Herbicide uptake by plants
Foliar applied herbicides
Effectiveness is influenced by the distribution/composition of the spray droplets and the characteristics of the leaf surface on which the spray is deposited.
Herbicides in the soil
Both foliar and soil applied herbicides may be present in the soil and absorbed through plant roots.
Water soluble herbicides are absorbed in water through root hairs and the area just behind the root tip.
Coleoptile and young shoot absorption
Some herbicides (such as triallate or trifluralin) act mainly through root uptake with some shoot uptake. These products can be volatile and need to be absorbed quickly to be effective. Non-volatile shoot uptake herbicides (that is, diflufenican and metolachlor) rely on a moist soil surface for best efficacy.
Stressed weeds are harder to kill than healthy, actively growing weeds. Stress is caused by lack of moisture, lack of oxygen due to water logging, extremes of temperature, nutrient deficiencies, insect pests, disease, a sublethal dose of herbicide from prior applications or residues and mechanical damage (that is, from tillage, slashing or grazing). Once a weed has been subject to stress it will not be adequately controlled by rates of herbicide that would otherwise be sufficient to control an unstressed weed, even after the stressed weed has apparently recovered from the stress. Additives may help control stressed weeds, but can be unpredictable.