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Control Experiments
Last updated: 17th November 2009


When dealing with invasive species there are three main ways considered for controlling them; physical, chemical or biological. Physical control is where plants are manually or with the aid of machinery removed from the site. Chemical involves the use of herbicides to kill the plants. Biological control is the use of another organism that naturally attacks the plant to be released to try and kill the whole population of plants. Biological control involved very long term experiments and is extremely expensive in the initial stages and without any guarantees of success. For this reason biological control was not included as part of the project, the main emphasis was on the use of herbicides, although removal was also used as several sites.
Each methodology has both positive and negative sides which must be weighed out for each situation. Herbicide treatments are generally considered more efficient and cost-effective than other methods (Carlile 2006; Motooka et al. 2002). The disadvantage of using chemicals is the potential negative impacts on the environment, affecting not only the target species but also other species in the neighbouring area. Physical removal can be largely labour intensive, meaning it could be hugely costly, it can in the case of Gunnera be difficult as it reproduces by vegetative means, the disposal of material removed will be problematic. But if feasible it is less damaging to the environment and an area will be ready to be used straight away.


Experiments

A set of experiments were carried out over several years to look at the possibility of using herbicide as a means to control Gunnera.

1. Greenhouse Experiment- "Concentrations of RoundUp"

Three different concentrations of the herbicide RoundUp were used to find out if the recommendations by the manufacture were adequate or if a stronger concentration is required. The following table shows the concentrations used.

Treatment Concentration Recommended RoundUpŠ
RoundUpŠ 1 20 ml-1 recommended
RoundUpŠ 2 50 ml-1 2.5 x recommended
RoundUpŠ 3 100 ml-1 5 x recommended
Control 0 ml-1 N/A


The results were measured by grading the plant health by visually estimating the effect on the leaves, until the plant was considered dead, i.e. all the leaves were brown and completely wilted.



Results


Figure 2.2. Mean plant health of Gunnera tinctoria plants in each experimental treatment: control (green diamonds), recommended concentration (blue triangles), 2x the recommended concentrations (yellow circles) and 5x the recommended concentration (red squares).

The results showed that irrespective of what concentration the plants were treated with the plant died. As the use of higher concentrations means a higher cost and effect to the environment it is recommended to:

USE THE RECOMMENDED CONCENTRATION


2. Greenhouse Experiment- Garlon vs RoundUp on plants with different leaf sizes

A second experiment was set up to observe which herbicide was more effective Garlon or RoundUp. Garlon is a selective herbicide, meaning it will not kill grasses, which could be useful to prevent killing other neighbouring plants in the field. Additionally the leaf area of the plants was incorporated into the experiment to try and establish if a certain quantity of herbicide is necessary for the herbicides to be affected.





The results showed that Garlon was a much faster acting herbicide, but the end result was the same, also the leaf area of the plant was found not to be important, so even small quantities of herbicide applied to the plant were sufficient to affect the plant. Despite the advantage of Garlon being a selective herbicide, it is a much more toxic herbicide which is not allowed to be used near watercourses and additionally is expensive and not widely available.
With the knowledge of these two experiments in the greenhouses the following experiments were carried out in the field, to test the use of herbicides on fully mature plants growing in the wild.


3. Field Experiment- Methods of Application


It was decided to continue the use of RoundUp, which up to now has been effective and is also widely available and also one of the few herbicides allowed to be used near watercourses, which is where much of the Gunnera is growing. The method of applying the herbicide was tested, as the common application method of spraying is not suitable for many of the sites where Gunnera is found. As the plant grows over two meters tall and with rhizomes growing along the soil surface it would be difficult to either manually or mechanically get access into the site and spray herbicide above the plants. For this reason and also for environmental reasons two more selective methods of applying herbicide were used.

1. Cut and Paint, this method involves cutting all the leaves and flower heads off the plant and applying herbicide straight to the cut.
2. Cut and Injection, this method is the same as above but additionally herbicide was injected into the rhizome, but drilling small wells into the rhizome and filling them with herbicide.


















Photos: (Left) Cut and Paint technique, blue dye used to show herbicide application to the cuts.(Right)Plot where plants were treated.

To test these methods, 3m x 3m plots were marked out in the field, all plant in these plots were counted and treated. Subsequently, when revisiting the plot the number of growing shoots were counted and compared to the initial number before they were treated.



The results in 2006 after the applications, the mortality rate (plants with no growing shoots were considered to be dead) was 96% for the C&P method and 93% for the C&I. In 2007 the mortality was 96% for the C&P method but was only 83% for the C&I method. In 2008 there was evidence of re-growth in both treatments and the mortality rates decreased to 61% for the C&P method and 75% for the C&I method.
From these results we can establish that both the selective methods of applying herbicide are effective, and despite being labour intensive, the quantity of herbicide used is minimal, reducing the cost and the impact to the neighbouring environment. It is also important to note that there was not a 100% success, and therefore it will be necessary to reapply. The reason for not having 100% mortality is probably due to the increased size of the plants compared to those used in the greenhouses and also possibly the effectiveness of the application, care must be taken to apply the herbicide straight after the cut is made, it must also be carried out on a dry day and all plants including small ones must be treated. There was regrowth observed particularly after a two years, so it will be important to check areas treated for re-growth to reapply the herbicide

Re-applications will be necessary after 1 or 2 years




4. Field Experiment- Timing of herbicide application.

As the more selective methods used in Field Experiment 1 are quite labour intensive, for large fields it may not be an ideal solution. When the leaves haven't reached their full height, in early spring, it would be possible to walk through the field and spray the herbicide. This experiment investigates the effectiveness of herbicide when applied at different times. The first application was made when the leaves first started to emerge, and subsequently two times after that as they begin to grow but are not fully expanded.


Photos: From left to right, showing the three stages of leaf emergence when applications of herbicide was made.

The results from this experiment found that the only application that was successful was the later application. The first two, despite the leaves dyeing back they managed in the same season to produce a whole new set of leaves. The third application had some success, but not to the same degree as those of Field Experiment 1. that was carried out in the late summer, when full growth had occurred.
The reasons for these results are not clear but it is possible that during the early stages of leaf growth, the movement of nutrient is mostly in an upward direction inside the plant and little herbicide is transported into the rhizome. During the later part of the growing season, the movement of nutrients maybe in a downward direction towards the rhizome, to store sugars over winter, therefore, resulting in the increased effectiveness of the herbicide.

Early applications are not effective, late summer applications are recommended


All these experiments lead to the production of an information leaflet and, management guidelines, the following is a summary of the recommendations.

Recommendations

1. Use Glyphosate based herbicide (RoundUp): considered less toxic to environment

2. If possible use more selective methods to apply herbicide: reduce environmental impact

3. Apply when plant is in full growth: higher success

4. Re-apply when necessary : one application will probably not be enough

5. Rhizomes physically removed must be disposed of safely : it can produce new plants

6. Continue to monitor in successive years for further re-growth