Question: I have a lawn weed problem and I was wondering if I should use a weed killer and herbicide blended product?

Answer: The so called “crab grass/weed preventer with fertilizer” products are marketed as convenient to use but that convenience may come at a price. The first cost is the product itself, which can be quite expensive. Another cost may be the decreased health of your turf.

In order to effectively kill weeds, those products which contain fertilizer plus pre-emergent weed killers must be applied before warm season weeds (like crabgrass) germinate, which may be in mid to late February. Unfortunately, that is much too early to apply fertilizer for all grasses except fescue or bluegrass. When fertilizer is applied that early, the grass may start to grow much too soon if we have a couple of weeks of warm weather. Past experience has taught us (remember the Easter weekend freeze of 2007) that a late frost can do a lot of damage to warm season turf grasses. Bermuda, Zoysia, Centipede and St. Augustine grass are all tropical grasses that are much better adapted to heat than to cold. These grasses can be easily “tricked” into growing too early in the spring. This can be true for any warm season grass, but according to Jim Jacobi, Extension Pathologist, it is especially damaging to Centipede grass.

The more prevalent “weed and feed” products contain a fertilizer plus a post-emergent weed killer. These products are normally applied to young developing weeds. In most years these weeds are most susceptible to herbicides during the same time the grass is in the “green up” stage. This is the correct time to fertilize but it can be a very bad time to apply herbicides. The reason it can be bad is because warm season grasses are most easily damaged or stressed at this stage of their growth.

You may be wondering if there is a correct time at all to use “weed and feed” products and that is a very good question. Those products that contain a post-emergent weed killer and fertilizer blends may have a window of usefulness after spring green up, when late germinating summer weeds are small. For instance, if you use a pre-emergent weed killer (without fertilizer) in mid to late February it may wear off enough that some later weeds emerge. These weeds could be controlled with a “weed and feed” in late May or early June, after the grass has passed the stressful green up phase.

There is a correct way to approach weed control and proper fertilization but it involves an extra bit of effort. Pre-emergent weed killers may be used by themselves at the time just mentioned. Fertilizer should then be applied by itself in late April to early May. Fertilization type and amounts should be based on soil test results, not guess work, so this is a good time to test the soil to see if additional lime is needed and to find the correct fertilizer for your lawn.

I have had people tell me they have done it the “wrong” way for many years without a problem. My response has always been I know people who live very unhealthy lifestyles and live to a ripe old age, but we seldom hear about all those who die young because the former is the exception, not the rule. For instance, we regularly see soil test reports indicating excessively high amounts of phosphorus and potassium. These lawns may go for a long time with no problem but problems may arise at some point down the road.

Even if you never see severe problems, you are at the very least wasting fertilizer and possibly polluting ground water, streams and rivers. We also regularly see lawns that are fertilized too early get hammered by a late frost. If you fertilize too early the law of averages will eventually catch up with the luckiest gardener.

Lastly, we see turf with unexplained lack of vigor and poor health problems that may indicate severe weakening by a poorly timed herbicide application.

For information on proper care of your turf grass visit www.aces.edu and search our publications area.