Answer: Based on your description I am not certain what is happening to your plants. It is possible that they were planted incorrectly and never became fully rooted. Or with clay soil there is the possibility of a drainage problem, especially if your yard is in a low area where water collects. With a watering system, it is also possible to overwater unintentionally, or to water too little and too often thus encouraging surface rooting which makes the plant susceptible to a variety of problems. (Overly wet soil will kill many plants because their roots do not get enough air. And with overly dry soil, they simply wilt, stress and eventually die.) Too, there might have been a disease or insect problem or you might have even overfertilized and caused fertilizer burn. There are so many different causes for plants to die.
I would suggest you consult with your local county extension to run some basic soil tests (this will check how much fertilizer you need to use and also whether or not you need to lime) and also to try to evaluate whether or not your soil is well drained. They should be able to help you troublshoot why you are losing so many plants and based on knowing that, help you with selecting new ones.
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