Last week I wrote about my war on weeds, but didn’t have enough room to discuss how herbicides actually work. They must come in contact with the plant. They must penetrate or be absorbed by plant tissues and must reach and disrupt a vital process or part of the plant. Some chemicals kill on contact. Others are soil-applied and move with water into the plant. Most systemic herbicides are applied onto the green leaves of the weed and are moved in the food-conducting tissues of the plant to wreak havoc on plant systems.
Different characteristic about plants affect herbicide performance. Leaf shape and surface can make a difference. Broad and flat leaves have more surface area for chemicals to land on, while thin and upright leaves such as grasses may be harder targets to hit with an herbicide. Most plants have leaf hairs that can interfere with uptake of chemicals. The more leaf hairs a plant has, the more difficult it is to get good contact between the herbicide and the leaves because the herbicide evaporates before it reaches the leaf surface where it can be absorbed.
When attempting to control plants with very hairy leaves, it is important to add an appropriate additive agent, such as a surfactant, to the herbicide to ensure the application actually reaches the plant tissues. This is an example of why it is easier to control plants in the seedling stage than in the vegetative or mature growth stages: seedlings have fewer leaf hairs.
Besides physical features of the plant, climatic factors such as water, temperature, humidity and wind also influence herbicide effectiveness. If you don’t apply enough water, either as irrigation or rain, after putting down a pre-emergent, you may see little success. Or, if you apply some herbicides and it rains too soon after application, there goes your success. Chemicals don’t work if the temperature is too hot or too cold.
Wind can blow your chemical away from the target plant.
Even products that are non-selective, supposedly killing everything green they touch, don’t kill all weeds. Selective herbicides designed to kill just broadleaf plants or just grass-like plants can sometime damage adjacent plants if the chemical volatilizes off as a gas or moves with irrigation water. Pre-emergents have to be applied before a seed sprouts or they don’t work. They also may fail if the herbicide layer is physically disturbed or the wrong product is used for the wrong weed.
Bottom line – always read and follow the label directions.