Hello Laura,

I love a mystery, and you have done a good job of detective work, trying to determine what might be going on with your zinnia and marigold plants.

The first three things we consider when there are issues with problems in the garden are soil, water (too much or too little), and light (too much or too little). If these are fairly equal in the areas you are growing your flowering plants, those that have thrived and those that have not, I can't help but wonder if the herbicide you mention is what is causing the problems. While you did not spray one of the areas that is showing a failure to thrive, perhaps that area was sprayed by the previous owner, or perhaps the herbicide landed on the third area by "drift."

The symptoms you describe could be caused by inadequate water or sunlight, but because you did not mention either of these as variations, I am suspecting they are not factors. The fact that your plants seem all to have come from the same sources would seem to rule out disease, though that could be a possibility, if there was a disease in the soil in those areas from previous plantings. The fact that only the plants in these three areas were affected also seems to rule out insect damage.

So, I am going to suggest that it is the herbicide that is the culprit. While herbicides used in home gardens are supposed to have a very short afterlife, it is possible that the herbicide is in the soil and is still doing what it is designed to do (kill or debilitate plants). The following website includes the symptoms of plants exposed to various herbicides.

https://hgic.clemson.edu/factsheet/herbicide-damage-to-landscape-plants/

This site (https://extension.psu.edu/persistence-of-herbicides-in-soil) addresses the length of time an herbicide can remain active in the soil, depending on the soil characteristics. I have included a quote of particular interest from the website below.

Soil Factors

Soil factors affecting herbicide persistence include soil composition, soil chemistry, and microbial activity. Soil composition is a physical factor determined by the relative amounts of sand, silt, and clay in the soil (the soil texture), as well as by the organic-matter content. An important chemical property of soil that can influence herbicide persistence is pH. The microbial aspects of the soil environment include the types and abundance of soil microorganisms present in the soil.

Soil composition affects herbicidal activity and persistence through soil-herbicide binding (adsorption), leaching, and vapor loss (volatilization). Generally, soils high in clay, organic matter, or both have a greater potential for carryover because of increased binding of the herbicide to soil particles, with a corresponding decrease in leaching and loss through volatilization. This "tie-up" results in decreased initial plant uptake and herbicidal activity. More herbicide is held in reserve, potentially injuring susceptible crops in the future.

In general, medium- and fine-textured soils with an organic matter content of more than 3 percent have the greatest potential to bind or hold herbicides and to injure sensitive rotation crops. Coarse- to medium-textured soils with a lower organic matter content (less than 3 percent), more typical of the soils common to Pennsylvania, are less likely to retain herbicides and to have carryover problems. Under the right circumstances, however, herbicide carryover can occur in any type of soil.

If the herbicide application is the reason for the problems, it sounds as if you can expect better results next year. Meanwhile, you could continue to experiment, perhaps planting something that is resistant to herbicides (many food crops have been manufactured to be resistant). If you believe something else is causing the problems, you are welcome to write to us again.

Good gardening to you!