Time to kill the weeds — but do homework first

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 these is to prevent summer annual weeds 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, the application of pre-emergent herbicides 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 are 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 act by inhibiting cell division (mitosis). Specifically, they inhibit the formation of cell walls and chromosome movement to daughter cells during mitosis. The cells do not complete division, and affected cells remain as single cells.

These 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; 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. Should the young seedling break the soil surface and start to photosynthesize, the herbicidal action stops the process, and it 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, nevertheless there is some flexibility if you miss your window of opportunity to apply. Barricade, however, can only be applied to well-established lawns. For lawns that are not well-established, Dimension is a better choice.

Other Pre-Emergent herbicides such as Pendimethalin, trade name Pendulum, have been the gold standard for lawn pre-emergence control. It offers up to 16 weeks of control and is a good product for perennial weeds. It, too, should only be applied to well-established, lawns. There are many other products that offer pre-emergence weed control in lawns.

Research on these pre-emergence products was obtained from Jared Hoyle, turfgrass specialist, Kansas State University.

Putting out the pre-emergent applications depends on many factors.

Many times, turfgrass managers center their pre-emergent applications around crabgrass germination. Crabgrass typically begins to germinate around April 22 in Northwest Ohio. According to the turf grass team with the Ohio State University, crabgrass germinates when the soil at approximately 2 inches deep reaches 54 degrees for seven consecutive days.

A phenology indicator 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 2 inches deep is 50 degrees, measured at the OSU Agricultural Research and Development Center in Hoytville.

Another way — though not always accurate — is to estimate the soil temperature by Lake Erie water temperature measured from Toledo five miles offshore and adding a couple of degrees. As of April 9, this reading was 47 degrees.

A final note: 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 is that the area was treated with a pre-emergent weed control product.

Regardless of the product you choose as a pre-emergent herbicide for your lawn always remember to read the label for the correct rate, turfgrass tolerance and lawn reseeding, before applying any application.