What kind of poppies do you have? How much are you watering? I am not an expert on growing plants in zone 7, so I am USUALLY inclined to think too much water, but other than that, without more info, I can't guess.

They are corn poppies, and I could be watering too much. In my zone it's best to plant poppies in the fall, but I planted in the spring. So I was concerned that they would get too hot too fast, and I have been watering alot. However, the ones I planted in our field next to our house also are turning yellow, and I've given them no attention at all. Once they germinated I basically let them thrive on their own. So back to square one I guess.

I don't have an answer as to why they are turning yellow BUT...

I am in zone7b/8a and my poppies always start to 'turn' right about now. I consider them a spring time plant along with the sweet peas. Once the weather starts to turn hot both poppies and sweet peas start to yellow and die. Sometimes I get another flush of poppies in the fall but mostly they are just spring flowers for me.

I originally thought they would re-seed in the same area and come up in the spring. However they seem to re-seed somewhere different each year -- I must be carrying the seed around on my clothing.

Thank you for the help! I think it may be the heat stressing them, it has gotten really hot here in the past couple weeks due to a heat wave. But now temp is getting back to normal. Also, I read recently that green veins/yellow leaves mean an iron deficiency, but that is usually alkaline soil, and my soil is acidic, so I don't know exactly what to think. The poppies havent bloomed yet, so hopefully everything will work out.

Yellowing of plant leaves, chlorosis, has many causes all related to soil nutrients. Your soil could have low levels of, Nitrogen, Potassium, Magnesium, Zinc, Iron, Sulfur, Boron, Copper, Manganese, or Molybdenum, or the soils pH could be off preventing the plant from uptaking something, or one or another could be in quantites that interferes with the uptake of others, a nutrient imbalance.
Why was the limstone added? What did a soil test say about adding some? Why was the rock phosphate added? What did a soil test say about adding some? Was a soil test done before anything was added?

I have not done a soil test, I have only done a ph test, and my soil is acidic. The soil in most of my yard is red clay except where it has been amended with compost. The rock phospate was mostly to help aerate the clay soil and the limestone was to get the soil to a higher ph level. I think I will get a soil test done, I have never had one before. Can you recommend a good place for testing?

Just to add to my last post--the flowers are growing near a cement wall, typically indicating a neutral or alkaline soil, but the test said it was acidic, and don't think its an iron deficiency because the other plants growing along with it (foxglove, candytuft) seem to be doing fine. I don't have nematodes which someone in my neighborhood suggested, because I have too many grubs for that! (Just added some milky spore to correct the grubs). Maybe a soil test is the only option?