FYI French marigolds are considered a trap plant for root knot nematodes. They have to be planted densley in order to be effective. You do have to monitor closely because the nematodes will quickly infest the marigolds. If they are not removed and replaced then the nematodes will move to your vegetable palnts.
https://www.lsuagcenter.com/en/lawn_gard ... garden.htm
Scroll down to trap plants for specifics.
BTW I would not be overly concerned about planting flowers in your garden. The department of agriculture strictly regulates the types of chemicals used on nursery plants (been there done that). Any chemicals will be in small amounts and will not be detremental to your garden. When I grew bedding plants for my landscaping business the only chemicals used were fertilizers - water soluble. Having dealt with many wholesale growers fertilizer versus other chemicals is the rule of thumb. There may be some plants (generally not bedding plants) that require other chemicals when growing for wholesale or retail production. I think you would be surprised at how few chemicals are used in wholesale growing. Chemicals are expensive. Chemicals are harmful to employees. Reputable growers avoid chemical as much as possible. As long as nursery stock is not being destroyed by an insect infestation growers just leave them alone.
Before you say that I do not know what I am talking about let me tell you that I do. I was in the landscaping business long enough to know my growers and develop a relationship with them. Chemicals are an unwanted expense. Just good business.
Any way don't worry about planting nursery/store bought flowers. The strongest chemical you may get will be slow release fertilizer mixed in the soil.
Sorry - guess I went on and on a little too much