Crabgrass Preventer vs Weed Killer

We were recently asked why there aren’t any products that had lawn food, crabgrass preventer and weed killer in one bag. The gentleman thought this would be a great idea since it would save a lot of time. His question reminded me that there is a lot of confusion about the differences between weed killer and crabgrass preventer. Let me try to straighten this out.

Crabgrass, unlike most lawn weeds, grows from seed that the mother plant produced the previous summer. The mother plant dies completely over the winter never to be seen again. In early spring (starting about now) the seed germinate and start to grow. Crabgrass preventer is an agent that dissolves and forms a coating on the surface of the soil. ANY seeds that try to sprout and push a root through that barrier are killed. The best way to time you crabgrass preventer application is to keep an eye on a forsythia bush that is growing in your yard or neighborhood. You MUST have your crabgrass preventer applied before the flowers have completely fallen off the forsythia. It is a convenient coincidence that crabgrass seeds germinate at the same time that forsythias are flowering.

Spring weather can be fickle but, if you use the forsythia as your clock, your crabgrass preventer will always go on at the right time. Remember that crabgrass preventer forms a thin film on the surface of the soil so make sure that all your raking is done before you apply crabgrass preventer. If you rake afterward, you will scratch up the barrier and crabgrass will be able to grow. Dandelions, plantain and other lawn weeds are perennial plants. They come up from a root system that was established the year before and they survive the winter. Because they are coming up from roots not down from seeds, they are immune to the effects of crabgrass preventer. Lawn weed killers do not prevent weeds, they kill existing weeds. Weed killers (liquid or granular) are basically plant growth hormones that, when applied at the proper time, cause the weed to die from the shock that occurs when the weed tries to grow faster that it is able to. Sort of like an overdose of stimulants. If weed killers are applied too early in the season (when nigh time temperature are below 50 degrees) they will not kill the weeds.

During the cool days and nights of early spring, the weeds are not growing rapidly enough for the growth hormone to cause the level of shock necessary to kill the weed. Every June people complain that the weed killer they used is “no good” or “doesn’t work”. The real reason it didn’t work is because it was applied WAY too early. Sure, you can look out in the yard and see those buggars out there growing but it’s too early to get rid of them unless you want to dig them out. Bide you time and strike when the iron is hot (or when the soil is warmer). The best time to apply weed killers will be during late May or early June. If you apply it sooner you are wasting time and money.

Another important thing to remember about weed killer is that they are absorbed ONLY through the leaves of the weeds. Because of this, weed killers MUST remain in contact with the leaves of the weeds for a minimum of 24 hours (48 is best) for the weed to absorb enough to be killed. Many who use liquid weed killers will soak and soak an area to be sure they get the weed killer into the soil. Any weed killer that isn’t actually touching the leaves is wasted. Since leaf contact is the name of the game with weed killers, don’t mow the lawn prior to applying weed killer. If you mow, you’re removing the leaves that absorb the weed killer. Keep an eye on the weather and make sure that you don’t apply weed killer when rain is expected within 48 hours after the application. Even a quick shower is enough to wash the weed killer off the leaves, rendering it ineffective.