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K-State Turf and Landscape Blog

Methods of Predicting Crabgrass Emergence

(By Jared Hoyle, KSU Turfgrass Research and Extension)

This morning when I cranked my truck up and headed to the office it was 25 deg F.  Just a couple days ago it snowed.  Just before that it was 80 deg F.  What is going on with the weather? (Every time I make that statement, someone says “Welcome to Kansas!”) With the fluctuating weather and temperatures I get the question “What date do I need to put my preemerge herbicide out on?”

Well this is a trick question because there is no correct answer for it.  Here are some of my thoughts on preemerge timing.

As soon as that first warm front comes through everyone gets excited and ready to work in their lawn.  I have even heard the comment that spring and summer are going to come early this year.  Are they right?  I don’t know but it is something to think about.

But as soon as we get excited about the warm weather we have another cold snap.  So we got all excited and talked about weeds germinating early and needing to get out our preemergence herbicides because the forsythias are blooming. But what happens when it gets cold?  Any crabgrass that “might” have germinated is now dead because of the cold temperatures. So this brings up a lot of discussion between myself and some colleges. Here are just some of the questions we are asked and a couple of comments.

Why do we recommend around April 15th for most of Kansas to put out a preemerge in our lawns?

– April 15th is about the time that we have our “last” frost/freeze. (This also shifts to April 30th in Western/North Western KS and to April 1 in Eastern/South Eastern KS.) This goes along with our concept that I was talking about before.  Even though it may be getting warm we are still might have a cold snap that would end up killing the crabgrass if it emerged. This is the same concept of why we don’t plant crops until after this date as well.  (Something I don’t know anything about but that is what I figure…..)

Should we use soil temperature at 1″ to predict when to put out preemergent herbicide applications?

-Soil temperatures are a great way of determining when to apply your preemerge herbicide.  Scientists say when soils reach a daily average of 55 deg F for about 5 days at a 1″ soil depth then it is time to put out your preemerge.  Well say you go out and measure your soil temperature at 1″  you are not going to get a daily average, you are getting a single point in time.  Does that really represent what is going on?  What if you are maintaining many different properties.  It is not practical to obtain that information.  Good news is that KSU has a website (http://mesonet.k-state.edu) that you can click your closest weather station and get daily maximum and minimum soil temperatures where you can then calculate a daily average.  Bad news it only give you a 2″ or a 4″ soil temperature.  But using the 2″ soil temperatures are going to give you a better idea of the daily averages then going and taking one measurement.

The forsythias are blooming but my redbuds aren’t. Do I still put my preemergent herbicide out?

– This is called phenology – the study of cyclic and seasonal natural phenomena, especially in relationship to climate, plant and animal life.  Many people believe that when the forsythias are blooming then crabgrass is germinating.  That is not 100% true.  When forsythias are in full bloom then we need to be getting “ready” to get our preemerge applied.  Even though we see that this is a good indicator, Dr. Fry and others reported in a study that ornamental plant flowering is not always a good way to predict crabgrass germination and emergence. (Fry, J., S. Rodie, R. Gaussoin, S. Wiest, W. Upham, and A.Zuk. 2001. Using flowering ornamentals to guide application of preemergence herbicides in the Midwestern U.S. International Turfgrass Soc. Res. J. 9:1009-1012.) There are some things to consider when utilizing phenology for crabgrass germination and emergence.  Not all forsythias will bloom at the same time.  It determines where that plant is located in the landscape.  There are micro climates in the landscape.  Think about plants located in roadway medians.  They are typically warmer due to cars and the concrete and asphalt in close proximity.  The same thing goes with crabgrass germination.  Crabgrass will germinated sooner in areas that are warmer, for example (next to sidewalks, bare ground, etc.).

Crabgrass emerging in bare ground earlier than turf

I have heard about growing degree days to predict crabgrass germination. What is that?

– Growing degree days (GDD) use air temperatures instead of soil temperatures within a formula to get a cumulative number of growing degree days.  Using base 50 deg F, once you get to about 200 GDD then crabgrass will start to germinate.  Don’t want to calculate GDDs, don’t worry there is a website that will do it for you. http://www.gddtracker.net  Just enter your area code and click on crabgrass germination on the right side and it will give you the total GDD. It will also show you a prediction for the next couple of days too. GDD do not go backwards, they only accumulate.

So to put this all in perspective using the prediction methods mentioned above this is what we got;

Today is March 14th so using the calendar model for Manhattan we would not put our preemerge out until closer to April 15th for Manhattan.

Using the 2″ soil temperature method because that is what is available to us our daily average soil temperatures (Deg F) have been;

9-Mar 47.6
10-Mar 45.4
11-Mar 43.65
12-Mar 41.15
13-Mar 39.65

With this method we would assume the 1″ soil temperature is higher but we are not to the 55 deg F mark yet. So we are getting close to putting out our preemerge herbicide.

In my back yard my forsythias are blooming.  So that means if I am using the phenology method I need to be getting ready to apply my preemergence.

Lastly, using the GDD model we are at 85 GDD. So I still have some time but getting close.  Remember though that you can get more than one GGD in one calendar day and if I look at the future (prediction on the GDD Tracker website) on March 20th Manhattan will be at 111 GDD. So just in 6 days we would have gained 26 GDD.

Now these rules, concepts, ideas are not bullet proof but it is something to think about when planning your lawn care program.  There is not magic date for anything that you do to you lawn.  You need to make sure you keep up with the temperatures, soil temperatures, precipitation, etc.  The more you know what is going on with your turfgrass the better you will be able to predict crabgrass preemergent applications.  Use more than one method.  This is going to give you the best idea of what to do!

Now by this weekend we are going to get a little warmer but again looking at the 10 day forecast it might get cold again…. Just something to think about.  I hope this got you thinking as a turfgrass manager and how this is going to help you choose products if you are going to have to get applications out earlier or even later after crabgrass has emerged.

Always remember to READ THE LABEL for the correct rate, turfgrass tolerance, and specific instructions before application!!!

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