Why are my flowers dying or failing to grow?

Question

Purchased the house a year ago. The previous owners had some flower beds but the flowers were not doing well. We added more flowers and added addition flower beds. When we went to dig the beds, we found the lots of clay. We add new soil in the new beds but not to the existing beds. We planted Coneflowers, Blue Salva, Marigolds, Impatiens, and Knock-out Roses. The Blue Salva died back at first but has come out strong in the last month. My marigolds are stunted and we lost at lot. The Coneflowers did not last long at all. The Coneflowers turned black and died. We live in Amarillo, Texas and I believe we fall in Zone 7. The house faces the north and flowers get full sun most of the day.

I know that we need to add topsoil to the beds but is there anything else that I need to do?

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Answer 1

Soil improvement is fundamental for plant health, so you are on the right track by adding topsoil. You can also improve the native clayey soil by incorporating compost or an enriched organic potting soil to increase soil organic matter content. Mulch the soil surface with the same.
https://www.domyown.com/dr-earth-home-grown-organic-potting-soil-p-9497.html

Soil testing and recommendations for mineral amendments based on actual deficiences found is the best way to go, or at least use a complete organic fertilizer like one of these:

http://www.soilminerals.com/Agricolas4-8-4_MainPage.htm

https://www.domyown.com/dr-earth-organic-all-purpose-fertilizer-p-9485.html