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I live in Ohio and I dormant overseeded in early January. So far, it has worked wonderfully. A lot of the new seed is coming up but some weeds have started to germinate as well. I normally put down some pre-emergent, like Halts, but I waited to let as much of the seed to germinate as possible. The snows and wet springs made this method of overseeding work fantastic in my opinion.

Does anyone know when is the earliest I could put the pre-emergent down? I would love to be able to use this dormant overseeding method every year, yet still prevent crabgrass and other weeds from popping up in late spring and all through summer.

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The time out period will be listed on the label but is commonly six weeks or more. With the climate changing quickly and causing ground temperatures to rise you have to be diligent to apply at the right time, too late does no good.

From this professional in Georgia

Crabgrass seed, you see, germinates 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-emergent herbicide.

Check to see if crabgrass is sprouting already and try a test patch that you have overseeded.

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It's tricky dormant seeding and then waiting for seed to germinate to put pre-emergent down. If it is warm enough for the new seed to germinate, it's warm enough for crabgrass to do the same.

A product such as Tupersan can be applied to prevent crabgrass without preventing the grass seed from germinating. It is expensive and only lasts for about 30 days, so then you could apply another round of pre-emergent but you probably will have to post kill crabgrass then.

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There are several modes of action in pre-emergent herbicides, but they all have one thing in common, which is targeting young, immature plants that are still dependent on the parent seed.

From my answer here:

I'd wait until the 3 leaf stage (where most of the seedlings have 3 blades each). This will ensure that they are finished taking nutrients from the old seed, and are mature enough to live independently. You could probably do it before that point, but when working with herbicides, safe is always better than sorry.

Doing that, you should also be in time to prevent the first crabgrass, as your seeded lawn should reach this stage far earlier in the spring, while the temperatures are still cooler.

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Sorry, I probably won't answer your question. My advice is to overseed after (weeks) you put your pre-emergent down. The point is that you cannot control when the weeds and crabgrass will germinate: when springtime temps turns ~50deg. If you overseed around that time, your grass will attempt to germinate at that time too. If you place a pre-emergent around then, it will stop both good and bad seeds from germinating.

  • Without a sprinkler system, I just don't see how effective overseeding in fall can really be. I would be betting on a rainy season, which would be 50/50. In Ohio, I am pretty much guaranteed that each spring is plenty wet and perfect for thickening my lawn. I am of the belief that a thicker lawn is more effective at reducing weeds than any chemical, so I plan on going this route each season; however, I am wondering how mature the new seedlings to be before I put the pre-emergent down. – Evil Elf Apr 30 '13 at 12:48
  • Hi Evil Elf, I overseed in the fall without a sprinkler system. Sure, I use the springer each day for the first 14 days. Also as an alternative, consider putting the grass seeds down after the pre-emergent, in the spring. That's what I do so. T – user446 May 1 '13 at 13:46

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