Diagnosing Herbicide Damage To Trees
By Dr Glynn Percival
2020-11-02
Diagnosing Herbicide Damage To Trees
By Dr Glynn Percival
Bartlett Tree Research Laboratory, Shinfield, University of Reading
Herbicides are an integral part of landscape maintenance within UK towns and cities and within publicly and privately owned gardens. However, over the past few years the Bartlett Tree Research Laboratory (BTRL) has recorded an alarming rise in the number of malicious herbicide poisoning of trees. Indeed in 2019 alone BTRL staff were involved in five expert witness court cases concerned with malicious herbicide poisoning and subsequent death of mature trees. Aims of this article are to highlight key factors professionals involved with urban tree care should be aware of when diagnosing herbicide damage in trees with a focus on the herbicide glyphosate.
Introduction
Herbicides have now become an integral part of landscape maintenance within UK urban landscapes to include publicly and privately owned gardens. For example, it is estimated over ₤100 million was spent on “weed and feed” herbicide products in 2014 by private homeowners to keep lawns free of broadleaf weeds such as buttercup, daisy, groundsel, chickweed etc. Herbicide weed control is also still the primary method of vegetation management in industrial areas, car parks, under power lines, along highways and other non-cropland areas, as herbicides are generally more cost effective and efficient compared to hand, mechanical, flame or steam methods of weed control. Over the past five years, however, the Bartlett Tree Research Laboratory has been involved in an ever-increasing frequency of malicious poisoning of mature trees by members of the general public. Possible reasons for this increase include the ease by which members of the public can now purchase highly concentrated or neat forms of herbicides such as glyphosate via the inter-net. Previously, neat herbicides were only available to workers with recognized spray qualifications such as PA1 and PA6. Many YouTube videos now exist showing how to effectively inject herbicides, especially glyphosate, into trees and at what concentration to inject to ensure a total kill.
Types of Herbicide
Herbicides are classified by i) the kinds of plants (dicotyledons, monocotyledons) they target and ii) when they are applied (pre-emergent i.e. before weeds are present) or post-emergent i.e. when weeds are visibly present). Post emergent herbicides are further classified on whether they are selective (kill only specific weeds), non-selective (kill all weeds), contact, residual or systemic. Non-selective herbicides can then be further classified depending on their mode of action such as protein/lipid inhibitors, effects on plant hormonal system such as auxin or gibberellins etc.
Pre-Emergence Herbicides
Pre-emergence herbicides are mainly residual herbicides that create a chemical or physical barrier on the soil surface which disrupt weed seed germination or kill emerging weed seedling. The following herbicides are registered for pre-emergent control of weeds around trees in urban landscapes. The names in parenthesis are some of the trade names these herbicides are sold under:
Flazasulfuron (Paradise, Chikara)
Propyzamide (GemStone, Kerb Pro Flo, Kerb Granules, Propyz) – A residual amide herbicide
Napropamide (Devrinol)
Isoxaben (Flexidor 125)
Metazachlor (Sultan)*
* Also has post emergent activity
Pre-emergence herbicides are generally not absorbed by roots and therefore safe to use around most trees irrespective of species. Despite this, pre-emergence herbicide labels should always be consulted prior to use. Drift contacting foliage can cause injury on nearby trees especially if their foliage is wet. Some pre-emergence herbicides may cause injury on trees if applied at exceptionally high rates (Photograph 1). Herbicide damage to urban trees through the use of pre-emergent herbicides is, however, rare and where damage does occur most trees generally have the capacity to recover from injury. The length of soil residual activity varies depending on several factors such as photo-degradation (exposure to sunlight), soil microbial degradation, solubility and volatilization. As a general rule 6 to 18 months of residual activity tends to be the “norm”.
Post-Emergence Herbicides
Post-emergence herbicides are applied after weeds have emerged. Post emergence herbicides are either, selective or non-selective and can be foliar or root absorbed, contact or systemic.
Selective herbicides
Selective herbicides control specific weed species, while leaving the desired crop unharmed. Within urban landscapes selective herbicides are extensively used for grassland/lawn management. These herbicides kill broadleaf (dicotyledonous) plants but are safe on most grass (monocotyledonous) species. Selective herbicides are referred to as growth regulator or hormonal type herbicides because their chemistry resembles naturally occurring growth regulators in plants. These herbicides are the principal ones that are mixed with fertilisers to form the "weed and feed" products commonly sold in supermarkets and garden centres throughout the UK. Examples of selective herbicides registered for weed control around trees in urban landscapes include:
2,4-D (Agricorn D 11, Depitox) – A translocated phenoxy herbicide
Dicamba (IT Dicamba) – A hormonal growth regulator herbicide
Dichlorprop-P (2,4-DP) - A hormonal growth regulator herbicide
MCPA (Agricorn 500 11, Agroxone) – A translocated phenoxyacetic herbicide
Mecoprop-P (MCPP, Landgold Mecoprop-P, Clovotox) – A translocated phenoxypropionic herbicide
Clopyralid (Blaster Pro)
At rates recommended for broadleaf weed control, these herbicides are not root active and their persistence in soil is generally less than one month with the exception of dicamba (3-4 months). Consequently, selective herbicides seldom injure trees when applied at their labeled rates and frequency. If drift from these herbicides comes into contact with non-target tree foliage, leaf distortion and browning can occur. Likewise on warm days (>26oC) certain formulations of selective herbicides can volatilize into a gaseous state and cause leaf injury to near-by trees. However most tree species can tolerate this form of injury and recover without any major detrimental effects on growth unless exceptionally high rates of herbicide were applied. In this instance selective herbicides can be absorbed through tree roots and cause severe injury. Shallow-rooted species and deciduous hardwoods are most sensitive while conifers tend to be more resistant. Similarly, applying selective herbicides on a too frequent basis, for example to ensure a weed free lawn (Photograph 2), can cause decline and death of sensitive tree species such as Japanese maple, Malus and Sorbus.
Non-selective herbicides
Non-selective or total herbicides are used to clear waste ground, industrial and construction sites, railways and railway embankments etc., as they kill all plant material with which they come into contact. Non-selective herbicides are absorbed through the plant foliage and have little or no root activity. These materials are safe around trees as long as drift does not contact the foliage, green bark or suckers. Non-selective herbicides tend to fall into one of two classifications:
Contact herbicides
Contact herbicides work by scorching or burning any foliar tissue they come into contact with. Contact herbicides that are registered for weed control around trees in urban landscapes include:
Pelargonic acid (Finalsan)
Acetic acid (New Way Weed Spray)
Contact herbicides whilst disfiguring to a tree (Photograph 3) rarely result in tree death unless repeat sprays are applied over 2-3 years.
Systemic herbicides
Systemic herbicides are absorbed by foliage or roots and translocated through-out the entire tree. Systemic herbicides registered for weed control around trees and for stump killing in urban landscapes include:
Glyphosate (Roundup, Ecoplug Max)
Pyraflufen-ethyl*
Triclopyr*
* Can only be used in combination with glyphosate and not as stand-alone herbicides.
In most cases herbicides are sold as combinations of pre, post emergence, contact and systemic. Some examples include glyphosate + 2,4-D, clopyralid + triclopyr, dicamba + MCPA + Mecoprop-P, triclopyr butotyl + clopyralid.
Herbicide Chemistry/Mode of Action
It is beyond the scope of this article to cover in detail all the modes of action by which herbicides cause plant death. However, the five commonest modes of action include:
1. Hormonal herbicides such as 2,4-D, clopyralid, which mimic the plant growth hormone auxin causing uncontrolled and disorganized growth.
2. Mitosis inhibitors, which prevent re-budding in spring and new growth in summer.
3. Photosynthesis inhibitors, which block specific reactions in photosynthesis leading to cell breakdown
4. Amino acid synthesis inhibitors (glyphosate), which prevent the synthesis of amino acids required for construction of proteins
5. Lipid biosynthesis inhibitors, that prevent the synthesis of lipids required for growth and maintenance of cell membranes.
For detailed information see: https://www.intechopen.com/books/herbicides-physiology-of-action-and-safety/modes-of-action-of-different-classes-of-herbicides