Environmental conditions can influence the growth and physiology of weeds and the action of the herbicide, as well as the interaction between the weed and the herbicide.

Water-soluble (hydrophilic) herbicides tend to be more strongly affected by environmental factors than oil-soluble (lipophilic) herbicides.

However, there can be large interactions between temperature and humidity and, unfortunately, past weed research has often not taken this relationship into account by varying and measuring both temperature and humidity within experiments.

Light

Light intensity has been found to affect the development of the plant cuticle and plant growth.

High light intensities can reduce the effectiveness of many herbicides by increasing the thickness and changing the composition of the cuticle. Thick cuticles usually impact on water-soluble herbicides more than oil-soluble herbicides.

High light intensities may actually increase the levels of control of soil-applied Group 5 herbicides by increasing plant transpiration, which increases xylem movement. Light is also required for mode of action Group 5 (for example, atrazine), Group 10 (for example glufosinate, Group 22 (for example, paraquat) and Group 14 (for example, carfentrazone). These modes of action work quicker when applied under high light intensity.

Some Group 1 ‘dim’ herbicides are rapidly degraded on the leaf and in the spray tank by ultra violet light, especially if applied in summer.

For some weed species, decreasing light intensity increases the ratio between shoots and rhizomes, which may lead to better control of perennial weeds through better herbicide interception and more phloem-transported herbicides translocating to the root system.

Low light intensities tend to reduce the rate of photosynthesis, which can affect the ability of the plant to regrow from the effects of a herbicide.

Temperature

Temperature has a major influence on herbicide effectiveness by affecting the development of the plant cuticle, plant morphology (that is, physical form and external structure) and physiology (internal processes).

For some herbicides, the rate of herbicide uptake increases with air temperature, although the total amount absorbed usually doesn’t change. For other herbicides (in particular glyphosate and glufosinate) increased temperature, especially coupled with low humidity, may result in leaf cuticle dehydration and hence substantially reduced uptake. 

Research on awnless barnyard grass (Echinochloa colona) in Australia has shown that, for both glyphosate-susceptible and glyphosate-resistant populations, higher temperatures (35°C /30°C versus 25°C /20°C) reduce the plant’s susceptibility to glyphosate by 2.5 times at approximately 70 per cent relative humidity.

Other physiological processes are also affected by temperature, depending on whether the plants have a C3 or C4/CAM carbon cycle.

High temperatures and low moisture levels cause the plant to stop transpiring by closing down its stomata. This causes a deficit of carbon dioxide in the plant, which shuts down the energy production within the plant and leads to a process called photorespiration.

C4 and CAM plants have an additional biochemical stage that allows them to store carbon from carbon dioxide as a four-carbon molecule to use for energy when the stomata are closed due to high temperatures. C3 plants grow best between 15°C and 30°C, with photosynthesis and growth declining above 30°C (lack of CO2). C4 plants keep photosynthesis operating above 30°C and at higher light intensities. Grasses and sedges make up about 80 per cent of C4 weeds.

Frost

Severe frosts can damage plant leaf surfaces, potentially reducing the uptake of herbicides.

Translocated herbicides, such as Group 1, 2 and 4 herbicides, can also have their effectiveness reduced when applied before or after a frost, most probably due to reduced phloem transport. Group 1 herbicides are very slowly translocated in the phloem, even under favourable conditions, so plants that have been stressed by frost will translocate the herbicide even slower. This is particularly evident with clethodim.

The effectiveness of any herbicides applied at lower rates (either intentionally or through poor application) will be greatly affected by any plant stress.

Humidity

Humidity has a major influence on herbicide uptake through three processes:

• amount of epidermal wax – plants grown under higher humidity have less epidermal wax than the herbicides must penetrate;

• aqueous path through the cuticle – higher humidity conditions hydrate the cuticle, making a continuous pathway for water- soluble (polar/hydrophillic) herbicides. Conversely, low humidity diminishes this entry pathway and reduces the uptake of hydrophilic herbicides such as glyphosate and glufosinate in particular. Oil-soluble herbicides are less affected by this aspect of humidity; and

• droplet survival (life of droplets) – low humidity conditions reduce droplet survival, decreasing t the amount of time that the herbicide has to pass through the cuticle and epidermis.

Adjuvants can have a big influence on droplet spread, droplet retention and droplet survival and thus the rate of movement of herbicides through the plant cuticle and epidermis. The type of adjuvant system required to assist leaf retention, spread and therefore optimal leaf uptake for a hydrophilic herbicide such as glyphosate is very different to the preferred adjuvant for a lipophilic herbicide, for example Group 14. These differing adjuvant requirements means that a compromise will be required if tank mixing.

Most adjuvants do not affect the actual inherent translocation capability or the toxicity of a herbicide’s active ingredient. However, ammonium sulphate (AMS) and urea ammonium nitrogen (UAN) have been demonstrated to improve uptake and cell membrane transfer for several weak acid herbicides.

Soil moisture

Moisture-stressed plants tend to have smaller leaves and thicker cuticles that contain more wax.

This can affect the amount of herbicide retained on the leaf, as well as the amount that is able to be absorbed into the leaf.

Moisture stress also leads to a decline in photosynthesis, respiration and transpiration in the plant. Herbicides reliant on transport within the xylem, such as those from Group 5, will be particularly affected.

All soil-applied herbicides will be greatly affected by moisture stress as they rely on the soil–water film to make contact with roots and emerging shoots of seedlings.

Herbicides that must be converted to an active form within the plant will also be strongly affected by moisture stress. Some examples of herbicides that are converted within the plant include esters and weak acid herbicides.

Wind

Wind, coupled with lower humidity can lead to faster drying of droplets, which reduces herbicide absorption.

Precipitation

Dew

Dew can reduce herbicide effectiveness by causing herbicide to run-off leaves. The level of runoff is influenced by leaf surface wettability, spray formulation (including adjuvants in the formulation or tank mixed) and application volume.

A dry leaf surface on some crop types may be able to handle more than 1000 litres per hectare without producing run off. Whereas run off from an already wet leaf surface can easily occur if the spray formulation has a low surface tension (that is a high surfactant loading) and the application volume is particularly high.

However, if spray can be retained on a wet leaf surface, the uptake of water-soluble herbicides will be maximised due to the fully hydrated cuticle.

Re-wetting of spray deposits by dew can also improve hydrophilic herbicide effectiveness as long as there is no run off from the leaves.

Rain

Rain has the greatest effect on herbicide application if it occurs soon after application.

The effect rainfall will have on the herbicide application is not just related to the total volume; rainfall intensity and duration are also very important factors.

Water-soluble herbicides tend to need longer rain-free periods, while oil-based herbicides need shorter rain-free periods because the oil-based herbicides are quickly absorbed by the waxes within the plant cuticle. Paraquat and diquat are the exceptions to this rule: while they are water soluble they are also taken up extremely quickly by plants.

Research in northern NSW that looked at the effect of glyphosate (450 grams/litre) rate and rain-fast period found that small awnless barnyard grass (E. colona) was effectively controlled with 255g active ingredient per hectare when simulated rain commenced 22 hours after application, 450g a.i./ha with rain commencing 10 hours after application, 675g a.i./ha with rain commencing four hours after application and 900g a.i./ha with rain commencing one hour after application.

The impact of rain on herbicide effectiveness varies greatly with weed species. Generally it is determined by the weed’s susceptibility to the herbicide (lower dose of herbicide needed to kill it) and the rain-fastness of the herbicide being applied.