Lawn care product suppliers offer a broad range of products that combine fertilizer with weed control. These combination products are generically referred to as "weed and feed." Though widely used by homeowners, their value is questioned by environmentalists and lawn care experts. Multi-tasking seems like a good idea, but optimum timing of the two products don't always match, more product than necessary might be used and money might be wasted.
Words like "preventer" and "pre-emergent" indicate that the product is designed to stop the germination of weed seeds. Many manufacturers offer a combination of fertilizer and pre-emergent herbicide and recommend it for an early spring application.
Selective herbicides kill broadleaf weeds, but not grass. The combination of selective herbicides and fertilizer is the most popular combination among the commercially available weed-and-feed products, with manufacturers recommending application over the entire yard in the summer and fall.
The proper time for pre-emergent application is in the very early spring, about the time the forsythia blooms in your area. If applied after the weeds have already germinated, the application will be completely ineffective. The University of Missouri Extension Service recommends the first application of fertilizer not be applied until rapid spring growth has slowed, about mid-May for much of the country. Products that combine pre-emergent herbicide and fertilizer will either apply the fertilizer too early or the pre-emergent herbicide too late.
Experts with the city of Austin, Texas warn that using selective weed control over the entire lawn wastes money and leads to run-off that ends up in our water supply. They suggest spot treatment of weeds and a separate application of fertilizer. Unless your lawn is completely over-run with weeds, you should spend less money, do your part for the environment and only spend a little extra time in the process. This practice is also suggested by the University of Missouri Extension in its guideline to integrated pest management.
The University of Missouri Extension suggests early fall as the best time to fertilize and also an optimum time to control broadleaf weeds. If weeds are heavily present in your lawn, this could be the one time of year when a combination product application could be beneficial and economical. If weeds are few and far between, spot treatment will be just as effective and more economical.
University of Missouri Extension experts write that the best weed control is a dense, healthy stand of turf. Following a program of carefully planned fertilization, aeration and irrigation, along with careful mowing techniques, will develop a lawn that needs little, if any, herbicide treatment.