Herbicides, or chemical weed-killers, always come with recommended rates of application, and it is forbidden by law to exceed these. Not only are herbicides toxic to humans and animals in stronger concentrations, but also they may damage the plants you're trying to grow. One strategy for prolonging the effectiveness of an herbicide is to take the recommended amount and divide it into two or three doses that can be applied a few weeks or months apart.
Types of Herbicides
Weedkillers can be divided into two categories, preemergence and postemergence. Those in the first category are effective at preventing weed seeds from sprouting. Those in the second kill the weed plants themselves, though their effectiveness is much greater against small, actively growing weeds than on larger, established plants or those that are dormant.
Effects of a Split Application
The recommended, labeled rates of application may be more than is needed in many situations, especially when weeds are small. Applying half or a third of the suggested quantity may eliminate weeds altogether or reduce them considerably, making a second application much more effective. It is not illegal to apply herbicides at a reduced rate as long as the total amount does not exceed the legal limit. The timing of split applications varies from product to product and from crop to crop.
One of the benefits of a split application is that less product may be needed to achieve the desired result. Another is the extended time of effectiveness, especially with preemergence herbicides. For instance, the chemical prodiamine keeps weeds from germinating for about two months. If two split applications are made, the weed deterrence is increased to four months. Another benefit is the reduction of chemical runoff into the water table. For instance, the University of Nebraska Cooperative Extension Service says that atrazine runoff from cornfields may be reduced by a third by using split applications.
A full application of weedkiller might be needed during a wet spring, when weed seed germination is high, or if there are large weeds with established root systems to kill. Many weeds germinate in cooler weather, and in abnormally cool summers an extra application may not be possible if you've exceeded the yearly maximum.
Treat all herbicides as toxic chemicals, and avoid contact with skin or breathing vapor. Read and follow all directions carefully. Every product has certain plants that it kills most effectively, so know the type of weed you want to remove before you choose an herbicide.