Gather Background Information
The first step in an IVM program is to gather information on the life cycle and habits of the noxious weed.
Gorse, Ulex europaeus, also called furze or Irish furze, is a spiny, evergreen exotic shrub which can grow up to 15 feet tall and 10 to 30 feet in diameter, depending on location. Branches end in a spine and are covered with green scale-like or prickly leaves 1/2 to 2 inches long. Showy yellow pea-like flowers, 1/2- to 3/4-inch-long, grow in clumps near the tips of its branches. The hard seeds are very small, shiny, and brown and are enclosed in 1/2 inch long hairy pods (Boyd 1985; Cook 1987; Hoshovsky 1986).
Young gorse plants have a compact, rosette-like form, thin triple leaflets, and no thorns. Adult plants are usually shrub-like but may also form mats or cushions in exposed coastal sites. Individual plants grow outward forming a central area of dry, dead vegetation (Hoshovsky 1986).
Gorse is extremely competitive, displaces cultivated and native plants, and impoverishes the soil. It creates an extreme fire hazard due to its oily, highly flammable foliage and seeds, and abundant dead material in the plant’s center. It not only increases the risk of fire, but also produces a hotter fire than most weeds (MacCarter and Gaynor 1980).
Because of various characteristics of the plant, the soil is often bare between individual gorse plants, which increases erosion on steep slopes where gorse has replaced grasses or forbs. Spiny and mostly unpalatable when mature, gorse reduces pasture quality where it invades rangeland. Gorse understory in forests interferes with cultural operations, increasing pruning and thinning costs (Balneaves and Zabkiewicz 1981).
Gorse is native to western and central Europe, where it has been cultivated for centuries as hedgerows, and in France as reserve livestock forage. Early European emigrants introduced gorse to more than 15 countries or island groups. In New Zealand, gorse was once planted on large estates for the provision of sheep feed on land too poor to grow other crops (Cook 1987; Markin et al. 1995; Bell 1939).
Gorse has become a major weed of agriculture and forestry on the West Coast of the U.S, as well as in New Zealand, northwest Spain, Tasmania and Australia, and at high altitudes in Hawaii (Gaynor and MacCarter 1980).
The geographical distribution of gorse depends primarily on temperature. It cannot survive in arid climates, or in continental regions where there are extremes of heat and cold. Day length may also affect its latitudinal distribution, as short-day conditions inhibit maturation and prevent thorn formation and flowering. Gorse will grow on most soil types, including acidic soils with less than 4% organic content (Zabkiewicz 1976; Hoshovsky 1986).
Gorse grows well in shady slopes with high soil moisture and good drainage. Look for gorse in areas with degraded soils or disturbed sites such as roadsides, pasture lands, gravelly floadplains, cleared forests, or other areas following a disturbance (Cook 1987; Zielke et al. 1992).
Gorse reproduces by seeds and can resprout from roots or root fragments. Resprouts and seedlings flower within six months to a year. Gorse flowers primarily from winter through early spring, but it can also flower at other times of the year. Seed pods mature as early as May in California but not until mid to late summer in Oregon and Washington. When mature, the seed pods explosively burst open, projecting seed up to two yards. The waxy seeds can also disperse down waterways or be transported by animals such as quail. Gorse can also spread by sprouting from buds on roots or by producing roots where lower branches are close to or in contact with soil. Seed counts in gorse stands are extremely variable, but annual production can average 400 to 500 seeds/square yard. A continuous mature stand of gorse could generate a seedbank containing 250 million seeds/acre.
Most seeds germinate after the first fall rains, roughly the same time as annual grasses and forbs. Seeds scarified by tumbling over rocks in waterways germinate quickly in fine soils (Zabkiewicz 1976; Rudolf 1974; Chater 1931; Partridge 1989; Zabkiewicz and Gaskin 1978;Ivens 1978; Popay and Adams 1990; Meeklah 1979; Bean 1980; Read 1998).
Special Challenges to Management
Its thick cuticle, prolific production of durable seed, and ability to resprout from stumps and roots make gorse difficult to control once established. Repeated control efforts over several years are typically required to suppress gorse (Zabkiewicz 1976).
Gorse seeds are extremely persistent in the soil. Water-impermeable seed coats allow them to remain viable in the soil for 25 to 40 years, creating a very sizable seedbank. Fire, soil disturbance, and/or moisture can stimulate germination (Hermann and Newton 1968; Zabkiewicz 1976).
The ability of gorse to fix nitrogen enables this plant to colonize and dominate areas with poor soils. Gorse plants extract and retain plant nutrients such as calcium, magnesium, and sodium, which changes nutrient dynamics and can impoverish the soil. It is not known what effect this may have in determining which species can replace gorse after control efforts are implemented. The plants grow best at a soil pH of 4.5 to 5, and consequently, the thick litter produced by gorse plants tends to acidify the soil. Gorse prefers moderate to full sunlight and can die if heavily shaded. The plant is moderately tolerant to frost when young and very frost resistant when mature (MacCarter and Gaynor 1980; Zabkiewicz 1976).
It usually takes time for gorse to become established at a site, but then it can quickly spread and displace other vegetation. However, when it travels down waterways, perhaps because of the scarification of the seed against rocks, it strikes very quickly and does not lay dormant for very long (Read 1998). In managed areas, gorse usually colonizes difficult terrain and inaccessible areas that escape routine mowing or cultivation (Parker and Burrill 1991).
Some questions, such as those below, can only be answered on site.
Set Realistic Goals for Your IVM Program
The answers to the following questions can help you set realistic objectives and goals.
Containment - keeping an established population of the weed from spreading to non-infested areas. This strategy is especially useful when time and money are in short supply or when the infestation is very large. For example, a barrier strip between infested and non-infested areas can be maintained and monitored so that adjacent lands remain weed free. In addition, measures that stop seed production will prevent further spread of the weed.
Reduction - reducing the area covered by gorse, or reducing its dominance. This strategy can also be used against new or established weeds, but it requires more resources and more time than containment.
The "Bradley Method" (see Appendix 2), developed in Australia, is a simple yet innovative strategy for natural areas that combines containment and reduction.
Eradication - completely eliminating the weed from the management area. This strategy usually consumes the greatest amount of time and resources and is applicable mainly to newly-invading weeds that are confined to a limited number of small areas.
Establish Monitoring Programs
When planning a monitoring program, keep in mind the context of your target weed: is it invading or has it already invaded?
Locate and record gorse infestations on a map. (Chapter 2 of the University of Northern Iowa IVRM Technical Manual contains a detailed discussion on how to map and inventory vegetation - see Bibliography). Note particularly sensitive areas on the map, such as critical habitat for threatened or endangered species, agricultural production areas, or areas subject to frequent disturbance and thus prone to invasion. Update maps at regular intervals.
Focus monitoring efforts on sites where gorse problems are most likely to occur (see Distribution). Encourage public sighting and reporting through an education or incentive program (see under Educate Vegetation Management Personnel and the Public).
Prioritize the sites you will work on. Make a realistic assessment of your weed management resources, keeping in mind the goals of your project and the cost of a follow-up program after any treatments. Without follow-up, your control efforts will be wasted. It is better to thoroughly control a weed at one or two sites than to use up resources to incompletely control the weed at many sites. If the weed is very widespread, try to determine where it poses the most serious economic, social, or environmental problem and concentrate on those areas.
Plan monitoring and treatment efforts to coincide with critical life stages of the weed. To use your resources efficiently, try to include monitoring with other planned activities in the area.
Maintain records of your monitoring activities.
Create standardized forms to make data collection easier and help remind you to gather all the information you need. Forms work best if they include labeled blanks for all pertinent information and allow the user to check or circle rather than having to write words or numbers (See Appendix 3 for some examples of forms).
Include information such as the name(s) of the person(s) collecting the data, the location, and date of monitoring; a qualitative description of the vegetation, such as the names of the plants or types of plants (native vegetation, annual/perennial weeds, trees, etc.) and stage of growth (germinating, flowering, setting seed, etc.); a quantitative description, such as percent cover, plant density, size of the patch, or if possible, the number of plants.
Note special conditions such as unusual weather events and record treatment history, including information on treatment applications (who, when, where, how, cost, difficulties, and successes). This will allow you to evaluate and fine-tune treatments.
Set Treatment Thresholds
Setting treatment thresholds includes prioritizing and balancing treatments with resources. Weeds will be treated when populations increase beyond a predetermined level. This level will largely depend on the characteristics of the site and weed. In some cases the level may be no weeds at all, and in other cases the number of weeds you can tolerate may be much greater.
Considerations for Setting Priorities
What is the level of the threat? Is the gorse population changing? Is it in an area where soils are frequently disturbed? Does it threaten agriculture, pastures, or rangeland? Is it encroaching on critical habitat for a rare, threatened, or endangered species? Is it displacing the best examples of native communities?
What is the size of the weed population? The opportunity for control is related to the infested area. Small patches can be more easily controlled than large infestations.
What resources are available? Do you have the resources
required for carrying out your goal?
With the advent of herbicides, prevention as a weed management technique has often been neglected; however, it is a practical, cost-effective, and extremely important part of noxious weed control.
General Weed Prevention Measures
Because most noxious weeds are pioneer species, a dense ground cover of desirable plants with a closed canopy will usually help prevent their establishment.
(Adapted from Fay et al. 1995).
Revegetation - Follow-up Weed Prevention
Establishing dense, competitive vegetation can help permanently replace weeds. Revegetation is critical in preventing weed infestations in areas where the soil has been disturbed or the vegetation removed. In some situations you may be able to encourage desirable vegetation that is already in place; but because of the aggressive nature of gorse, it is more likely that you will need to thickly sow seeds of desirable, competitive plants.
Prior to revegetation, sprouts from root crowns and seedlings must be killed. Return visits to control sprouts must continue until the gorse seedbank is depleted, because gorse seedlings do not seem to be inhibited by even a dense growth of annuals or shrubs. According to Gray (1996), in many areas it is not worthwhile to reseed until the seedbank is depleted, as efforts to destroy new gorse sprouts and seedlings will also destroy the young growth of any desired vegetation.
Some areas may be cultivated and farmed to control gorse.
Gorse thickets are first burned under conditions conducive to a hot fire
(low relative humidity, high ambient temperatures). After the land is cleared,
it can be farmed with hay or other grains for a few years, after which
it can be grazed again. MacCarter and Gaynor (1980) recommend oversowing
a pasture mixture that can of grow quickly into both the ashes and dead
gorse. They also report that the combined effect of competition of white
clover, Trifolium repens, and the symbiont Rhizoctonia fungi
will prevent gorse establishment in situations of extreme competition among
pasture species and de-foliation caused by grazing livestock (Hoshovsky
Apply Management Methods
No individual method will control gorse in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. The treatment methods described in this section will help you to design an integrated program that will suit the circumstances of your particular situation.
Biological control does not aim to eradicate weeds, but to keep them at low, manageable levels. After their introduction, biocontrol agents can take 5 to 10 years to become established and increase to numbers large enough to reduce the density of the target weed. Once established, effective biological controls provide an inexpensive, long-term, and non-toxic means to control weed populations. Since insects have specific requirements for growing and thriving, it is important to match the insect to the weed management site. Understanding these requirements will help you integrate the insects into other weed control efforts. When you release biocontrols, continue using other control methods on the perimeter of the release site, but avoid using them where they might adversely impact the insect population.
The information provided below is only a summary. For more information consult Biological Control of Weeds in the West (see Bibliography) or contact commercial weed biocontrol insectaries (see Insectaries).
Larvae or pupae of A. ulicis can be found inside
brown and hardened seed pods from early to late summer. If you want to
encourage the weevil, locate an infested gorse plant that can be allowed
to persist. These gorse plants become a nursery for A. ulicis and
help insure that it will be present in sufficient numbers to help suppress
Numerous studies indicate that grazing by goats can effectively control gorse, and that they are less costly to use than mechanical and chemical control methods. Goats can graze areas too steep or uneven for machinery, and do not pose the environmental dangers inherent with herbicides (Wright 1927; Harradine and Jones 1985; Radcliffe 1982; Hoshovsky 1986).
Goats and sheep have also been used to control gorse. Destruction of mature stands by cutting or burning, followed by high-density goat grazing (60 to 75 goats/acre), followed by a rotation of goats (25 goats/acre) and sheep (10 sheep/acre) may be most effective. Rotational grazing with cattle, then sheep, then goats is also effective. Cattle trample and kill some plants and make access easier for sheep and goats. Sheep then eat all the best forage, leaving the goats nothing to eat but gorse (Bell 1941; Wright 1927; Radcliffe 1982).
New growth of gorse is soft and palatable to livestock. In late spring and summer, however, these new shoots and seedlings become hard and inedible. Since goats prefer younger parts of the plants and will not eat plant parts greater than 4 years old, mature gorse plants must be removed. Experience shows that Angora and Spanish goats provide the best control. Graze the area for at least five consecutive years to exhaust the soil seedbank (Cook 1987; Daar 1983; Bell 1941).
Since the goats will eat almost any plants, exclosures made of portable electric fencing may be needed to protect patches of native or desirable vegetation within the grazed area. Stocking rates will vary depending on the amount of gorse available for forage and the appetites of the goats. Goats must be cared for properly and their fencing maintained to prevent the animals from escaping to graze non-target plants. For these reasons, it is best to hire a handler with vegetation management experience rather than attempt to maintain or manage a goat herd yourself. And whether you use goats or sheep, survival of seedlings is lowest with a long grazing rotation and highest with continuous stocking (Hartley and Phung 1979; Hartley et al. 1980).
Physical controls for gorse consist of removing plants by hand, with machinery, or by burning. After plants have been removed, the gorse seedlings sprouting from the seedbank must be treated for at least 5 consecutive years. Once the seedbank has been depleted, desirable vegetation should be encouraged to fill any gaps left by treatments (see the Revegetation section above). Mulching can be used as needed after removal. Stems of gorse plants can be chipped and used as mulch as long as care is taken not to include root fragments. Root systems should be removed completely and disposed of by composting or burning off site (Balneaves and Zabkiewicz 1981; Hoshovsky 1986).
Where gorse infestations are relatively small, manual removal can be effective. Manual removal can be applied selectively to individual gorse plants without disturbing other vegetation, and can be used on steep or uneven terrain (Bell 1939).
When beginning a hand-removal project, flag the treated areas so they can be identified for follow-up treatment. It is easiest to work in relatively small areas of infestation. When faced with dense and/or extensive stands of gorse, it is best to divide them into grids (with flags, stakes, etc.) so that workers can thoroughly weed smaller areas before moving onto the next grid. The grid system also facilitates dividing work activities between those pulling and those removing the debris.
Seedlings young enough to still have cotyledons can be killed by grubbing them out or cutting below the 2 seed leaves. Young plants can be pulled by hand or with a weed wrench—a hand-operated tool that acts as a lever and pulls plants out by the roots. The plant should be gripped at the base or the stem may break off, leaving the root system intact. Take care to remove the root crowns. If not thoroughly removed, root crowns will resprout repeatedly. Pull plants before they have a chance to set seed and when the soil is moist in order to facilitate the removal of root crowns and root systems. Along the Pacific Coast, November through March are optimal times for this activity. Manual removal must be complete to be successful. Missed portions of grubbed plants will resprout, and new seedlings develop from the seedbank (Hoshovsky 1986; Cook 1987).
Plants larger than six inches in diameter at the base should be cut to the ground using a blade, pruning shears, hand saw, powered brush cutter, or chain saw. Carefully pull out roots to minimize soil disturbance. To minimize resprouting, cut plants when they have just started to flower. At this time, food reserves in roots are low and seeds have not been produced (Hoshovsky 1986).
Plants that are cut and have not yet set seed can be left on the site to help smother regrowth but should be chipped or composted prior to replanting efforts. Root crowns or root fragments may resprout and should always be removed from the site or destroyed; do not include them in chipped or composted materials (Hoshovsky 1986; Ivens 1978; Fuller and Barbe 1985).
Mechanical control involves the use of machinery to cut gorse from a large area. This method is not selective and can damage desirable vegetation and disturb soil. Mech-anical removal may be limited to areas of even terrain with few obstacles. As with manual removal, control efforts should be timed to pre-empt seed production. Mechanical control efforts must also include follow-up treatments (mechanical or other) to kill shoots from resprouting plants and seedlings sprouting from the seedbank (Hoshovsky 1986).
Controlled burns have been used to manage gorse worldwide for decades. If correctly timed, burning will reduce gorse biomass, destroy seeds still on the plants, kill seedlings, and reduce the number of years subsequent treatments will be needed to exhaust the seedbank (Balneaves and Zabkiewicz 1981).
Temperatures of 100°C (212°F) or more sustained for 15 minutes are required to kill gorse seed. During a burn, this degree of heat penetrates only about 1/2 of an inch of the soil surface. Slightly lesser temperatures actually break gorse seed dormancy, causing increased germination. Seeds heated to 80°C (176°F) for fifteen minutes will give 100% germination in the laboratory in about 12 days (Zabkiewicz and Gaskin 1978).
According to MacCarter and Gaynor (1980), burning can be improved by crushing the gorse first with a trac-layer tractor, then burning against the wind.
Root crowns that are not destroyed in the burn will re-sprout unless killed or physically removed with a weed wrench. Resprouts will not produce seed for at least a year and are vulnerable to chemical controls. Burning must be followed by control of new seedlings annually until the seedbank is depleted (Zabkiewicz 1976; Hoshovsky 1986).
Because of the potential hazards, use burning to control gorse in areas too large for manual or mechanical means. Fire should not be used in areas where forest vegetation is mature, since burning will tend to encourage gorse and other shrubs to dominate (Zabkiewicz 1976; Cook 1987).
For effective and safe burning treatments, plan the burn to coincide with times of low fire risk. Consult with the local fire department officials to plan the logistical details of the burn, including appropriate weather conditions and safety precautions. After the burn, remove roots and be careful to disturb as little soil as possible. Moist soil will facilitate removal of burned gorse. Alternatively, crowns may be killed by spot-treating sprouts about six months after the burn. Burning should be followed with revegetation if native vegetation is not vigorous enough for re-establishment (Hoshovsky 1986).
In Oregon, forest managers use fast-growing tree species to shade out gorse, reducing or sometimes eradicating an infestation. The technique has also been used successfully in New Zealand, and to a limited extent in Hawaii. Planting acid-tolerant, fast-growing trees in gorse thickets may eventually shade out gorse without further management efforts. However, site preparation involves killing existing gorse plants, and follow-up treatments to kill regenerating gorse until the trees are tall enough to form a canopy (Hermann and Newton 1968; Markin et al. 1995; Sutherland 1954).
Although the details of this treatment method vary, the basic principle is the same: small areas of open space are created within the gorse patch and tree seedlings are planted in these areas. To create the open spaces, gorse plants and their root crowns are removed by hand either in a checkerboard pattern, parallel rows, or at random. Removing the plants randomly or in a checkerboard pattern will result in a more natural appearing tree cover, but rows allow easy access for planting and maintenance. In general, this technique works best when soil disturbance in the gorse patch is kept to a minimum when clearing a planting area (Hermann and Newton 1968).
Since tree seedlings will be vulnerable to rodents sheltering in the gorse patch, they should be protected with a barrier such as a tree tube or wire cage. Tree seedlings should be at least 18 inches tall and 5/16 of an inch thick at the base or they may have difficulty competing with the gorse. The gorse must be kept cut back so it does not overtop the tree seedlings. It takes 10 to 15 years for the shade from the trees to kill the plants and it is unlikely that all the gorse will die (Hermann and Newton 1968).
In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply them in a judicious manner.
The following links discuss the primary considerations when using herbicides: Use Herbicides Properly, Herbicide Information Resources, Criteria for Selecting an Herbicide, and Application Methods.
Applying herbicide to plants when gorse is most susceptible (preferably before seeds are produced) is crucial to the effectiveness of the treatment. Gorse is sensitive to foliar herbicide treatments during periods of active growth prior to or after flowering. These periods are early summer, early fall, and mid-winter. William et al. (1996) suggests that the plant is most vulnerable just after flower drop in the spring or prior to flower drop when the plant is actively growing. Do not allow resprouting shoots to grow more than 6 inches before herbicide treatment (this greatly decreases the amount of herbicide needed to kill the sprouts). To increase effectiveness, apply herbicide in late afternoon (as long as wind is not a problem) to take advantage of sap movement. Plants larger than 3 to 4 feet high should be cut back or burned and allowed to resprout before herbicides are applied. Balneaves (1980) suggests crushing and burning in the spring, followed by an herbicide application at 4-8 weeks and at 9-12 months later. Once the soft young stems begin to harden, a surfactant may be needed to help the herbicide penetrate the waxy stems (Meeklah 1979; Read 1998).
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