Using A Tarp To Kill Grass

Asked May 16, 2018, 4:25 PM EDT


I had asked a question about using a tarp to kill weeds so I can plant grass and was recommended black plastic at least 4 mil thick. The answer said it would be more efficient in the summer when the sun is hotter. About how long will this process take if I begin it now?

Also, they noted that some weeds would not be killed and recommended Round-up (glyposate) even though I said I don't use chemicals and have an organic garden nearby. I've heard that there is a strong vinegar solution that you can buy for gardening, would this do the trick? If so, what is the proper application?

What is the best method for removing the dead weeds and root systems? I know that tilling is not recommended anymore as it disrupts important elements of the soil. Is a landscape rake or spade fork sufficient? Do all the roots need to be removed or is there no threat once they are killed this way? If so can I just plant grass seed without taking out the weed debris?

Is there any concern of any chemicals from the plastic leaching in the soil? At some point in the future I may convert this space back to a vegetable garden.

Thank you for your time.

Alpena County Michigan

5 Responses


Your earlier question/answer is correct with one change. Research into solarization has shown clear plastic 1.5-2mils thick is more effective than black plastic. I could find no research that indicated the plastic was leaching chemicals into the soil. Organic farming does use this solarization method. Solarizing takes 4-8 weeks and should be done in the warmest part of the season. For the Alpena area that is typically mid-June through mid to late August. Here are a few references for you.

The perennial weeds, like thistle, will likely survive solarization and will need to be removed by other means. When using herbicides, Perennial weeds are best killed in fall when they are sending energy back into the root. They can be dug up any time of the year. It is best to remove any weed before it sets seed. Here are weed ID references—-

If the root is truly dead, then leaving it in the soil will actually help add nutrient back into the soil as the roots decompose. Remove the dead tops of weeds to reduce the amount of seed they may be holding.

There are several steps recommended when seeding a lawn. Fall seeding takes advantage of the typical rains and cooler temperatures. Key is to work in any soil amendments first, getting fresh viable seed, getting good ‘seed to soil’ contact, and keeping seed moist until it is germinated and growing. The seed to soil contact is an issue for you if you haven’t removed enough dead weed material from the surface. Here is a link with details, should you need it—-

Good luck with your project! Thank you for using Ask an Expert.

Thank you for your response. The last ask an expert response said black plastic, so unfortunately I purchased some as I had limited time to start this project. Will this still work in a semi-effective manner? Will it take significantly longer?

I did some more hunting and did find some dependable info on black plastic. Black won’t heat the soil as much as clear and takes about the same time to kill weeds; this quote is from

“Perennial weeds can be controlled before planting by using glyphosate or covering the area for one to two months with a 6-millimeter thick, black plastic. I like to put the black plastic down in the fall and by spring the area under the plastic is clear of weeds”

And this quote from a vegetable garden article by the Univ. Of Delaware Extension—-

“If you have a fallow section of the garden, even for a few weeks prior to the next successional crop, black plastic can kill existing weeds by excluding light and raising the soil temperature, so that section of the garden is ready for planting when you are ready to use it.”

So, you can still get weed kill, but you won’t necessarily get all the benefits of soil solarization such as killing fungal pathogens and weed seeds. If the black material isn’t returnable, go ahead and use it. Don’t worry about the mil thickness. I’m sorry for the confusion between ‘weed suppression’ and ‘soil solarization’.

When your new lawn area becomes dense it will naturally suppress weeds. Remember to mow no shorter than 3inches, so that the roots will become longer and the turf becomes thicker. Very short grass has very short roots, leading to a thinner lawn.

Thank you; I appreciate your time!

You are very welcome, glad to help