Thanks for the question.

Sounds like you have a significant project in front of you, Cutting the sod and then flipping it over was not a bad idea. You will have retained the top soil and also contributed to the organic content of the site. I assume that you do not intend to plant any veggies in 2021 but want to prepare for a 2022 planting. Yes indeed, in this process you have probably moved weed seeds around. For this reason water the area extremely well before laying on the plastic. Seeds need water to germinate and you will then be encouraging them to do so ahead of the solarization process. I believe solarization with clear plastic is the best bet. Be sure that all edges are secure to the ground. Clear plastic will allow more sun light to enter and this will increase the temperature under the plastic. Essentially you will be creating a situation not unlike that seen in global warming. Keep this plastic on for at least six weeks. Some individuals leave the plastic on throughout the winter while others remove in before the first snowfall. It really is your choice as by late fall everything should have been killed. Take a look at the following:

http://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html - :~:text=In general, transparent or clear,trapping as clear plastic does.

https://www.csmonitor.com/The-Culture/Gardening/diggin-it/2011/0822/Soil-solarization-A-chemical-free-way-to-get-rid-of-weeds-and-unwanted-grass

https://extension.umn.edu/yard-and-garden-news/use-suns-energy-kill-weeds

When spring 2022 arrives, you may want to lightly rake up as many roots as you can. Applying an inch or two of top soil could also be done. It would make it easier to plant your veggie seeds. Rake this top soil into the underlying upturned sod. A soil test at this point would be a wise step as it would indicate what additional nutrients, if any, should be added. See:

http://soiltest.cfans.umn.edu/testing-services

I would hold off on using any pre-emergent herbicide in spring 2022 as this might interfere with vegetable germination. If any weeds appear, just remove them by hand.

Good luck on this endeavor.

This is so helpful, thank you! I have a couple of clarifying questions.

You suggest watering the soil deeply before laying down the plastic because this is necessary for seeds to germinate. Would this encourage the weeds seeds to germinate as well? I don't want that, correct? You are correct that I'm eyeing either a late-Fall 2021 or early-Spring 2022 planting, so I don't intend to sow any seeds intentionally until then.

You also suggest using clear plastic for solarization. The plot in question gets a good deal of sun each day so I think this should work well. However, is it too early to reach the temps needed? I read that temps beneath the plastic need to reach 115 degrees or more in order for weeds to be killed, possibly even as high as 140 degrees. If I lay down plastic down when highs are in the 70's in Minneapolis, will it get hot enough? If it's best I wait until we have consistent 80's, any suggestion on what I should do instead? Perhaps black plastic just to keep the light away in the meantime?

Thanks for your response and your excellent questions.

With regard to your first question, we do want weed seeds to germinate prior to solarization. Seeds themselves are not always destroyed by solarization, especially if they are an inch or two under the soil’s surface. By germinating prior to solarization, the seedlings resulting from these seeds will be destroyed during solarization. The worst time for their germination is when you start sowing your veggie seeds. This wetting will not stimulate all weed seeds to germinate. Some have to go through a period of cold temperatures. These may germinate next spring. This is why I indicated that you may still see weeds in spring 2022. However the more weeds that can be stimulated to germinate now and subsequently killed via solarization will mean fewer germinating weed seeds next spring.

As to your second question, the solarization process is based upon elevated temperatures under the plastic. The attaining of such temperatures is faster and more intense under clear plastic. Solarization is based upon the conversion of light energy into heat energy. The more light passing through the plastic means greater temperatures under the plastic. Simple example: here in northern Minnesota we have been having air temperatures in the 60’s to low 70’s. I have a thermometer inside my car. When I park my car in the sun and all windows are rolled up, the interior temperature of the car often attains 90 degrees or higher. I’m sure that you have experienced hotter temperatures inside your car than exist outside of the car. Think of the plastic as simulating an enclosed car parked in the sun Black plastic is black because the plastic itself absorbs light. This is why it has the black color. The sunlight that is absorbed by the plastic cover means that less light passes through the plastic and onto the soil. This leads to lower temperatures under the plastic. I would recommend putting the clear plastic on now and not use any black plastic. Don’t be too influenced by the outside air temperature. The concern should be what the temperature is under the plastic. In other words, the temperature under the plastic depends more upon the amount of sunlight passing through the plastic than upon external air temperature. If you go with clear plastic now, put a thermometer on the soil under the plastic so that you can see what the inside temperature reaches. You will be amazed.

Great questions and please send any further comments/questions to us. Good luck