Contolling weeds in a bushland setting
Every year, weeds cost Australia over $4 billion dollars, in control and lost production. Our primary production and unique environment are under threat from invasive weeds. Environmental weeds are encroaching on important wilderness areas with a profound effect on biodiversity.
Since the arrival of the first Europeans with their crops and ornamental plants, more than 28,000 exotic plants have been brought into Australia, a few accidentally but most deliberately. Now, more than 2500 species of introduced plants are established in the wild. (Info courtesy CSIRO Entomology Dept)
How to prioritise weeds
Many areas of grazing property and bushland contain a variety of weed species but due to time and financial constraints they cannot all be removed at the same time.
When prioritising weed control, it is important to take into account the following:
Rate of spread - is this weed known to spread rapidly and take over large areas in a short time? It is a higher priority to remove species that can rapidly disperse as opposed to those species that are not rapidly increasing their range.
Number of plants- It is important to tackle a weed if it is currently in low numbers. It may then be possible to eradicate an entire species rather than tackling one that is already well established.
Effects on the landscape - Some weeds are known to dramatically change the environment if they are introduced to an area. Impacts may include dense shading, changing the pH of the soil or reducing water flow. It is important to control these weeds before they start effecting surrounding vegetation.
Difficulty to control - It is important to consider how successful control attempts are likely to be. Is the weed accessible for control? Are there important indigenous plants close by? Will several applications of herbicide be required? If a plant is difficult to control and not known to spread rapidly it may be considered a lower priority for eradication.
Impact on rare or threatened plants, animals or habitats - if an important species or habitat has been identified it is important to control any weeds threatening its survival as a priority. Location of the weed - it is important to control a small outbreak of a weed if it occurs in native vegetation as opposed to a large population in a mainly weed infested area.
Timing
The timing of weed control is as important as the technique used. Variations in climatic conditions do allow for opportunities to control weeds, however, there are also associated risks. Good timing of weed control is used to achieve the maximum level of control and causing the least amount of disturbance or using the least amount of chemical.
Timing must take into consideration:
Rainfall - it is important to consider when it last rained and when it is likely to rain again. Hand removal of weeds is easily done when the soil is damp from rain. Some chemicals however will not work if rain falls during or shortly after application. Unseasonal rain can open a window of opportunity for weed control. For example, summer rain may cause some annual weeds to grow allowing for easy removal when other native plants such as orchids and lilies are dormant.
Wind - it is very important not to apply chemical sprays when it is windy as it can easily lead to damage of non-target plants.
Ecological timing considerations -
Seasonal variations mean that weed control may need to be earlier or later than previous seasons. In addition, some seasons may provide windows of opportunity. For example, a significant summer downpour may give Broom a growth spurt, making it more susceptible to herbicide during summer when native ground flora is dormant. By using herbicide which is selective, a woody weed killer (triclopyr) can be applied without killing native grasses.
The most effective weed control will take place when the weed species is actively growing. The timing of this will vary for each species.
The majority of susceptible flora such as orchids and lilies are present during winter and spring. If it is possible to control a particular weed species at other times of the year it will avoid causing damage to these indigenous plants.
Weed control techniques: grassy and broadleaf weeds
There are two main removal types physical and chemical removal these are discussed below: The following methods outline those techniques that do not require chemicals to achieve results. In some cases these methods can be used in combination with chemicals, such as follow up spray after brush cutting.
Hand removal -
Removing weeds by hand is a simple yet important task. The following guidelines should be adhered to:
Hand removal should only be attempted if the species is suited to this technique or is small enough so that removal does not cause significant soil disturbance. This is best undertaken in June or July when the soil is damp allowing for easy removal. In permanently wet areas it may be best to wait until August or September to allow easy access.
A knife or screwdriver can be used to push into the soil at the base of the plant. Gently lever the handle towards the plant and slowly remove the knife. Repeat around the base of the plant until the weed is loose enough to remove. It is important to leave the soil as undisturbed as possible while ensuring all of the weeds root or bulb system is removed.
Collect all the removed weed material in a plastic bag and move off site to avoid any seeds or bulbs from re-shooting.
Cover or solarisation –
Covering weeds is a technique used in situations where a grassy weed has formed a dominant mat across an area. The idea is to prevent any sunlight from reaching the plants causing it to slowly die through lack of photosynthesis. Ensure there are no indigenous plants in the area to be covered
Using black plastic, cover the entire area where the weed is present and firmly secure down the plastic with tent pegs or similar.
Leave in place for approximately two weeks.
For best results remove the plastic during a sunny day that will assist in the killing process by burning the plant.
Remove all plastic and pegs from the site.
Grooming or brush cutting –
Grooming or slashing is a large-scale version of brush cutting and usually undertaken by contractors with the required machinery. The technique however is somewhat similar to brush cutting. In both cases the purpose is to remove the bulk of the weed biomass by cutting the plant as close to the ground as possible. With some species this may significantly reduce their chances of survival, other species however may require a follow up spray with an appropriate chemical. By first grooming or brush cutting however, the total area to be sprayed is greatly reduced and the chemical can be targeted to a specific area.
This method is also useful in removing flower heads of weeds to reduce the amount of seed entering the environment.
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Herbicides in weed control: Are they safe?
Applying chemicals in the natural environment, particularly around waterways carries a certain amount of risk.
There are many concerns in the community about the excessive use of herbicides. The Natural Resource Management Board is aware of these concerns and does not advocate the use of herbicides without a clear ecological benefit.
There are several good reasons for using herbicides in bushland weed control.
The scale of environmental weed control problems is so large that if herbicide use in bushland was abandoned there would be an inevitable and very severe decline in the quality and eventually quantity of remaining indigenous vegetation
Herbicides can be used effectively and safely by skilled operators and avoid impacts upon native vegetation and the broader environment
Herbicides offer the only effective treatment for removing large infestations of tenacious and aggressive invasive species, e.g. Three-cornered garlic or African feather-grass.
Herbicides are very cost effective and labour-efficient.
It is very important that all chemicals are used in accordance with the manufacturer’s guidelines including safety precautions and dilution rates.
In regard to herbicide uptake by weeds or native plants it is important to consider the following principles:
The type of plant; woody, herbaceous, etc
Growth stage of plant
Season
Chemical concentration
Method of application
When using chemicals in a bushland setting the following precautions should be adhered to:
Before using a chemical, complete a test plot, (a small area without any significant indigenous species). The results of the test plot should be assessed before continuing with any application. Remember that most chemicals take up to four weeks for full results to show.
Avoid deliberate or accidental contact with any natives. Although certain chemicals may not harm native grasses and shrubs, it pays to be cautious. Flag all susceptible ground flora such as lilies, orchids, ferns, etc. These can then be covered with bottles, buckets and boxes to prevent accidental spray drift.
Avoid applying chemicals to the point of run-off, as this will lead to the chemical being applied at a higher than recommended rate. Remember that both the mixing process and the amount of chemical mixture applied per unit area controls the rate of chemical application.
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Chemical types
Residual: active in the soil and are absorbed through root structures of the plant. They are designed to keep areas free of all or some weeds for a specific time period, usually between three and twelve months. The chemical's principal mode of action is uptake through root systems and has an expected half-life in the soil of between three months and one year
Systemic: are transported within the plant to the roots, storage organs, stems and foliage. They usually are not absorbed through the soil, but may remain active in the soil for up to six months.
Non-systemic or knockdown: destroys only the plant tissue that comes into contact with the herbicide. Effective only for annual species and the young growth of perennial species.
Within these groups, herbicides can be Selective or Non-selective depending on the metabolic pathway of the plants they are designed to kill.
Selective herbicides affect some plant types but not others. They might be broadleaf selective, grass selective or woody weed selective.
Non-selective herbicides have the potential to affect any type of plant. Therefore non-selective herbicides are not suitable for use in stands of native grasses
Some brand name examples include:
Roundup®: non-selective systemic herbicide
Brushoff®: selective systemic herbicide
Garlon®: a selective systemic herbicide
Metsulfuron Methyl (e.g. Brush Off®) - Metsulfuron Methyl is a broadleaf selective systemic herbicide, which can be used in specific situations for spot spraying in bushland. At low rates it will not kill native grasses and will not affect many native shrubs like wattles, unless they are completely covered. Use of Metsulfuron Methyl for spot spraying is generally not recommended in areas of high understorey diversity.
Metsulfuron Methyl is residual in the soil for up to six weeks, and is absorbed by roots and bulbs; therefore it should not be used in areas with native orchids, lilies and other susceptible ground flora. This chemical is useful in areas where broadleaf weeds (e.g. Salvation Jane, Sour sob and Bridal creeper) are growing amongst mostly native grasses with other native ground flora.
In areas dominated by native grasses (e.g. Themeda, Danthonia, Stipa), it can be used safely for spot spraying. Take care in identifying native grasses, as some plants similar looking to grasses may be sedges or lilies, including genera: Lepidosperma, Lomandra, and Gahnia. The use of this chemical is often better left to persons experienced in bush care.
Glyphosate (e.g. Round Up®) - Glyphosate is systemic non-selective herbicide and will affect all species of plant (broadleaf, woody and grassy). Glyphosate can be residual in the soil for up to four weeks and can be absorbed by roots and bulbs. It can be safely sprayed late in the year, when susceptible native ground flora (i.e. orchids, lilies, ferns, etc) have become dormant. However, Glyphosate will kill any non-dormant species (e.g. native grasses) which are still growing. Glyphosate is very useful in a wipe on situation. Round Up Biactive® is a Glyphosate product that contains no surfactants or penetrants and is therefore suitable to use in wet areas.
Triclopyr (e.g. Garlon®) - Triclopyr is used mostly for cutting and swabbing large woody weeds in bushland situations. It can also be diluted with water and sprayed to selectively kill blackberry, broom and bridal creeper.
Use of Triclopyr for spot spraying is suited to relatively degraded sites and not areas of high understorey diversity. Triclopyr is residual in the soil for up to six weeks or more, and is absorbed by roots and bulbs. In areas dominated by native grasses (e.g. Themeda, Danthonia, Stipa) it can be safely used for spot spraying. The use of this chemical is often better left to persons experienced in minimum impact bush care. It can be safely sprayed late in the season, when susceptible native ground flora (e.g. orchids, lilies, ferns, etc) have become dormant and are not active.
Surfactants and penetrants
When applying chemicals in a spot spray or wipe on situation, it is recommended to use a surfactant or penetrant.
Surfactants allow the herbicide to disperse over the leaf surface but do not help in penetration into the plant tissue. Surfactants are useful when using a wipe on chemical to spread the chemical.
Penetrants (Organo-silicon penetrants, e.g. Pulse) help the herbicide to enter the plant tissue even through woody stems.
Care must be used in their application and is often better left to people experienced in bush care. In creek-lines and bogs, the use of surfactants has the potential to harm native aquatic fauna, e.g. frogs. Do not use chemicals containing surfactants or penetrants in these situations. Be sure to read the label and seek further advice if you are uncertain as to the safety of chemical use near a watercourse.”
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Wipe on
Wiping on chemicals suits a situation where there is potential for chemical run-off or spray drift to harm native plants, particularly native orchids, ferns, lilies and other ground flora. Application of the chemical is made by using a wick-wand or paintbrush to paint the leaves of plants. This suits broadleaf weed species such as Ribwort and Salvation Jane For larger species (such as Watsonia) a rubber glove covered by a cotton glove can be dipped in the chemical and wiped over the plant. Care must be taken to ensure no chemical touches skin or surrounding vegetation.
Cut and swab
Cut and swab is one of the most common weed control techniques used in bush care for controlling small woody shrubs through to large trees. The chemical is applied in a very localised area, minimising any off target damage or movement into the soil. There is still potential to harm small ground flora by trampling, therefore cut and swab techniques should be undertaken before or after spring if possible. For those species to be controlled using cut and swab technique, the following should be undertaken:
Using loppers or a chainsaw (depending on the size of the species) remove the entire trunk to just above ground level.
Ensure all of the green material or small shoots are removed until a single stem is left.
Using a small paintbrush, weed wand or similar apply a small amount of chemical to the entire area that has been cut.
It is crucial to apply the herbicide within 30 seconds of making the cut to ensure the plant absorbs the chemical.
Where possible, remove large branches off site (without dragging or trampling over native plants) to avoid covering surrounding vegetation. If this is not possible, select an area free of native plants to stockpile the branches and allow them to slowly break down.
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Frilling and ringbarking
Frilling and ringbarking are similar to cut and swab techniques but may be used in situations where the removal of the entire plant may cause significant structural damage to soil.
This is particularly useful if a weed's root system is contributing to erosion control or bank stability or when the specimen is too large to be removed. Frilling and ringbarking are effective due to the majority of sap traveling through a plant in the thin outer layers of the bark.
Therefore if a poison is introduced to this outer layer it is quickly transported throughout the plant. To successfully kill a plant using this technique the following must be undertaken:
Choose a part of the stem (or exposed root if it is present) that is as close to the ground as possible
Using a small hatchet, make scrapes into the bark approximately 10 cm long (vertically) and deep enough to penetrate and expose the soft tissue.
Continue this process around the entire circumference of the stem where possible.
Using a small paint brush, weed wand or similar apply a small amount of chemical to the entire area that has been cut.
It is crucial to apply the herbicide within 30 seconds of making the cut to ensure the plant absorbs the chemical.
Drill and fill
Drill and fill is quite similar to frilling or ringbarking whereby the plant will be killed while left standing. Drill refers to the action of using a power drill to drill holes into the stem of a plant in several different locations. These holes are then filled with an appropriate chemical within 30 seconds of the hole being made. This method is a lot less time consuming than frilling as the drill does most of the hard work. This may require the use of a larger quantity of chemicals to fill the hole as opposed to painting over a scrape.
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Spray application
Spot spraying suits areas which are ecologically degraded and contain a low density of herbaceous ground flora. It can be an effective method provided all of the above principles are adhered to. Spray application can range from a small handheld spray bottle to a backpack spray or large spray tank mounted on a vehicle. Regardless of the size of the container, it is extremely important to double-check the rate of dilution. If all factors suit the application of herbicide using the spray technique (timing, climate) and the above principles have been followed, the herbicide can be safely applied.
It is important to consider starting and finishing points to ensure that sprayed areas are not trampled, potentially wiping chemicals on off target species. Please also ensure you have followed the guidelines in regard to the site's potential for inundation with water.
This information has been prepared by -
Caroline Dorr
Sustainable Landscapes Officer
Adelaide and Mt Lofty Ranges NRM Board
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