Now is the time to apply pre-emergent herbicides to prevent weed seeds from developing and growing into mature plants in our lawns. The reason we use pre-emergent herbicides for lawns is to prevent summer annual weed and some perennial weed seeds from growing and competing with our grass in the lawn. Some of these weeds include crabgrass, goosegrass, and dandelions. If the weeds have already germinated and are actively growing in our yards, the application of pre-emergent herbicides on these plants will not be effective.

For those who do not know how a pre-emergence herbicide works, here is a very short explanation. Most of our over-the-counter turf grass pre-emergent herbicides are in the Dinitroaniline family and further divided into the subfamily of Methylaniline group of herbicides. The common active ingredients of these products include trifluralin, benefin, pendimethalin, prodamine, and dithiopyr.

Dinitroaniline herbicides are very lipophilic (oil loving) and tend to concentrate in high lipid areas of very young seedlings and their corresponding new roots. It is in these areas they exert their toxic action. Therefore, susceptible young seedlings are killed by inhibiting cell division in root cells, which stops normal root growth. This inhibition leads to plant dehydration due to severely restricting the root system size, and function. Likewise, should the young seedling break the soil surface and start to photosynthesize, the herbicidal action stops this process, and the young seedling dies.

With few exceptions, these herbicides have no effect on existing plants, so they must be applied before seed germination. But like everything in life, there is an exception. Dithiopyr, trade name Dimension, and Prodiamine, trade name Barricade, can kill crabgrass if it is young, past the cotyledon stage. Cotyledons are young seedlings that have grown and have from two to three leaves. These two herbicides, have some residuals for continued pre-emergence activity on crabgrass. It does not last as long in the lawn as some of the other pre-emergent herbicides but there is some flexibility if you miss your window of opportunity to apply. Barricade, however, can only be applied to well- established lawns.

So, when do you put out the pre-emergent applications for lawn weed control? Well, it depends on many factors. What weeds are you trying to control? Where are you located in Ohio? Many times, turfgrass managers center their pre -emergent applications around crabgrass germination. Crabgrass typically begins to germinate around April 22 or a little later in NW Ohio. Additionally, weather varies from one spring to the next, and with it the timing of crabgrass germination. According to the turf grass team with the Ohio State University, crabgrass germinates when the soil at approximately 2 inches deep reaches 54° F for seven consecutive days. A phenology indicator plant that we can use to let us know when this has occurred is when the Redbud tree starts to bloom, and the Forsythia’s yellow flowers begin to fade. Applications need to happen before the above events occur. As of April 9, the average soil temperature at two inches deep is 52° F measured at OSU’s Agricultural Research and Development Center in Hoytville.

Pre-emergent herbicides do not last forever once applied to the lawn. Microorganisms and natural processes begin to gradually break them down soon after they are applied. If some products are applied too early, they may have lost much of their strength by the time they are needed. Pendimethalin, trade name Pendulum, has been the gold standard for lawn pre-emergence control. It offers 16 weeks of control and is a good product for perennial weeds. However, it should only be applied to well- established lawns. For lawns that are not well-established Dimension is a good product; however, control will be less than 16 weeks. A final note to keep in mind: pre-emergent herbicides cannot tell the difference between a good seed and a weed seed. Often, we hear complaints of newly seeded lawns that are not germinating and growing. The underlying problem was the same area was treated with a pre-emergent weed control product.

Mention of trade names or commercial products in this article is solely for identification purposes and does not imply recommendation or endorsement, nor is criticism implied of similar products not mentioned.

Always remember to read the label for the correct rate, turfgrass tolerance, lawn reseeding, and other specific instructions before applying any pre-emergence application to the lawn.