This is my first time here, and asked this question under the "Is it invasive" thread, then realized that I probably should have started a new thread, so I apologize for repeating.
I live in Grapevine, Tx, between Dallas and Ft. worth. A neighbor has seedlings (numerous!) of Cow Pen Daisy. I was considering planting a few in my perennial bed. I can deal with a prolific reseeder, but recently read that this plant produces chemicals that kill other plants that grow too close. Well...how close is too close? Would this be a problem for established plants, or just other seedlings or newly planted things? Does anyone have info on this?

I answered you on the other thread and copied to this one
I grow Cowpen Daisy and I love it. I have never heard of it being detrimental to other plants.
It is an annual and it grows fast but is very easy to pull, I just thin out what I don't want.
Since it grows fast it might shade out other slow growing plants nearby, but you can control that part.
I think the benefits in nectar and larval food far outweigh any drawbacks this plant might have.

Thank you! Several websites called it allelopathic, one was nrcs.usda.gov, but I appreciate your actual experience. I'm going to go ahead and try some in my perennial bed.

You are welcome, I hope you will enjoy it.
I had never heard of it being allelopathic, but then I know I haven't heard of everything.