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The Loam Ranger - Plant poisoning
Dear Loam Ranger,
As a council officer I get called out to look at trees that the caller claims have been poisoned. What clues should I look for?
The deliberate killing of trees in either public or private areas without approval is illegal and is a particularly selfish and immoral crime, distressing to those who love trees and the green and leafy suburbs and towns in which they live. While prosecutions for cutting down trees illegally do occur, rarely is it possible to obtain the standard of proof required for prosecution for poisoning. We have, however, obtained direct evidence of the intentional killing of trees with herbicides and other means and would like to share our experience.
We get many requests, from both residents and councils, usually in suburban areas, to prove that their trees or other plants have been poisoned. These are often accompanied by more or less overt accusations directed at neighbours. While we have certainly had positive diagnoses of herbicide poisoning, the great majority of such requests either clearly do not involve herbicide or are too difficult to diagnose. This article summarises our experience.
Reasons for tree and plant death
Before we can conclude with certainty that a tree has been poisoned, we must consider the much more likely reasons for tree death first. Apart from anything else, herbicide testing is expensive. The most common causes of rapid decline and death in trees in particular are:
- soil level changes, particularly soil build up around the trunk
- cut and fill over more than 20% of the rootzone
- excessive wetness resulting from drainage changes causing more run-on uphill or periods of unusually high rainfall
- root disease, especially in wetter than usual conditions
- severe insect attack, either foliage or boring insects.
Finally, much less common is accidental or deliberate administration of poisons.
Symptoms of herbicide poisoning
The symptoms of herbicide poisoning can broadly be divided into two types, dependent on the type of herbicide:
Systemic herbicide (e.g. glyphosate)
- overall yellowing and wilting followed by leaf drop and death
- wilting and leaf drop within 2 weeks with a lethal dose
- lingering and possible recovery with a sublethal dose
- recovering buds stunted and yellow or whitish.
Contact herbicide (e.g. picloram; broadleaf weed killers):
- burning or blackening of contacted foliage or massive leaf drop
- where roots have been poisoned, symptoms only on the side where roots have been affected
- mushy black lesions in the bark, often at the crotch of trunk and branch
- weak, epicormic shoot growth in the case of a sublethal dose.
Where to look for signs of herbicide application
To deliberately kill a tree with herbicide is surprisingly difficult, and success usually depends on the efficiency with which the herbicide is applied.
Foliage application: Difficult with a large tree. Usually only the lower (accessible) foliage is affected, unless a very high concentration of systemic herbicide has been applied.
Stem application: Look for drill holes or cuts to the cambium (ringbarking). Herbicide that has been painted on works only on thin bark.
Root application: Look for signs of digging and cuts on roots or smelly soaked soil. Broad-scale spraying of soils is usually a very inefficient method of killing trees but may kill other vegetation.
What to test
When everything else has been eliminated, we can test both soil and foliage for herbicides. We have had successful diagnoses in the following circumstances:
- Foliage from a tall eucalypt that dies suddenly in front of a block of flats.
- Sulphate of ammonia buried next to damaged roots of a tree on a development lot (intended to kill by salt stress).
- Glyphosate (e.g. Roundup) in soil from beneath a group of trees in a view line.
- Picloram (e.g. Tordon) in drainage water from paved areas around street trees.
When we have diagnosed chemical poisoning, we have also found one or more of the following circumstances in association with the diagnosis:
- Accidental error in public open space maintenance.
- Rapidly growing trees in front of neighbours’ views.
- Properties for sale with views a major selling point.
- New neighbours moving in or redevelopment to take advantage of views.
- Public planting of trees in view lines, particularly in front of high-rise units.
- Redevelopment applications where tree preservation orders are given or anticipated.
- Neighbourly dispute over tree retention.
Never have we obtained a positive diagnosis where these factors do not occur, despite the most common accusation being neighbourly vindictiveness. Our advice is: do not assume the worst until all the most obvious things have been eliminated. You may just make neighbourly relations even worse with hasty conclusions!