Herbicides And Trees
By Dr Glynn Percival
2022-01-04
Herbicides And Trees
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. 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 are routinely used for landscape purposes within the UK to include publicly and privately owned land. Indeed, it is estimated over ₤100 million was spent on “weed and feed” herbicide products in 2014 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. Over the past five years, the Bartlett Tree Research Laboratory has been involved in several cases of malicious poisoning of mature trees.
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.
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. Commercially available herbicides that act by this means include:
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. However, drift contacting foliage can cause injury especially if the leaves are wet. Some pre-emergence herbicides may cause injury on trees if applied at exceptionally high rates. 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.
Image 1 Pre-emergent Herbicide Damage
Post-Emergence Herbicides
Post-emergence herbicides are applied after weeds have emerged. Post emergence herbicides include:
Selective herbicides
Selective herbicides control specific weed species. 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. 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 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)
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, however, occur. If contact does occur most tree species can tolerate this form of injury and recover without any major detrimental effects on growth.
Non-selective herbicides
Non-selective or total herbicides 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:
Image 2 Contact Herbicide Damage
Contact herbicides
Contact herbicides work by scorching or burning any foliar tissue they come into contact with. Examples include:
Pelargonic acid (Finalsan)
Acetic acid (New Way Weed Spray)
Contact herbicides whilst disfiguring to a tree 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.
Glyphosate
In all expert witness cases the BTRL has been involved with to date, glyphosate has been shown to be the “herbicide of choice” used to maliciously poison trees. Glyphosate is normally applied as a foliar spray where it is rapidly absorbed and transported through-out the entire tree. Glyphosate inhibits a plant enzyme involved in the synthesis of amino acids. Glyphosate can also be absorbed minimally through roots as glyphosate adsorbs strongly to soils and is degraded by soil microbes.
At normal diluted quantities it is highly unlikely that any form of glyphosate poisoning via the roots would exist. If neat glyphosate that does not represent normal use patterns was drenched around the root system of a tree however, then poisoning via the roots is possible. Depending on the size of the tree and weather conditions glyphosate takes 4-20 days to be completely distributed through-out a tree. Decline and death then occur over the following 2-8 weeks depending on weather conditions (hotter drier conditions promote a quicker tree decline and death).
Image 3 Low Dose Glyphosate Damage
Image 4 Drill Holes and Neat Glyphosate Pouring
DIAGNOSITIC PROCEDURES
Individuals may be able to confirm or discount the possibility of herbicide injury by examining plant symptoms and injury progression. Based on the authors experience the following approach is recommended when investigating any suspected/malicious tree poisoning.
1. Determine the timeline of tree decline and death. A rapid decline in tree health i.e., weeks or months is associated with herbicide application. No natural and/or climatic factor exists that would cause such a rapid decline in tree health and death within such a short time frame.
Whilst it is appreciated urban landscapes/private gardens can create an environment that is naturally hostile to tree biology (soil compaction, waterlogging, drought, de-icing road-salts) the timeline for tree decline and death caused by these problems is generally 2-5 years.
To put into context Dutch Elm Disease, the most devastating disease UK trees have ever encountered would take a minimum of 6 months to kill a mature Elm tree with the “norm” being 2 years
2. If the tree is not dead look for symptom patterns and document/photograph the severity of symptoms. Patterns of injury may help identify the type of herbicide used (Table 1). Photograph the tree from all angles. Continue to report and photograph symptoms through-out the growing season if possible. Take a large number of quality photos including close-up photos. Record the date and location of each photo. Aerial photos may also help to show the pattern and severity of herbicide damage.
3. Observe adjacent plants. Glyphosate contaminated soil will detrimentally influence surrounding plants i.e., if you suspect herbicide damage on a specific plant, it is likely that adjacent plants will show similar symptoms within the same time frame. Most herbicides used in urban landscapes can move in the soil, especially in the direction of water flow.
3. Inspect for drill holes and/or wounds exuding resin. These wounds tend to be black in colour. Measure the size of the drill holes to negate the possibility of tree decline by naturally occurring bark boring beetles.
4. Collect bark/shoot/twig samples*. Ideally tree tissue that is dying rather than dead. If the tree is dead collecting tissue samples is still worthwhile. Focus sample collection around any drill hole sites. Sample plant tissue from areas where symptoms are intense. Plant tissue samples should be packed in dry ice and sent to a reputable analytical laboratory immediately after sampling.
5. Collect soil samples* from 0.5 meters away from the tree trunk. Collect soil from around the trunk i.e., north, south, east, and west. It is extremely important soil samples are collected! Depending on soil type (clay, loam, sand etc.) glyphosate can still be detected in soils up to 200 days after application. Collect soil samples to a depth of 30 cm. Aim not to sample too deep because it may dilute any herbicide residues.
*For court cases positive confirmation of the actual herbicide used to kill/damage a tree requires laboratory testing of live/dying/dead tissue and/or soil. Use an officially recognized analytical laboratory for this purpose else submitted evidence may not be recognized by the court. Although chemical analysis is expensive it is the only means to provide a positive identification of some of the herbicides that damage plants. In addition, ensure continuity of samples can be proved from collection, to posting to receipt by the laboratory.
6. Observe surrounding trees especially if they are of the same species for any forms of pest and disease ingress. Such information will prove useful if asked in a court of law whether decline was caused by a naturally occurring biological agent rather than an herbicide.
REMEDIAL TREATMENT OF HERBICIDE INJURED TREES
Short term (3-4 weeks after herbicide application)
Irrigating and/or spraying the plant with water can aid recovery by leaching root-active herbicides away from the root zone. However, care should be taken not to wash herbicide residues into the root system of nearby uncontaminated plants. Likewise, activated charcoal or biochar incorporated into the soil may help bind herbicide residues and minimize injury.
Activated charcoal/biochar must be applied as soon as possible following herbicide poisoning, preferably at the early onset of symptoms. Activated charcoal/biochar is applied at 150 times the amount of the active ingredient per 4000 sq metre (1 acre) of the applied herbicide. For example, if 100g active ingredient of an herbicide were applied per acre, then 100 x 150 or 6kg of activated charcoal/biochar would be necessary to deactivate the residue. Herbicides which have shown to be effectively adsorbed by activated charcoal/biochar include: 2,4-D, atrazine, amitrole, dicamba, dichlobenil, chlorthal-dimethyl, diuron and picloram.
Applications of sugars to trees as a root drench or soil injection have been shown to confer a degree of tolerance to some herbicides by maintaining levels of leaf chlorophylls, carotenoids, and photosynthetic efficiency. In the case of herbicide poisonings apply sugar at 50 g per litre of water per square metre of soil under the canopy. Calculate the area under the canopy using the equation πr2 i.e., 3.14 x radius x radius. Consequently, if the area under the canopy is ten square metres, then drench with ten litres of water containing 500g of dissolved sugar.
Long Term
If branch dieback results after herbicide application, pruning should be delayed for a least a year to fully assess the extent of the injury. This will avoid additional pruning of dead branches that may result from continued decline.
If signs of tree recovery become apparent, then “light” fertilising in conjunction with mulching and sugar application are advisable to aid in the recovery process. If the poisoned tree bears an edible crop, eating the fruit or nuts is not advised in the first year after recovery.
If tree removal is required, then replanting the site should be attempted only after herbicide residues have degraded (further soil analysis maybe required for this purpose) or if contaminated soil is removed and replaced with fresh soil.
Other Images
