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Turfgrass Science

The winter of 2014 was so cold that we won’t have to worry about crabgrass this year (April Fool’s!)

Spring is finally in the air. With the warmer weather we will soon be seeing beautiful flowers and nice green grass, but we also get a few things most people are usually not too thrilled about. One of the things that most people dislike about this time of year is that crabgrass is going to start germinating soon. But how do we know when to get a pre-emergent herbicide out to make sure we nip the crabgrass in the bud?

April 15th has traditionally been the cutoff date for getting a pre-emerge out in central KY (a few days before that in western and southern KY and a few days after that for eastern and northern KY), but with years like this one where it stays cold much longer than normal, how do we figure it out?

Crabgrass will germinate when 1 inch soil temperatures are between 57-64 F so if you monitor soil temperatures, you will know when temperatures start approaching this range to make your application. Keep in mind that south and west facing slopes are going to reach these temperatures earlier than other areas and locations in cities will warm up more quickly than in the country. If you do not monitor soil temperatures, you can extrapolate temperatures for your area by going to the UK Ag Weather site and looking at data collected closest to where you live.  They are also monitoring soil temperatures at a 2 inch depth so their reporting will be somewhat cooler than what you would see at 1 inch (http://wwwagwx.ca.uky.edu/ky/data.php).

If you use growing degree days (GDD) you can approximate when crabgrass will germinate by using the GDD formula:

GDD = (max daily temp + min daily temp)/2 – base temperature

A couple things to understand about this equation:

1. Base temperature = 50 F

2. In the early season you may calculate negative numbers—these are just considered 0 GDD

A study out of the University of Maryland showed that crabgrass first germinates with as few as 42-78 GDD but peak germination will occur between 150 and 225 GDD. As of March 31st in Lexington, we have only accumulated a total of 28 GDD so if you use this model as a prediction of crabgrass germination, you still have some time to apply your pre-emerge. UK Ag Weather has also taken care of this for you and if you plug in your county, the website will calculate GDD for you where you live (http://wwwagwx.ca.uky.edu/dd.php).

The final method you can use to get an approximation of when crabgrass will germinate in Kentucky is to pay attention to what other plants are doing. For instance, when I see daffodils showing up, I know spring is on the way. One of the best plants we have as an indicator for crabgrass germination is forsythia. Although it is not always perfect, a good rule of thumb is to get your pre-emerge application out before the forsythia blooms begin to drop (not when blooms appear). This typically means you have between 10 to 14 days to get an herbicide out before crabgrass begins to emerge. Currently in Lexington, forsythia is just beginning to bud, but within the next few days I expect it will be in full bloom. If we use this as our time frame, April 15 would once again be the time to shoot for to have the pre-emerge applied.

Forsythia buds just beginning to break. Photo taken yesterday (3-31-14) in Lexington.

The same forsythia bushes as above in full bloom April 2013.

For information on crabgrass control, see “Weed Control for Kentucky Home Lawns” (www2. Ca.uky.edu/agc/pubs/AGR/AGR208/AGR208.pdf).

Newly germinated (left) and young crabgrass (right). Photo on the left from Virginia Tech Turfgrass Extension and photo on the right from Purdue Extension Entomology.