Answer: Based on your description I would suspect poor rooting is the culprit. If they did not root out into the surrounding soil this would cause stunting and eventually death, often it is slow growth followed by a sudden demise much as you described. Sometimes plants fail to root due to soil incompatibility (eg planted into hard clay with a smooth sided hole where the hard clay acts as a container underground) or due to being rootbound at planting and the roots continue growing in that original circle and eventually the plant strangles itself. There are also some pest and disease problems that could cause them to fail to thrive and eventually die off as a result. You might want to work with your county extension and/or professionally trained nursery staff to try to determine what happened and take whatever steps are needed to prevent it happening with your replacements. I'm sorry you've had trouble with your plants.
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