Q.what is killing my impatiens?
I have soil mounded up around a large maple tree with impatiens planted all around. I do this every year and it is always thick, full, poppin’ and just fabulous. But this year they are dying off one by one. They didn’t fill out and I don’t know if something is eating them or what. I am faithful about keeping them watered and fed. I am very sad.

Certified GKH Gardening Expert
This article may offer some clues: https://www.gardeningknowhow.com/ornamental/flowers/impatiens/impatiens-problems.htm