Killing Climbing Asparagus

One of my Waiheke jobs is poisoning an infestation of Climbing Asparagus or Krulkransie (Asparagus scandens) on a private bush section. Asparagus scandens originally hails from the Western Cape of South Africa. In New Zealand it invades forest remnants and disturbed areas. A. scandens  chokes understory species, preventing their regeneration, and can ring-bark older trees. When the understory has gone and the larger trees eventually fall, it's curtains for the forest ecosystem as we recognise it.

A. scandens rampant in the understory of regenerating  native bush.

This invasive is shade tolerant and although it may not produce much of a seed bank, it compensates by producing seedling banks. Small plants may hang around for ages, inconspicuous on the forest floor until going berserk during their spring/winter rapid growth phases (though not as much as some other invasive vines). To top it all off, A. scandens is tuberous with 87-94 % of the biomass (by dry weight) of the plant occurring in the below ground organs (Timmins & Reid 2000). This plant is so successful at persisting and spreading that i doubt that even old-growth forest would offer much biotic resistance to its incursions.

The property is a large one and much of the work is done on slopes leading down to the coast so i use a 10 litre backpack sprayer. The poison of choice is Glyphosate (N-(phosphonomethyl)glycine) originally (before the patent expired) marketed solely as Roundup by its developer - that gargantuan fuck-up of a corporate citizen, Monsanto. 

Shaded area indicates the size of the property where i spray Asparagus scandens. A. scandens has spread throughout much of the understory.

For most of us mammals, sensitive individuals notwithstanding, Glyphosate is a comparatively harmless herbicide with an acute mammalian toxicity sufficiently low that table salt (sodium chloride) has a lower LD₅₀. For those who don't know, LD₅₀ is the median lethal dose in mg/kg (or (micrograms) µg/kg or (nanograms) ng/kg depending on the lethality) required to kill 50% of a tested population, so a lower LD₅₀ means higher toxicity. Table salt has an LD_{50 of}  3000mg/kg when orally introduced to rats whereas Glyphosate has an LD₅₀ of ~ 5000mg/kg.

For comparison here are a few other LD₅₀'s,  all are based on oral introduction to rats unless otherwise stated: caffeine, 192 mg/kg; nicotine 50 mg/kg; ricin 22 µg/kg (note: micrograms, but intravenous); dioxin 20 µg/kg (note: micrograms); lectin bearing sap from some Adenia species < 2 µg/kg (this maybe recorded toxicity to individuals, not an LD_50). The toxic action of lectins is similar to that of ricin and is a fascinating tale in itself.

Very few chronic effects of Glyphosate have been observed - it's (so probable in practice definitely) non-carcinogenic, non-immunotoxic, non-neurotoxic. As far as endocrine disruption is concerned most studies say no, a few studies say maybe, in high doses. Given the findings of one reasonably tight looking bit of research (Gasniera et al. 2009) it may be more about the formulation of a spray than Glyphosate alone. Of course, there are trade-offs; Glyphosate does persist in the environment as a pollutant, though largely inaccessible to anything due to its aggressive binding to soil. Reservations about very large scale agriculture use over long periods of time may be warranted - if not for their toxic effects on most of us, for problems with degradation of soil quality. However, in some circumstances, given certain temporal scales, this may still be the lesser of a range of possible evils.

In the case of my climbing asparagus control efforts spray is the only reasonable choice. Hand pulling, root digging, are energetically indefensible and pinetarsol isn't going to cut it. So what are some the trade-offs in my case. Well, Asparagus scandens doesn't have the good graces to grow in isolated patches. It's dispersed by birds who will often shit out the seeds beneath tree boughs where the invasive will germinate. This suits A. scandens just fine because as the name suggests, it climbs using established plants as scaffolding.

This means when i spray there are residual kills of understory native species. To begin with this used to distress me a lot. Though still cautious,  i'm now hardened. The goal is to preserve the ecosystem as a whole. Practical limitations on energy available for weed eradication mean attention to individual plants would be at odds with ecosystem survival. *Warning - Geek Alert* Coming in and spraying at this stage is the equivalent of casting an Armageddon spell at the end of a Heroes of MM battle you are otherwise going to lose.

 

The areas shown in the above video (sry about the quality) are of the easiest kind to spray. The Asparagus coverage is so thick almost no care is required at all. When i mention scaffolding survival in the video i mean surviving my spraying activities, not surviving the untreated Asparagus infestation itself - that's not on the cards for the unfortunate plants. What is more problematic/stressful are recently-invaded, dense groves of native saplings formed in light gaps. Here the weed is well established but is intertwined among the natives and has not yet over topped them. These areas are still to large for hand pulling to be energetically defensible and in any case there would be no way to get the roots and tubers out without killing the saplings. Here i have to work carefully to limit spray contact with as many natives as possible.

There is strong resistance on Waiheke to the use of human-synthesised chemical sprays. The local authorities use rancid coconut oil on grass verges and it works very well. However, the sustainability of this practice  - the where and how of production and the impacts on other peoples communities are important questions. I don't believe New Zealand has a thriving coconut oil industry.

My partner has recently begun work for a Waiheke based charitable trust doing weed eradication work. Some people take such exception to this weed trust's work that they target the work vehicle for sabotage. In the area i'm working there is a strong contingent of community chemophobes who fear poisoning from chemical inputs (human-synthesised or "unnatural" chemicals, i presume) and those who erroneously (perhaps) believe weed removal is resulting in an increase in slippage. It may be these folks who have engaged in such charming and potential lethal direct action as loosening of wheel-nuts.

Spraying is a choice, one that i am prepared to make in this circumstance. The consequences of not spraying would be to lose these forested areas with a great deal their constituent plant and animal biodiversity. If there are people in the community/stakeholders who feel so strongly that they will accept (or even welcome) those consequences, they need to be prepared for some tough conversations with people who share my response to the issue.

References

Gasniera C, Dumontb C, Benachoura N, Clair E, Chagnonb M, Séralini G. 2009. Glyphosate-based herbicides are toxic and endocrine disruptors in human cell lines. Toxicology 262  184–191.

Glyphosate Fact Sheet. http://environmental commons.org/glyphosate.pdf Accessed 10/06/2011.

Neurotoxicity, Immunotoxicity, and Endocrine Disruption with Specific Commentary on Glyphosate,
Triclopyr, and Hexazinone: Final Report.  http://gis.fs.fed.us/foresthealth/pesticide/pdfs/seratr01_43_08_04.pdf Accessed 10/06/2011.

Timmins S, Reid V. 2000. Climbing asparagus, Asparagus scandens Thunb.: a South
African in your forest patch. Austral Ecology  25 533–538