Lawn – Timing Pre-emergent Herbicide

One of the sourest phrases that can be directed at a child or adult is “I told you so!” Whether you touch a hot match, leave tools out in the rain or lift cinder blocks all afternoon, someone is usually ready to exclaim “I told you so!” when you complain about the experience.

In my line of work, I’m forced to reluctantly deliver “I told you so!” when gardeners ask me about their problems with annual weeds. The theory of controlling them is simple: maintain a vigorous turf so they can’t get established … and use a pre-emergent herbicide if they get ahead of you.

Bob K. wrote me recently to ask about controlling crabgrass. “We’re having pretty good success with our bermudagrass lawn but the crabgrass is overtaking it.” he related. “We put down pre-emergent (click for sources) herbicide twice this spring but now we have about fifty percent crabgrass in our lawn. Will the crabgrass die over the winter and not be a problem next spring?”

I didn’t use the “I.T.Y.S.” phrase when I replied…. because he probably followed my timing instructions last spring. For several years I have repeated the mantra “Summer weeds: March 15. Winter weeds: September 15.”

If you battle weeds in your lawn you know what I mean: Put out your crabgrass preventer in mid-March and your winter weed preventer in mid-September.

March 15, September 15.” It rhymes, it’s easy to remember and that’s what I’ve always recommended.

Bob’s question, though, made me do a little research. He put down a pre-emergent (click for sources) twice this spring and still didn’t get good control. Why?

I realize now that my rhyme will have to be discarded.

Crabgrass seed, you see, germinate when spring soil temperatures tend upwards from 50 degrees F. Weed scientists use 52 degrees F. at one-half inch deep as the trigger point for their application of pre-emergence herbicide.

Dr. Gerrit Hoogenboom oversees a network of 50 weather stations across Georgia for the University of Georgia College of Agricultural and Environmental Sciences. I was able to get from him the soil temperature data from Cherokee Town and Country Club in Dunwoody. What I found shocked me!

The data show that soil temperatures reached 52 degrees F. on Feb. 24 in 2000, Feb. 25 in 2001, Mar. 14 in 2002 and Mar. 4 in 2003.

My timing advice has been wrong! To get good crabgrass control in Atlanta you should apply the pre-emergent on March 1, not March 15. Otherwise the seed will sprout beforehand….and most pre-emergent chemicals do not control crabgrass seedlings.

In Bob’s situation, the crabgrass will die this winter – but the seed it has already dropped will sprout next April and he could have even more of the weed next year.

So what should he do?

All of the following products give excellent control of crabgrass in summer and annual bluegrass in winter:

benefin (Amaze, Balan, Crabgrass Preventer)
bensulide (Betasan)
oryzalin (Surflan)
pendimethalin (Halts)
dithiopyr (Crab-Ex)
prodiamine (Barricade)

The key is the timing. Pre-emergent herbicides MUST be applied BEFORE crabgrass seed germinate. If Bob and other lawn owners choose one of the products above and put it out on March 1, 2004. I think summer crabgrass control will be much better.

My second (September 15) date still stands. Put out a pre-emergence control for annual bluegrass, henbit or chickweed on that date and you’ll avoid my winter weed “I told you so!” next spring.

REMEMBER: Read all herbicide labels thoroughly to be sure the product can be used on your particular turfgrass and that it controls the weed(s) you are trying to manage.

annual bluegrass in bermuda

annual bluegrass in bermuda

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