Reasons for the Failure of a Herbicide to Work Properly

If the herbicides you use aren't working properly, you may only need to look at the label a little more closely, and wait for the right conditions before spraying, to begin to win the gardener's perennial war on weeds. Herbicides differ in the kinds of plants they control and at what stage of growth. They won't work effectively if you apply them incorrectly or don't follow other required instructions. Unsuitable environmental conditions may also prevent them from working.

Unsuitable Herbicides

For effective weed control, use a suitable herbicide at the appropriate stage of weed growth. Some herbicides are selective, which means they only kill certain types of plants. For example, dicamba and fluazifop are selective: dicamba kills broad-leaved weeds but not grass, and fluazifop kills grass but has no effect on broad-leaved plants. Post-emergent herbicides are only for use on living weeds while pre-emergent herbicides prevent new weeds growing. Glyphosate is nonselective, killing most plants, but won't prevent new weeds appearing, while trifluralin kills weed seedlings germinating in soil but has no effect on plants.

Failure to Follow Instructions

Applying a herbicide incorrectly or failing to follow up the application as directed can lead to poor weed control. Post-emergent herbicides are most effective when you spray them over all leaf and stem surfaces -- one quick spray contains insufficient active chemical to kill most plants, especially if the plant has hairy leaves that prevent chemicals reaching a wide surface area. Pre-emergent herbicides require irrigation after application to activate them, so insufficient watering or rainfall reduces their effectiveness. They also only work within the top 1/2 to 1 inch of soil, so hoeing, digging or dragging objects across treated soil disrupts the weed-killing barrier.

Environmental Conditions

Herbicides require optimum conditions to work effectively. Some chemicals are active only within a certain temperature range, and if it rains soon after application, herbicides wash away before plants absorb them. Spraying on a windy day may mean the herbicide ends up on desirable plants, or you, rather than the targeted weeds. Dusty conditions prevent herbicides contacting plant surfaces, and some herbicides, such as glyphosate, won't work as well on cloudy days or if plants aren't actively growing. Topdressing the soil with fertilizer reduces the effect of pre-emergent herbicides, according to James Atland at Oregon State University.

Improving Herbicide Performance

Using a herbicide that contains more than one chemical, diluting it at the correct rate, adding a surfactant, and crushing or cutting plants before application all improve herbicide performance. Even nonselective herbicides don't kill all plants, so choose a product with two active ingredients to increase your chances of success, and don't try to stretch it further by mixing it with more water than recommended, if using a concentrated solution. When controlling hairy-leaved plants, add a surfactant, such as a few drops of dish detergent, that helps herbicides stick to plant surfaces, and damage plants through cutting or crushing before applying to help the herbicide penetrate plant tissues.

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Jenny Green


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