How long to wait after overseeding before applying crabgrass preventer? - Ask Extension
I live in Denver and overseeded/ fertilized my lawn in October after core aeration. Can I apply a crabgrass preventer with fertilizer this spring? Is...
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How long to wait after overseeding before applying crabgrass preventer? #549621
Asked March 28, 2019, 11:31 PM EDT
I live in Denver and overseeded/ fertilized my lawn in October after core aeration. Can I apply a crabgrass preventer with fertilizer this spring? Is there any chance I would inhibit seeds that didn’t germinate last fall?
Jefferson County Colorado
Expert Response
Do you have a specific crabgrass preventer/preemergence herbicide in mind? And also, which species of grass did you plant last fall?
The general recommendation is to wait until new grass has been mowed three times before applying a preemergent herbicide. Conversely, after applying a preemergence herbicide and depending on the product, it is generally recommended that one wait 2-4 months before seeding again.
I am linking a factsheet for controlling annual grassy weeds in lawns which CSU Extension has put together: https://extension.colostate.edu/docs/pubs/garden/03101.pdf
And here is one more sheet on general lawn care: https://extension.colostate.edu/docs/pubs/garden/07202.pdf
Ok, thanks. The product is Scotts Turf Builder with Halts Crabgrass Preventer Lawn Fertilizer and the seed planted was a Kentucky bluegrass mix. Typically, I see crabgrass appear before I have needed to mow three times.
You were spot on earlier - the big question is whether the seeds which you overseeded with have germinated. Depending on when in October you seeded, I think it is certainly possible that the seedes germinated. Did you mow following the overseeding? I would also be curious to hear if your lawn is beginning to green up.
I did a little bit of research into 'Scotts Turf Builder with Halts...'. It appears that the active ingredient is Pendimethalin; this product is indeed likely to inhibit bluegrass seeds which have not yet germinated.
And just to add a little more information - If you were to apply Pendimethalin, the recommendation is to wait 8-16 weeks before reseeding again.
I overseeded the first week of October and I did mow once or twice afterwards. Yes, the lawn is beginning to green. I have attached pictures. I assume then that most of the seeds germinated and I should apply the Scotts after the third mow?
Because this past October was so warm (60s, high of 70), I think the seeds are likely to have germinated; however, from the picture we cannot see any seedlings and so you'll have to use your own judgement.
But now is an optimum time to apply preemergence herbicide.
While it is likely in my opinion that the seeds have germinated, there is another product which you might be interested in either for now or future use: Tenacity (active ingredient: mesotrione). This product has a minimal impact on bluegrass.
This CSU Plant Talk discusses Tenacity (mid page): http://planttalk.colostate.edu/topics/lawns/1531-bermuda-grass-lawns-gardens/
For a bit more information, I am including a document from OSU on this product: http://archive.lib.msu.edu/tic/holen/article/2012may14.pdf
For a bit more information, I am including a document from OSU on this product: http://archive.lib.msu.edu/tic/holen/article/2012may14.pdf
Lastly, I am including the following link (Iowa State Extension) for the pictures. The product works by affecting pigment which turns weeds white and kills them: https://www.extension.iastate.edu/turfgrass/blog/nimblewill-control-kentucky-bluegrass-turf
Great! Thanks for your help.
