterhouse, B.M (2003). Know your enemy: recent records of potentially serious weeds in northern Australia, Papua New Guinea and Papua (Indonesia). Telopea 10, 483.