Crabgrass Pre-Emergent Timing/Question

terps02(z7 MD)March 22, 2010

As spring starts and the Forsythias begin to bloom, habit tells me it's time to put down a pre-emergent to control crabgrass.

It seems everyone says that Forsythia blooming is an indication of crabgrass germination. It's that time in Maryland.

I'd like to discuss that. It seems to be that crabgrass doesn't do much until it's hot outside. I don't recall seeing the first evidence of crabgrass until June (at the earliest). So, why put down the pre-emergent in March? If someone were to put down one application of Halts, wouldn't it be better to wait until late May so the application would last all summer?

Does anyone have any pictures of crabgrass in April or May?

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garycinchicago(Z5 Chicago IL.)


A few points -

Halts (Pendimethalin) also prevents germination of poa annua, which germinates earlier than crab grass.

Pendimethalin has a residual of @ 90 days which takes you into June.

Forsythia bloom at or about a 55 degree soil temperature. Crab crab germinates once temperatures rise above 60 degrees - See
The forsythia bloom is just a visual aid to get off the duff and out into the yard doing spring time requirements.

    Bookmark   March 23, 2010 at 11:52PM
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dchall_san_antonio(8 San Antonio)

The beauty of using forsythia is that your conditions are different from the conditions in the next town over. Forsythia is what we call a "whisper of Nature." She is telling you, "It's time!" in Her subtle way. You just have to understand Her language.

    Bookmark   March 24, 2010 at 10:35AM
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terps02(z7 MD)

I understand that the forsythia blooms are a signal.

Let me ask another question.

I think the majority of people apply Halts (or any Pre-em) one time in early spring. Let's say Halts lasts 90 days. That means that most people have no crabgrass protection for the entire summer. I think most crabgrass grows in the summer (when it's hot). Thus, a crabgrass problem!

If a homeowner were to apply Halts one time per year, when should he/she do it? Like I said above, I think the majority of people do it right around now. I always have. Now that I think about it a little more, I think it's not the best timing.

If you had to rank these three options in order of best crabgrass prevention, how would you do it? Using Zone 7 as a common location, where the forsythia generally bloom right around the first day of spring.

A) Halts on the first day of spring
B) Halts on the first day of summer
C) Halts on first day of spring and summer

I say C, B, A.


    Bookmark   March 24, 2010 at 6:27PM
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andy10917(NY 6a)


I think that you may be crediting crabgrass with being a thinking creature, which it is not. Or you may be thinking that Pre-M "prevents" germination, which it does not - even though the name is "pre-emergent".

Crabgrass seeds germinate when there is adequate moisture (think April showers) and proper soil temperatures. The Pre-M does not "delay" the germination, and the crabgrass seeds can't really "outlast" the Pre-M's effectiveness. The Pre-M works by preventing root formation after the grass has actually germinated. With no root system, it wilts and dies before you even see it. No second chances, no waiting for the Pre-M to lose it's effectiveness. It's germinated and dead. The only crabgrass that makes it is if the conditions for germination don't happen before the Pre-M barrier breaks down. If you want to bet that we might not get 60 degrees and wet weather before 90 days from now, than put down more at the beginning of Summer. I'm saving my money to use the additional Pre-M in very late Summer to prevent the Poa Annua germination period (which will overwinter and be NEXT Spring's headache).

    Bookmark   March 24, 2010 at 8:31PM
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terps02(z7 MD)

Andy, Thanks.

Maybe I'm not thinking about crabgrass seeds and germination correctly.

How long after "germination" would you expect to see the first sprout?

Do you think that all crabgrass seeds germinate at approx the same time (within a few weeks) or do you think it's spread throughout the warm season?

I think that the first sprout happens within days of germination. I also think that most of the crabgrass that I see is in the mid to late summer. Which makes me think that the seeds germinate throughout the entire season, not just when the soil hits a certain temp.

Can you tell that I'm over analyzing? I'm good at that!

    Bookmark   March 24, 2010 at 9:07PM
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andy10917(NY 6a)

I don't think it is spread out by more than a few weeks when conditions are right. Nature doesn't work to spread things out - if crabgrass has a 90-120 day lifespan in which to compete for light, food and water and grow seed for next year, what would the value be in delaying it when delays decrease the chances of success? Think "natural selection" - when conditions are optimal, why wait? For the lower rainfall and stressful heat of June and July, when other grasses are mature and good competitors?

    Bookmark   March 24, 2010 at 9:34PM
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dchall_san_antonio(8 San Antonio)

Crabgrass also needs moisture to sprout. The problem in the early spring is many places get a little rain every day for a week. That's all it takes. If that happens in the summer, and your grass has bare spots, then the crabgrass will sprout then, too. If you grass has filled in and covered the bare soil, then the chances of seeing crabgrass sprouts are a lot slimmer.

    Bookmark   March 25, 2010 at 12:22AM
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I have jsut a few crbgrass plants in my lawn. Very few. NY has had a very wet spring. Is it too late to put a pre-emergent on? And second can it be applied to a wet lawn or even in the rain? Thanks

    Bookmark   April 26, 2011 at 7:16PM
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