Many of our retail stores have an herbicide aisle. It is very frustrating for homeowners to go through the time and expense of buying and applying a herbicide, only to discover that the area is still full of weeds. Worst yet to hire a private pesticide applicator and still have the weeds.

Human nature leads us to blame the chemical first, our actions second. Unfortunately, there are many reasons for weed control failure and most of the time, it is not the chemical’s fault.

First of all, where do these weeds come from? In most cases the weed seeds are already in the soil just waiting for the right environmental conditions to germination. When this happens, you have weeds. Also, weed seeds can be brought in on pet’s hair, bird droppings, shoes and clothing, running water, container grown plants, mulch, fill soil, wind and rainfall. In other words, it is harder to keep weeds out.

Weed control chemicals are called herbicides. Postemergence herbicides are weed killers that control weed after they come up. Rainfall or irrigation too soon after applying a postemergence herbicide can prevent the chemical from working. Most postemergence herbicides available for use in lawns and ornamentals are absorbed through the leaves, thus requiring rain-free period to be effective. The rain-free period requirement depends on the product used.

Preemergence herbicides, those that control the seedling before it emerges, timing and preparing for the herbicide application is the largest reason for weed control failure. The most common reasons for failure are that the preemergence herbicides are applied before the area is cleaned up or they are applied too late. Timing is important so preplanning matters.

Preemergence herbicides will not control weeds after they have germinated. To affectedly penetrate and provide a barrier to stop weeds from geminating, the soil must be void of weeds and debris.

Drought following the application of preemergence herbicide can result in a failure, since all preemergence herbicides require moisture for activation. Many preemergence herbicides can wait for rainfall a week or more, however, only after it is activated will it control the weeds. Supplemental watering may be needed.

The reverse situation of excessive rainfall is not a problem since the solubility of most preemergence herbicides is extremely low. Most remains in a very thin layer on top of the soil, moving only a fraction of an inch from the surface.

Longevity of herbicides in the soil is also affected by physical or chemical characteristics of the herbicide and its application rate. In general, higher rates of application will provide longer control. Conversely, applying too little will result in losing control of the preemergence herbicide sooner than expected. Thus, ensuring that the rate applied matches the rate needed for control which is just as critical for preemergence as for postemergence.

Not using the correct rate when applying the herbicide can affect the performance of the chemical. The rate, as well as the timing of herbicide application has to be matched to the weed control problem.

Herbicide labels usually specify a range of application rates because annuals and small weeds usually are controlled by lower rates than perennials and larger weeds. Roundup generally kills annual weeds at any stage of growth, whereas perennials are most effectively killed close to the flowering stage. However, do not wait so long that weed seeds set and the seeds are scatter.

Mistake in mixing the correct amount of the chemical or incorrect sprayer calibration can lead to failure with many chemical applications. Don’t guess by just pouring in what you think is a fluid ounce. Use the correct measuring tool, such as a teaspoon or other measuring device when adding the chemical. Calibrating the sprayer correctly also helps in applying the right amount of chemical.

Jim Coe lives in Lawton and writes a weekly gardening column for The Lawton Constitution.