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how to kill tall weeds in back yard that won't be harmful for the lawn I want to plant #765654

Asked August 04, 2021, 9:59 AM EDT

My new residence is almost finished. the back yard is a mass of tall weeds. They will be cut down tomorrow and the site will be leveled for future planting. Is there a spray I can use today that will keep the weeds from coming back, but not keep the lawn I want to plant from growing? THANK YOU

Douglas County Colorado

Expert Response

Hi, Sara,
A commercial herbicide containing Glyphosate will kill all actively growing plants. However, it needs a day or so to work, and if your weeds will be cut down tomorrow, it likely will not have time to kill the plants prior to mowing. It is also a very potent chemical, and needs to be applied in a narrow temperature range to avoid becoming an aerosol, killing other plants and trees in the area.
I'd suggest that you wait until the area has been mowed and leveled. Then proceed with soil preparation (see link, inserted.). New construction sites often are compacted as the contractor finishes the job. Good for the foundation - bad for the roots of plants. Consider adding good top soil from a reputable garden center, and tilling the area to loosen the soil before laying sod or planting seed.
https://planttalk.colostate.edu/topics/soils-amendments-composting/1605-soil-preparation/

Hope this was helpful!

Kind Regards, Replied August 04, 2021, 12:59 PM EDT
Hi, Sara,

A commercial herbicide containing Glyphosate will kill all actively growing plants.  However, it needs a day or so to work, and if your weeds will be cut down tomorrow, it likely will not have time to kill the plants prior to mowing.  It is also a very potent chemical, and needs to be applied in a narrow temperature range to avoid becoming an aerosol, killing other plants and trees in the area.  

I'd suggest that you wait until the area has been mowed and leveled.  Then proceed with soil preparation (see link, inserted.). New construction sites often are compacted as the contractor finishes the job.  Good for the foundation - bad for the roots of plants.  Consider adding good top soil from a reputable garden center, and tilling the area to loosen the soil before laying sod or planting seed.


Hope this was helpful!

Colorado Master Gardeners of Douglas County

2021 Virtual Helpdesk
Questions answered daily by email!


On Wed, Aug 4, 2021 at 10:00 AM Ask Extension <<personal data hidden>> wrote:

Hi Douglas County Master Gardeners,

Question #0035585 has been assigned to you by CSU E.

how to kill tall weeds in back yard that won't be harmful for the lawn I want to plant

My new residence is almost finished. the back yard is a mass of tall weeds. They will be cut down tomorrow and the site will be leveled for future planting. Is there a spray I can use today that will keep the weeds from coming back, but not keep the lawn I want to plant from growing? THANK YOU

From: sara valentine
Submitted: 08/04/2021 9:59 AM


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Kind Regards, Replied August 04, 2021, 1:04 PM EDT
THANK YOU so much for your prompt answer! So glad to have your good advice : )) 

On Aug 6, 2021, at 8:04 AM, User <<personal data hidden>> wrote:



Begin forwarded message:

From: "Ask Extension" <<personal data hidden>>
Subject: Re: how to kill tall weeds in back yard that won't be harmful for the lawn I want to plant (#0035585)
Date: August 4, 2021 at 11:04:21 AM MDT
To: "sara valentine" <<personal data hidden>>

Dear sara, here's the response to your question:

Hi, Sara,


A commercial herbicide containing Glyphosate will kill all actively growing plants.  However, it needs a day or so to work, and if your weeds will be cut down tomorrow, it likely will not have time to kill the plants prior to mowing.  It is also a very potent chemical, and needs to be applied in a narrow temperature range to avoid becoming an aerosol, killing other plants and trees in the area.  

I'd suggest that you wait until the area has been mowed and leveled.  Then proceed with soil preparation (see link, inserted.). New construction sites often are compacted as the contractor finishes the job.  Good for the foundation - bad for the roots of plants.  Consider adding good top soil from a reputable garden center, and tilling the area to loosen the soil before laying sod or planting seed.


Hope this was helpful!

Colorado Master Gardeners of Douglas County

2021 Virtual Helpdesk
Questions answered daily by email!


On Wed, Aug 4, 2021 at 10:00 AM Ask Extension <<personal data hidden>> wrote:

Hi Douglas County Master Gardeners,

Question #0035585 has been assigned to you by CSU E.

how to kill tall weeds in back yard that won't be harmful for the lawn I want to plant

My new residence is almost finished. the back yard is a mass of tall weeds. They will be cut down tomorrow and the site will be leveled for future planting. Is there a spray I can use today that will keep the weeds from coming back, but not keep the lawn I want to plant from growing? THANK YOU

From: sara valentine
Submitted: 08/04/2021 9:59 AM


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The Question Asker Replied August 06, 2021, 10:12 AM EDT

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