Hello,

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. 

http://www.thisland.illinois.edu/57ways/57ways_15.html

https://ir.library.oregonstate.edu/downloads/cv43nz30q

https://aggie-horticulture.tamu.edu/vegetable/files/2013/09/soil_solarization.pdf

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—-

https://www.extension.umn.edu/garden/yard-garden/weeds/annual-grass-and-perennial-weed/

http://www.msuturfweeds.net

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—-

http://msue.anr.msu.edu/resources/establishing_a_new_lawn_using_seed_e2910

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

http://msue.anr.msu.edu/news/get_an_early_start_on_vegetable_garden_weed_control

“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.