© Karen Foley 2018
Anybody using herbicides should have some understanding of how they work or, in other words, their mode of action. Knowing the way herbicides act inside plants helps the person using them to apply them properly and to explain the results obtained.
To kill weeds, herbicides must reach the appropriate site or sites of action within the weeds where a lethal reaction will occur. A herbicide must enter into the weed plant, move by diffusion or other means within the plant, escape being detoxified by the weed and finally attack some vital plant process important to the weed’s survival.
Herbicides enter plants through roots, stems and leaves. They exert their toxicity in many different ways and can be broadly divided into leaf-acting and soil-acting herbicides.
Leaf-acting herbicides may be further sub-divided into contact and translocated herbicides, according to whether they act by contact only or are absorbed and moved (translocated) within the plant. Contact herbicides e.g. paraquat kill only those parts of the plant that they touch. They are used mainly as directed sprays (i.e. onto the weeds) for controlling seedling or annual weeds and do not eradicate deep-rooted perennial weeds unless used repeatedly. Leaf-acting, translocated herbicides e.g. glyphosate, move through the sap conducting tissue (phloem) and are carried with the sap stream from the leaves to regions of active growth. Soil-acting herbicides are absorbed by roots and move through the water conducting tissue (xylem) to the actively growing parts of the plant.
Because leaf-acting herbicides move from the site of application to other parts of the plant they are often effective against weeds with deep roots. Weed foliage must be present if these herbicides are to be effective and it is futile to apply glyphosate or paraquat to bare soil. Similarly, it is bad practice to apply leaf acting herbicides at very high volumes (weak concentration) which result in excessive run-off of the spray. Many leaf acting herbicides have no action through the soil and any chemical reaching the soil is wasted. Thorough wetting of all foliage with a translocated herbicide is unnecessary because the herbicide will move within the plant. With contact herbicides good spray coverage is essential for effective control.
Plant physiological processes
Some of the most Important physiological processes that are affected by herbicides are listed below:-
Penetration and movement of herbicides within plants
A series of barriers have to be overcome between the time the herbicide is applied and the weeds are killed.
Plants absorb herbicides both through their upper and lower surfaces. The lower surface (epidermis) is usually penetrated most easily and particularly through open stomata (leaf pores), natural fissures or other breaks in the cuticle (the waxy protective layer on the outside of the leaf). Sprays with the addition of suitable wetters readily enter plant leaves through open stomata.
If a herbicide enters a leaf through the cuticle, it enters by diffusion. When weeds are under pressure, e.g. through age, drought or poor growing conditions, the cuticle tends to be thicker and herbicides may enter the leaf less readily. This helps to explain why glyphosate works better when weeds are growing well and why some surfactants may be beneficial as they improve penetration through the waxy covering. Not all surfactants are useful with particular herbicides as some may damage cell tissue and this could impede absorption of the herbicide and translocation from the leaf.
All herbicides are affected by weather conditions. Some generalisations are:-
Herbicides enter plants via the roots just the same way as nutrients. This may either be by passive or active mechanisms (requiring energy). If passive, the herbicide enters with absorbed water and may continue to move with water through the plant mainly in the water conducting tissue (the xylem). Active uptake involves entrance into the protoplasm and movement via the symplast system.
Light rainfall usually improves the effect of root absorbed herbicides as the chemical is incorporated more effectively through the root absorbing zone of the weeds. Moist soil is essential for good results with simazine [*] and, more particularly, isoxaben. On the other hand, heavy and prolonged rainfall may leach herbicides, such as propyzamide, from the weed germinating zone, especially on light soil and reduce its herbicidal effect.
Movement from site of uptake to site of action
Herbicides applied to weed foliage can be divided into those that mainly affect sprayed leaves e.g. paraquat and those that are translocated (moved) to other parts of the plant e.g. glyphosate. Once inside the leaf, further movement of a translocated herbicide must occur. This usually takes place in the water conducting stream (the xylem) or in the sap conducting stream (the phloem). A few herbicides, such as fluazifop - butyl move in both streams - in the water conducting tissue when absorbed by the roots and in the sap conducting tissue when absorbed by the leaves.
Contact herbicides do not usually translocate from treated leaves but simple diffusion can account for limited herbicide movement in some cases, e.g. paraquat moves a short distance in the xylem stream.
A number of factors can affect a herbicide during translocation from the point of absorption to the site of phytotoxic action in sensitive tissue. For example, simazine is converted to an inactive form in the leaves of some woody plants and this, along with the greater mass of the crop plant compared with germinating weed seedlings, may well be more important factors than ‘depth protection’ in accounting for the tolerance of these plants to simazine.
Application of herbicides to plants grown under protection
Special care is needed where herbicides are used under glass or polythene.
In this environment, day
temperatures are higher, plants grow more rapidly,
leaf cuticle is thinner and foliage is softer.
In general, there is a greater risk of damage from
doses of leaf acting herbicides that would be safe in the open.
For example, Butisan S is increasing in popularity as a treatment for
nursery stock when foliage is firm and not actively growing.
Soft young foliage is liable to be injured and small scale testing
of this or other approved herbicides is necessary before general
applications are made on plants grown under protection.
In addition, there is a greater risk of damage in protected environments from the volatilisation of certain herbicides such as oxyfluorfen.
Improving results with some important herbicides
Herbicides vary greatly in their properties and mode of action. These are summarised in Appendix 1 and set out in more detail below. In all cases it is important to follow label recommendations carefully.
Fluazifop butyl (Fusilade)
This herbicide is effective against scutch (Elymus repens) and many other perennial grasses, Unlike most of the herbicides available for nursery stock , which only control weeds in the germination stage, fluazifop butyl controls established grass weeds selectively in broad leaved crops.
The herbicide is absorbed by both roots and shoots and gives best results if applied in warm weather when soil moisture is adequate. Grass control is likely to be poor if applied under dry conditions as uptake by roots and leaves will be reduced (thick cuticle, closed stomata, poor distribution in soil). It is essential to add a suitable non-ionic wetter such as Agral 90 or Triton -100.
Glyphosate (Round up, Touchdown, Stirrup and many other products)
Target weeds must be at the right stage for spraying. They should be healthy, have green leaves and be actively growing. Weeds that have died back (sap stream no longer flowing) or are suffering from drought stress (thick cuticle and closed stomata) are not susceptible.
The effect of glyphosate is greatly enhanced on many perennial weeds when it is applied at the stage when the weed foliage is fully expanded and the movement of food material in the plant is strongly in a downward direction. Glyphosate is highly effective against bracken fern when applied in late July/early August when food material begins to be transported from fronds to rhizomes. The herbicide effectively ‘hitches a ride’ with the plant food to the extremity of the root system.
While the late summer/autumn period is a good time to control many perennial weeds with glyphosate, extra care is needed to avoid damage to nursery stock through accidental leaf wetting for the same reason. Malus species are particularly sensitive and large trees can be severely damaged if leaf wetting occurs at this period.
In general, better results are obtained with low volumes of application around 11 gallons/ac (120 litres/ha) as with this volume there is little or no run-off onto the soil where glyphosate is not effective.
Use of surfactants with glyphosate
Many different non ionic surfactants, mineral oils, vegetable oils, polymers, acidifiers, and buffers have been added to glyphosate in an attempt to improve rainfastness or increase absorption. In general, cationic surfactants (ethoxylated tallow amines) such as Frigate, Ethokem or Excell are of most benefit. However Mixture B, a non-ionic wetter/spreader containing 500 g/l nonyl phenol ethylene oxide condensate and 500 g/l primary alcohol ethylene oxide condensate can give improved results against difficult weeds. Mineral and vegetable oils tend to be antagonistic to glyphosate
Although alkaline water can result in the breakdown of some pesticides this does not occur with glyphosate as the addition of the herbicide acidifies the spray solution. In practice, the pH of the spray solution has no effect of the activity of formulated glyphosate.
Isoxaben (Flexidor, Gallery)
This herbicide enters weeds seedlings through the hypocotyl (that part of the stem between the developing root and the seed leaves. This means it is safe on vegetatively propagated nursery stock that has already broken bud but it will only control weeds in the germinating stage. It is a highly insoluble herbicide (solubility only 1 - 2 ppm compared with simazine 3.5 ppm) so that it does not penetrate deeply into the soil.. As a result it is safe on light land but will not control deep germinating weeds. Because of its low solubility it will not creep round ‘clods’ so that if used on field grown crops, the land must be well prepared. For good results, isoxaben needs rain or irrigation after application to get it into the soil (5 - 10 mm within 10 days) otherwise germinating weeds will not be well controlled.
It is not effective on peat or soil with a high organic matter.
This root absorbed herbicide moves rapidly into leaves where it interferes with photosynthesis. It only controls germinating weeds - seedlings and established weeds are not affected. The margin of crop safety is not high and lenacil should not be used on substrates with a high sand content or on outdoor crops growing on soils lighter than loamy sand. Weed control will be poor if soil or substrate is dry but excessive rain or irrigation after application or high temperatures can result in crop damage.
Metazachlor (Butisan S)
Like isoxaben, metazachlor is very insoluble (solubility < 1 ppm) so that it is only effective against weeds if applied to firm, moist, clod free soil or substrate in a volume of water greater than 220 litres/ha (20 gallons/acre). Its effectiveness is reduced if applied to peat or soil with more than 10% organic matter. It is liable to cause plant damage if used on sands, very light or poorly drained soils. Young, soft crop foliage in spring and early summer is also susceptible to injury and stock sprayed at this period should be watered to wash off the spray residue from the foliage. Because of its insolubility, Butisan S is only effective when applied before weed germination.
This highly persistent herbicide gives good control of a wide range of species over a prolonged period. Immediately after application it begins to evaporate and is broken down by sunlight. In field crops this can be avoided by incorporation to 25 ml within 30 minutes. In nursery stock , where incorporation is neither desirable or feasible, napropamide should be applied during the winter months (November - March) or needs to be watered into the soil. All soil or substrate disturbance must be avoided after application.
Napropamide is not effective on soils with more than 10% organic matter. Although it is a relatively safe herbicide and has been used successfully on a wide range of nursery stock, plants with golden or yellow foliage tend to be more susceptible and should be subjected to small scale trials first.
Normally used as a granular formulation, this herbicide forms a toxic layer at the surface, is taken in by underground shoots and affects weeds and crop shoots as they emerge from the soil. Crop plants producing shoots from below the compost surface, e.g. Yucca filamentosa, should not be treated and care should be taken to avoid granules being funnelled into the heart of plants with a rosette habit of growth. Application should not be made to plants with wet foliage as the granules will stick to the leaves.
Ronstar should be applied to containers as soon after potting as possible and before the stock begins to produce soft active growth. Containerised plants and stock in the open ground should be watered shortly after application to activate the granules on the surface but granules should not be mixed into the soil as this will reduce the weed control effect. Even application is essential for good weed control.
Ronstar granules should not be used on substrate with a high sand or other non organic content.
Goal causes rapid phytotoxicity in plants by damaging cell membranes and causing tissue desiccation. The herbicide is absorbed by both young leaves and roots but is not systemic so that there is little or no movement from the site of uptake. Its solubility is very low (<1 ppm) and it binds strongly with the soil so that it is virtually immobile in the soil and leaching is unlikely to occur. Goal has good residual properties against germinating weeds but has also good contact effect on many weeds e.g. cleavers (Galium aparine) after emergence.
Under moist soil conditions (usually after rainfall), Goal can produce phytotoxic vapour or ‘soil lift-off’. This can damage young developing foliage in some situations, although well developed leaves are unlikely to be injured.
This herbicide is closely related to napropamide (Devrinol), the other residual amide used on nursery stock. Like Devrinol, Kerb is effective through root uptake and kills many broadleaved weeds in the germinating or seedling stage. Established grass weeds, including scutch and annual meadow grass, are also susceptible.
Best results are obtained with winter application to fine, firm, moist soil or substrate, as under warm, moist conditions the herbicide has little residual effect. For this reason, Kerb is usually applied to field grown stock between early October and late February to control scutch and other sensitive weeds, although applications at other times of the year can be effective against susceptible weeds such as annual meadow grass.
Gramoxone acts in the presence of light to desiccate the green parts of all plants with which it comes into contact. After application, penetration through the leaf surface occurs almost immediately. Absorption and speed of action is increased by high light intensity but Gramoxone may translocate to some extent under very dull light conditions so that it can also have a marked effect on weeds and green parts of crops during the winter.
The site of action of Gramoxone is in the chloroplast, which contain chlorophyll in the plant cells and is a key factor in the photosynthetic systems of green plants. Gramoxone works on the photosynthetic membrane system; the paraquat ion reacts with free electrons produced by this system to give the ‘free radicle’ form. Oxygen rapidly reconverts this free radicle and in that process produces superoxides which destroy cell membranes, allowing water to escape and desiccating affected tissue.
Except in winter, speed of cell destruction is usually too rapid to allow any measurable translocation. Consequently complete coverage of the foliage and growing points is required for annual weed control. Creeping buttercup is also well controlled by winter/ spring applications but most other perennial weeds will resprout from the base.
Gramoxone penetrates plant foliage so rapidly that rain falling within a few minutes of application (or light rain during spraying) has no affect on its herbicidal activity.
Simazine (many products)
Simazine is only taken in by plant roots and, for practical purposes, there is no leaf uptake. Solubility is very low (only 3.5 ppm) and the herbicide is strongly adsorbed on clay colloids and organic matter. The affect of simazine on crops and weeds is determined to a large extend by the soil type. It behaves very differently on different soil types being held at the soil surface on clays, peats and clay loams but can leach and cause plant injury in light sands and in sandy substrates.
At doses normally used for selective weed control, simazine is only effective when applied to firm, fine, moist soil or substrate. Such applications will only control weeds in the germination stage and so the soil must be clean and free from established weeds. From a purely technical viewpoint, simazine can be applied at any time of the year. For environmental reasons, applications should not be made in the late autumn or winter or in very cold weather when microbial activity is low and when there is a greater risk of simazine leaching into ground water.
Where simazine is used regularly, strenuous efforts should be made to prevent any surviving weeds from shedding seed as resistant strains of groundsel and other species will develop.
Calibration of knapsack sprayers for herbicide application
Accurate application is essential if herbicides are to be used effectively and this means that accurate calibration of sprayer and nozzles is imperative. A sprayer does not need to be calibrated each time it is used, but checks should be made periodically to ensure that accuracy is being maintained. A simple method of calibration is given in Appendix 2.
Appendix 1. Main herbicides used on woody ornamentals - 1997