Sedum with Crown Rot
have numerous Autumn Joy sedum. Some are doing beautifully while others are turning brown at the base and falling over. I can pull them out and there are no roots. This was during a period of little rain and I haven't watered them.
Sedums are susceptible to crown rot. This fungal disease causes the damage you described. Plants growing in poorly drained wet soil are most often affected. The fungus may have moved into the planting bed during wet spring weather. The extremely hot and dry weather in summer can further stress the plant making it more susceptible to this disease. Remove infected plants and replace with another unrelated perennial. Make sure the remaining plants receive proper care. You will need to water during extremely hot and dry weather. Water the top 6 inches of soil, allow the top few inches of soil to dry, and water again.