Gardening with Craig Everett
Horticulturist with OSU Extension in Wood County
Pre-emergent (PRE) herbicides prevent weed seeds from developing and growing into mature plants. For those who do not know how a PRE herbicide works here is a very short explanation. They do not keep the seed from germinating but kill the young germinating plant. With few exceptions they have no effect on existing plants, so they must be applied before seed germination.
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. Weed seed that has germinated and is actively growing, the pre-emergent herbicides will not be effective on these plants.
But like in everything in life there is an exception. Dithiopyr common name Dimension and Prodiamine common name Barricade can kill crabgrass if it is young, at a two to three leaf stage, and yet have some residual for continued PRE activity. It does not last as long in the lawn as some of the other PRE herbicides but there is some flexibility if you miss your window of opportunity to apply. Barricade can only be applied to well established lawns.
So, when do you put out the PRE application for lawn weed control? Well, it depends on many things. What weeds are you trying to control? Where are you located in Ohio? Many times, turfgrass managers center their PRE applications around crabgrass germination. Crabgrass typically begins to germinate around April 22 or a little later in Northwest Ohio.
Additionally, weather varies from one spring to the next, and with it the timing of crabgrass germination. According to Jeff Stachler Ohio State University Agriculture Educator in Auglaize County Crabgrass germinates when the soil at approximately 2 inches deep reaches 54° F. for seven consecutive days. An 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 flowers begin to fade.
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 sixteen weeks.
There are many other products that offer pre emergence weed control in lawns. Research on these pre emergence products was obtained from Dr. (Jared) Hoyle Turfgrass Specialist Kansas State University.
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, and specific instructions before any pre emergence application to the lawn.
