When is the Right Time to Apply a Pre-Emergent?

The short answer…

Before weeds germinate. 

Ok, maybe we need the long answer –

The window for applying the first pre-emergent application is broad. 

• If you have already had your first application, it was done at the right time. 

• If you have not had your first application, you are NOT too late. 

Summer annual weeds begin to germinate when soil temperatures reach 55 degrees.  Mid-March is the normal time for soil temperatures to reach 55 degrees in central Oklahoma.  We have experienced some years where soil temperatures didn’t reach 55 degrees until the last few days of March and we have experienced springs where soil temperatures reached the summer germination point in early March. 

How do pre-emergent herbicides work? 

Pre-emergent herbicides create a blanket over a lawn preventing seeds from germinating on the soil surface.  Pre-emergent weed controls are designed to stop weed seeds from germinating.  So, as long as they are applied before weeds germinate, the timing is perfect.   

To be on the safe side, the best practice is to have the first application of pre-emergent herbicides on your lawn by the first week of March.

Last week in covering February Lawn & Landscape Tips we started by pondering the question: 

Do you think of February as the end of winter?  

Or, do you think of February as the beginning of spring?

The reality is February can go either way.    

Sometimes February feels like winter is never going to end.  Other times February feels like spring has arrived with bulbs, shrubs and trees bursting to life. 

Because crabgrass is an annual grassy weed, without a spring pre-emergent even the best lawns will have crabgrass this summer.

With mostly 60 and 70 degree days in the forecast for the next few days we are on the edge of spring fever.  But, the first feeling of spring can quickly turn to lawn anxiety if you haven’t applied the first spring per-emergent yet.    

It is OK!  You haven’t missed the window for a pre-emergent.  Soil temperatures are still well below 55 degrees.

The timing of the first application is not about air temperature; it is all about soil temperature.

The current 3-Day Average Soil Temperature in the Oklahoma City area is 45 degrees, 10 degrees below the temperature required for summer annual weeds to start germinating.

If you have already had your first application, it was done at the right time.

And, if you have not had your first application, you are not too late!

With the occasional winter-like cold snaps February is known for we can expect soil temperatures to range between the mid 40s and low 50s for the next few weeks. 

So, when is the right time to apply a pre-emergent herbicide to prevent summer annual weeds? 

Anytime in January, February or early March before weeds germinate.

There is one very important factor when it comes to the effectiveness of a pre-emergent no matter when you apply it…. Water! 

A critical factor in the effectiveness of a pre-emergent application is water.

Watering in the application activates the herbicide creating a barrier.  A best practice is to water within 2-3 days of when the application is made.  But, quality herbicides still retain effectiveness as long as they are watered in within 10-14 days.    

There are a lot of factors that go into creating your best lawn, but the most critical at this time of year is the application of the spring pre-emergent before weeds germinate.

One indicator that soil temperatures are warm enough for summer annual weeds to start germinating is bright yellow blooms of forsythia.

Even though we have had plenty of spring like days, forsythia has not started to burst with yellow yet.

Two More Important Weed Control Details:

1. Pre-emergent herbicides are not designed to control existing weeds in your lawn.  If you have weeds in your lawn now, a post-emergent grassy weed killer and/or a broadleaf weed killer must be applied.  Weeds currently present in lawns now are winter annuals best prevented with the fall and early winter pre-emergent applications.  Control now is possible, but the sooner an application is made, the better the results will be.

Best Practice – A clean lawn in January and February is the result of effective fall pre-emergent applications.

The best way to have a weed free lawn in February is to never skip the fall pre-emergent applications.

2. The pre-emergent barrier gradually weakens over the season from foot traffic, mowing and periods of heavy rain.  The edges of the lawn are the first to break down.  Additionally, summer annual weeds continue to germinate throughout the season.  It is recommended that a second pre-emergent application be made mid-March to early May for season long prevention.

Best Practice – A second pre-emergent application 6-8 weeks after the first application will provide season long prevention of summer annual weeds.

So, when is the right time to apply a pre-emergent herbicide?

If you have received the first application of the season, you did it at the right time.

If you have yet to receive the first application, you perfect…there is still time to prevent summer annual weeds!

If you have any questions about pre-emergent applications, please give us a call!

Lorne Hall

Hall-Stewart Lawn & Landscape

(405)367-3873