With the recent introduction of the Biosecurity Act, there is now more emphasis to think about our action in terms of weed spread and dispersal. The act specifically focuses on the shared liability relating to containment and control of weeds.

There is a significant and unresolved conflict between the retention of trees of species that are invasive and ecologically-damaging but are also recognised for their cultural, historic or aesthetic significance.

Camphor Laurel is one such species. They were planted extensively for amenity or cultural reasons but the species readily invades natural areas, impacting on biodiversity and ecosystem functions. Individual trees can generate copious progeny annually through seed production and dispersal.

Killing such trees will certainly stop seed set but this may result in community angst. There are instances where the removal of such trees is curbed by community or historic values. Protestors may only have their thoughts on a few issues, such as shade or the loss of very old picturesque trees, however we must consider that the seed from some of these invasive species may be transported long distances via birds and deposited in other areas.

Is it possible to preserve these trees whilst preventing them from producing seed?

Chemicals can be used to Modify Growth

We are all too familiar with herbicides and their primary role to kill weeds. However, there are many herbicides that have been used to modify the growth of plants without the aim of death. Other active ingredients (non-herbicides) have also been identified to alter plants growth for a desired outcome.

New Project

A federally funded project commenced in July 2017, courtesy of the Department of Agriculture and Water Resources and the lead agency MidCoast Council and other collaborating agencies. The primary aim was to undertake some research in the following two years to scope a handful of chemicals to de-flower or prevent fruit from developing on African Olives and Camphor Laurels. There is potential to use this technique, if successful, on other species. However, as there is limited project time it was decided to stay focused on these species.

African Olive

African Olive is a garden escape plant and has become a serious weed in bushland. It can further spread to and heavily impact upon agricultural land. More than 4,000 hectares of dense African Olive infestation has been identified across the western Sydney region alone. African Olive was listed as a key threatening process to biodiversity by the NSW Scientific Committee in 2010.

It is estimated that African Olive is having a negative impact on at least 25 endangered ecological communities as well as 13 threatened flora and 4 threatened fauna species in NSW. African Olive has further been listed in the Global Invasive Species Database. African Olive out-competes established native vegetation, casting dense shade which prevents the regeneration of native plants. Infestations can alter the floristic structure and habitat value of remnant bushland areas.

Camphor Laurel

Camphor Laurel is considered a threatening weed under similar listings to African Olive. They have the ability to adapt to the disturbed environment, have prolific seed production and a rapid growth rate as well as a lack of serious predators or diseases, they also possess many specific attributes which enhance its weed status.

Camphor Laurels are ecosystem changers. They have a tendency to form single species communities and exclude most other tree species, including desirable native vegetation. They have a very dense, shallow root system which, when accompanied by the shading provided by the canopy, suppresses the regeneration of native seedlings. They have the ability to replace and suppress native vegetation and have an allelopathic effect on other species.

Interim Results from Year 1

The list of potential chemical candidates for testing was rather lengthy and after an extensive literature review the list was trimmed down to three chemicals for Camphor Laurels and two for African Olives. Growth habit of the weed plays a large role into determining the type of treatment selected and how it is applied. A species like African Olive is often multi-stemmed and would be impractical for stem injecting whereas the single stemmed Camphor Laurel trees are ideal for a range of chemical deliver systems.

A long dry period of weather from winter to spring played havoc on the flowering times and synchronicity of Camphor Laurels and African Olives. Fortunately significant rains fell in early October to rejuvenate the weeds, however flowering was still not ideal. Timing of treatments was closely linked to flowering, namely near early-mid flower bud opening stage.

Assessment of treatment impacts on flowering or fruiting capacity of the weeds was undertaken in March and May 2018, but careful consideration was made to foliage changes. An ideal treatment is one that suffers no foliar damage while completely aborting reproductive issue.

The interim results from the African Olive experiments suggest this species is rather difficult to selectively control flowering/fruiting without severely affecting foliage.

The best compromise appeared to be treatment A with two times concentration that reduced fruit setting by 90% with a foliage damage score of 3 out of 10 which equates to some very noticeable symptoms from which the plant will take some time to recover. Four times concentration reduced fruit setting by 98% but with much more severe foliage damage. There is scope to apply various rates around this two times rate in the second year of the project, to better fine tune treatment outcomes.

It appears treatment A (same treatment for African Olives) was the most suitable for temporarily sterilising Camphor Laurels. It subtly made the foliage paler whilst significantly reducing reproductive capacity. Treatment B achieved very little. Treatment C will be tested in year 2 at much lower rates due to excessive foliage damage in year 1.

Concluding Comments

The second and last year of testing will be focused on getting consistency and robustness of the likely treatments that may be considered for registration or permits. The key to success is developing a treatment that can be easily and evenly applied that doesn’t leave obvious scarring of bark while achieving near perfect seed set control and barely noticeable effects on foliage. Timing of treatments could be investigated in subsequent projects, however there is only enough time to investigate rate responses of treatment A.

Fingers crossed for a better season than 2017-18.

This is a shortened version of an article in the Autumn 2018 edition of A Good Weed, the newsletter from the NSW Weed Society. Here’s hoping this idea can be extended to many more weed species, in particular privet.