Application and effective use of herbicides

© Karen Foley 2026

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 the weed plant, move within it, escape detoxification, and finally disrupt a vital process essential to survival.

Herbicides enter plants through roots, stems, and leaves. They can be broadly divided into leaf-acting and soil-acting herbicides.

Leaf-acting herbicides may be further subdivided into contact and translocated herbicides. Contact herbicides, such as paraquat, kill only the plant tissue they touch. They are mainly used for controlling seedling or annual weeds and do not eradicate deep-rooted perennials unless used repeatedly.

Translocated herbicides, such as glyphosate, move through the phloem and are carried from the leaves to regions of active growth. Soil-acting herbicides are absorbed by roots and move through the xylem to actively growing tissues.

Because leaf-acting herbicides move from the site of application, they are often effective against deep-rooted weeds. Foliage must be present for these herbicides to work; applying them to bare soil is ineffective. Excessively high spray volumes causing run-off should be avoided.

Penetration and movement of herbicides within plants

Several barriers must be overcome between application and weed death.

Shoot penetration

Herbicides enter leaves through upper and lower surfaces, most easily through the lower epidermis and open stomata. Wetters can improve penetration, but unsuitable surfactants may damage tissue and reduce absorption.

Weeds under stress develop thicker cuticles, reducing herbicide uptake. This explains why glyphosate is most effective when weeds are actively growing.

Weather effects include:

• High temperature and low humidity – reduced control due to closed stomata and rapid drying.
• High humidity – improved control as droplets persist and stomata remain open.
• Warm but moderate temperatures – generally optimal conditions.
• Rainfall – reduces glyphosate efficacy but has little effect on paraquat.

Root penetration

Root uptake occurs through passive or active mechanisms. Light rainfall improves efficacy by moving herbicides into the root zone, while heavy rainfall may cause leaching.

Movement to site of action

Some herbicides remain near the point of contact, while others translocate via xylem or phloem. Simazine, for example, may be detoxified in tolerant plants.[*]

Improving results with key herbicides

Herbicides differ greatly in properties and mode of action. Label recommendations must always be followed.

Fluazifop-butyl (Fusilade)

Effective against perennial grasses such as Elymus repens. Best results occur in warm conditions with adequate soil moisture. A non-ionic wetter is essential.

Glyphosate (various products)

Target weeds must be actively growing with green foliage. Application is most effective when downward translocation is strong, typically late summer. Care is needed to avoid crop damage.

Use of surfactants with glyphosate

Cationic surfactants generally provide the greatest benefit. Alkaline water does not reduce glyphosate activity in formulated products.

Isoxaben (Flexidor, Gallery)

Controls only germinating weeds. Requires rainfall or irrigation after application and is ineffective on high organic matter soils.

Lenacil (Venzar)

Controls germinating weeds through root uptake. Crop safety margins are narrow and soil conditions are critical.

Metazachlor (Butisan S)

Effective only when applied before weed germination to moist, firm soil. Young crop foliage may be damaged in spring and early summer.

Napropamide (Devrinol)

Highly persistent and effective but must be applied in winter or watered in. Avoid soil disturbance after application.

Oxadiazon (Ronstar)

Forms a toxic surface layer affecting emerging shoots. Granules must not contact wet foliage.

Oxyfluorfen (Goal)

Acts by damaging cell membranes. Low solubility and strong soil binding minimise leaching.

Paraquat (Gramoxone)

Acts rapidly in light, disrupting photosynthesis. Complete foliage coverage is essential.

Simazine [*]

Absorbed only by roots. Effectiveness depends strongly on soil type and moisture.