Identification And Control Methods Of Gorse In Tasmania

Native to Western Europe, gorse was introduced to Australia in the early 1800s for its ornamental and hedge plant values. It is now found in almost all states and grows in a wide range of soils.

In Tasmania, gorse is widespread and negatively impacts the ecological and agricultural values of the state. The importance in controlling the spread of this weed of national significance (WoNS) cannot be understated and requires a joint effort from private landholders, land care organisations and local government.

This article will focus on gorse identification features, environmental impact of the weed, and various methods of controlling the spread of the weed.

Gorse Identification

Gorse is a robust, prickly perennial shrub in the family Fabaceae, and can reach a height of over two metres.

Flowering is present throughout the year, and peak flowering season is late winter into spring. Petals are a highly recognisable bright yellow and the pea-flowers emit a coconut scent. Leaves resemble spines and vary in colour from grey-green when young to dark green when mature.

Gorse seed is classified as a legume or pod, is purplish-brown in colour and covered in fine hairs. Each pod contains either two or three black seeds which the pod ejects in hot weather or heat brought on by fire. Once ejected, seeds remain viable for up to 25 years. Germination of seeds is often brought on by fires and mature plants are known to regrow from the roots after being completely scorched.

Seed is spread not only by ejection from the pod but also by livestock, birds, water, vehicle tyres and the transportation of soil.

The lifespan of a Gorse plant is around thirty years, and the hardy nature of the plant means it can survive in extreme weather conditions. This is especially true in Tasmania where the climate is similar to parts of Britain and Western Europe.

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Given that Gorse plants generally always have some flowers on them, identification of the weed is easily possible. There are, however, some native species with similar features. Golden Shaggy Pea (Oxylobium ellipticum), Prickly Beauty (Pultenaea juniperina) and Golden Peas (Aotus ericoides) are all native shrubs of similar height and have yellow pea flowers. All of these species have some orange or red colouring in the centre of the flower that distinguishes them from Gorse and lack the coconut scent of Gorse flowers. When young, several Acacia species such as Prickly mimosa (Acacia verticillata) also resemble Gorse, this is due to the spiny leaves and absence of flowers due to age.

Impacts Of Gorse

Fire Hazard

Tasmania is an area naturally prone to bushfires and both native bushland and farmland are at risk. Gorse, when alive, is highly flammable and dense infestations of the weed increase the chance of fires, particularly when near buildings and infrastructure. In natural areas, gorse can alter the manner and frequency of bushfires, which can be detrimental to native flora.

Native vegetation

Gorse infestations reduce floral diversity of native species due to outcompeting or smothering seedlings, preventing species regeneration. If not controlled within a native bushland area, gorse could completely exclude any native groundcover and small shrubs, as well as the seedlings of canopy species. Several native Tasmanian plants are at risk due to gorse competition, including South Esk Pine (Callitris oblonga), Turnbridge Leaf Orchid (Prasophyllum tunbridgense) and Midlands Wattle (Acacia axillaris).

Agriculture

Invading Gorse has the ability to reduce the carrying capacity of both plant crops and grazing pastures. When infestations increase in density, livestock movements and access are often restricted and new crop growth is excluded. This equates to high control costs to landholders or land managers when undertaking measures to control the spread of the weed.

Roadsides, recreation and aesthetic values

When Gorse invades roadsides, it competes with, and excludes, native roadside vegetation and reduces driver visibility due to its dense growth. It is also a fire hazard and could act as a point of ignition.

Gorse restricts access to land and impacts recreational value of an area. Dense infestations are seen as unsightly and negatively affect the aesthetic value of a region, which could impact the region’s tourism and economy.

Gorse Control Methods

The methods used depend on the long-term objectives for the infested area. Most often a combination of methods will be required for best results. Like other invasive weeds, the sooner action is taken, the higher the chance of successful control. This section will introduce several Gorse control methods.

Mechanical removal

Mechanical clearing broadly encapsulates methods such as mulching, grubbing and root raking. Regarded as an effective primary control against gorse, these methods and are often combined with other techniques for best long-term results.

• Mulching is usually performed with an excavated-mounted mulcher and involves cutting off the plant at ground level which then get processed into a mulch. The mulch then acts a seedling suppressant and helps reduce further infestations.
• Grubbing is performed with an excavator, the gorse is broken off at the base, as close to the soil level as possible. Effective on hard, mature plants.
• Root raking is using excavator with a root rake fitted is used to pull bushes out of the ground, including large root systems.

Herbicide spraying

Several registered chemicals have proven efficacy in gorse control, including Grazon^® Extra and Garlon^® 600. These can either be sprayed directly to the leaves of the plant or applied to stumps shortly after cutting.

• Direct spraying of large infestations is an option for when the goal is to stop any further infestation while leaving the treated plants to die in situ. This often occurs when land managers want the dead gorse to remain, either as a visibility screen or as potential habitat for native birds and mammals such as Wombats and Bandicoots. This method does not restrict the possibility of later mechanical removal if dead plants are no longer desired. Considerations when spraying herbicide include factors such as bee activity, chance of off-target damage, wind speed and direction, and locations of nearby waterways.

Cut-stump treatment

• This method involves cutting the plant at the base and painting the stumps with herbicide in order to prevent further growth. Cutting is done either mechanically or with hand tools. Hand tools are generally used for small infestations or where access with machinery is not possible. It is also useful in areas of high biodiversity value as there is less soil disturbance and off target damage as line spraying and mechanical removal.
• Cut-stump method can also be applied in conjunction with mechanical control. An example would be painting leftover stumps immediately after the use of a forestry mulcher.

‍Revegetation Works

An important aspect in follow-up control of gorse is reducing the seed bank within the soil. The implementation of a revegetation program is an excellent example of follow-up control. The planting or direct seeding of native suppress will aid in suppressing any new gorse seedlings and will improve the biodiversity and aesthetic values of an area.

Grazing

The only livestock animals known to graze on gorse are goats and sheep. Goats are known to graze on seedlings as well as harder mature trees, while sheep generally only graze on younger, softer plants.

Grazing does help with controlling the spread of gorse but is only effective as long as the animals are present. The gorse would likely grow back with greater intensity after being grazed. Best implemented alongside mechanical methods such as mulching, whereby the livestock graze on any new seedlings. This can also occur if the gorse has been burnt and the livestock feed on sprouting seedlings and soft new growth.

Burning

Fire should not be used as a control method on healthy, dense gorse infestations as gorse is known to regrow after being burnt and fire promotes seed germination.

Burning could be implemented on smaller, manageable infestations, where secondary methods such as grazing and herbicide use are used to control new seedlings.

Biological controls

This involves using biological agents which would naturally affect the health of a weed. Biological controls are generally slow acting and do not eradicate weeds entirely. Research is ongoing into several potential biological controls which may be effective against gorse in Tasmania, these include:

• The gorse spider mite (Tetranychus linearius) form large colonies on gorse plants and feed on the leaves of mature plants. There has been some success introducing this species in parts of Tasmania.
• Gorse seed weevil (Exapion ulicis) has also been introduced to some parts of New Zealand and Tasmania. The larvae of the weevil attacks gorse seeds, but only in warmer months which has limited the success rate on controlling gorse.

Closing Advice To Landholders And Land Managers

As gorse is declared a Weed of National Significance, landholders have a legal obligation to reduce the spread of the weed where possible.

An integrated gorse control plan should seek to target the original infestation, with regular follow-up methods aimed at reducing the seed bank within the area. Good practice when undertaking a dense infestation is to start with the least affected areas and methodically work towards the larger gorse patches. When any new growth or seedlings are observed, action should be taken to reduce the risk of the weed spreading.

While dense infestations and regrowth can appear overwhelming, control methods are available and, when utilised correctly, reduction and long-term eradication of gorse in an area is feasible.

How Can We Help?

Tasmanian Tree Care takes a holistic approach to land care and weed management. Whatever you have planned for your site, we have the tools and knowhow to boost the value of your land while preserving the natural environment. Read more about our land management service here, or contact us for any questions you may have.