By Alina Suchanski

Reading time: 11 minutes 

Conservationists hate it, beekeepers love it and farmers just put up with it. Gorse evokes strong feelings amongst its supporters as well as opponents.

Gorse (Ulex europaeus) first arrived in New Zealand in the early 1800s. Seed was brought in by English settlers to grow plants for hedging and it was recorded by Charles Darwin during his voyage through New Zealand waters in 1835 to be growing in hedges in the Bay of Islands.

This spiny shrub (<2-3 m tall) has woody, erect or spreading stems which are many-branched in younger plants but become bare at the base as the plant gets older. Its leaves are reduced to spines, which are deeply furrowed. Pea-like yellow flowers are followed by hairy seed pods which turn black when mature and explode to release seeds.

 

Gorse quickly established itself and soon spread out of control, taking over vast areas cleared for agricultural use, but Worsley (1999) says that “settlers failed to recognise the threat, and gorse seed continued to be imported and plantings deliberately established into the 1900s”.

Origins and spread

A member of the pea family, gorse is an extremally hardy plant. It tolerates hot to cold temperatures, high to low rainfall, wind, salt damage, grazing, and poor soil. The plant produces massive numbers of long-lived seeds that explode out of their pods, spreading up to 5 m from the parent plant. Seed is also spread by soil movement, road graders, contaminated machinery, animals, boots, and stock food; invading roadsides, waste land, farms, quarries, forest tracks, metal dumps, fire breaks, exotic forests, landslides, and riverbeds.

The seed can lie dormant on the ground for up to 50 years, germinating quickly after the mature plants have been removed. Unfortunately, most methods of removing adult gorse plants, such as burning or bulldozing them, create ideal conditions for gorse seeds to germinate.

In addition, gorse does so well in New Zealand’s mild climate that it flowers both in summer and in winter, producing seed twice a year, unlike in Europe from where it originates.

Gorse is not a fast grower (15-30 cm per year) but will grow to 2.5 m and has been popular for intruder-proof hedging because its spines are so vicious. It’s also very good for windy, exposed, coastal sites and historically was used as a windbreak which could be cut to provide animal feed. Gorse hedges and windbreaks established on the Canterbury Plains since the 1850s have a combined length of 300,000 kilometres.

There is an opinion that the real problem with the plant’s out-of-control spread began during WWI when thousands of Kiwis, many of them farmers, joined the army and were sent to Europe as part of the ANZAC corps. Unattended gorse hedges were allowed to self-seed, with young seedlings ‘marching’ up hills and out to riverbanks, colonising any ground previously cleared for farming. This happened again during WW2, leading to a situation where total eradication of the species became virtually impossible.

After WW2, New Zealand entered a period of vigorous land development with the aim of increasing exports. The campaign against gorse intensified, particularly in North Island hill country.

Today, gorse is one of the most recognised aggressive invasive species in New Zealand.

Opinions about gorse

Despite its ubiquitous presence, the opinions about gorse are divided and the prickly plant has its supporters as well as opponents.

Gorse advocates the claim that as a legume, it fixes nitrogen providing valuable nutrients to other plants, and its dense root structure helps combat erosion. Its enemies point out that overproduction of nitrogen can lead to waterway pollution, and its dense habitat outcompetes native plants and hinders plantation forests.

Beekeepers see gorse as a valuable source of pollen for their bees, especially in early spring when few other natural pollen sources are available. The goat farming industry also perceives gorse as a valuable forage plant for their animals.

A somewhat unexpected endorsement comes from the ‘green’ camp. Many conservationists believe gorse to be a useful nursery for native bush regeneration, as older gorse bushes provide ideal conditions for seeds of native, shade-tolerant species to germinate and grow. Because it needs full light to germinate, gorse cannot regenerate significantly under its own shade. Native species such as fuchsia, wineberry, lemonwood and five-finger, together with taller trees such as totara, matai, kahikatea or beech, grow up through the ageing gorse canopy, overtop it, shade it, and eventually replace it. A deliberate policy of disturbing gorse as little as possible will rapidly lead to its demise. This technique is working successfully at Hinewai Reserve on Banks Peninsula.

Problems

Gorse now occupies large areas of hill-country, considerably reducing pastureland available for grazing by livestock. It also causes severe competition with young forest trees and makes access to forests difficult for pruning and thinning operations. Over summer, the foliage of gorse can become quite dry, making the plant susceptible to fire. This creates risks of damage to forests and houses situated close to stands of gorse. Gorse is able to re-establish quickly once a fire has been through a stand as the species carries large quantities of seeds with tough outer coats.

Control

The attempt to control gorse biologically in New Zealand was one of the earliest undertaken worldwide. Insects were seen as a low-cost alternative to poison, lessening the economic impact on landowners.

The gorse seed weevil was imported from England in 1928, and widely released between 1931 and 1947. But in response to the New Zealand climate, gorse adapted to form seeds in both spring and autumn. The gorse weevil was only active in spring, leaving the autumn seeds untouched.

Manual controls of grubbing, cutting, and bulldozing were time-consuming and expensive. Fire, grazing, and the use of herbicides were considered the best solutions. While carefully managed burn-offs can result in near-total destruction of an infestation, more often than not a thick cover of gorse seedlings appears soon after burning, as fire helps to break the dormancy of the seeds and provides nutrients for growth. The cost of clearance was also prohibitive, especially on marginal land or extreme terrain.

As with many scrub weeds, gorse soon regrows from dormant buds on stumps if shrubs are cut with chain saws or slashers without prior herbicide treatment. Regrowth from buds can also occur after fires. Although gorse can be killed using herbicides, many of these sprays are not very effective, because the shape of the leaves (thorns) and the thick cuticles on the spines help prevent absorption of herbicides.

The failure of these control methods led to a review in the 1960s of the role of biological agents in European gorse control. Yet, it was not until the early 1980s, that the introduction of further biological control organisms was reconsidered.

The variety of attitudes held towards gorse has affected the direction of research into its elimination. Approval to introduce biological controls was finally given in 1989. DSIR (now AgResearch Limited) imported the gorse pod moth, which is now established in the North and South Islands. In Canterbury, observation suggests that the gorse pod moth and gorse weevil are jointly destroying between 90-100% of spring/summer seed crop, and the gorse pod moth is destroying about 15-20% of the autumn/winter crop.

In addition to this, between 1989 and 2001, five foliage feeding control agents were introduced: the gorse spider mite, gorse thrips, gorse soft and hard shoot moths, and the gorse colonial hard shoot moth. Foliage feeders suppress growth by damaging the gorse plant, lowering its ability to photosynthesise, flower and produce seeds.

Results have been mixed, but in general neither the seed-feeding nor foliage-feeding insects are doing enough damage to be viable as a stand-alone control agent.

The concept of biological control of gorse using fungi was raised in 1995, with several species identified as potential mycoherbicides. One of these – Fusarium tumidum, a naturally occurring gorse pathogen – was selected for development.

The solution 

The very nature of gorse as a pioneering, fast-growing, short-lived shrub means that it can only survive where the land is constantly open and disturbed. Undisturbed, it grows vigorously for the first few years, but then slows to a relative standstill.

The Department of Conservation recommends the ‘Method Of Least Disturbance’ (or MOLD), followed by succession – letting native plants take their course. Succession can result in the natural replacement of weeds by native plants. A long-term solution, this might be a great option for those who have the patience.

 

Sources: 1. New Zealand Plants and their Story: Proceedings of a conference held in Wellington, RNZIH 2. Gorse, The Hedge Cutter 3. Gorse in New Zealand, Wikipedia 4. Weedbusters 5. Gorse, Massey University 6. Weed control methods, DOC

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