My dahlias have weak stems - flowers are lovely, everthing is green and healthy but the stems are very tiny and weak, and cannot support the flowers - they fall over. I bought them from the store as a fully grown plant, and it wasn't drooping at the store 2 weeks ago. It sits in my sunroom where there is filtered light by 2 open windows (great airflow). No disease, and looks healthy, but the stems are so spindly, they cannot support any flower, no matter how small.
I water every other day, the soil never really dries out, and I also spray the leaves to keep them moist once a day.
They were not drooping at the store when I first bought the plant, and it was already a fully grown plant with blooms when I bought it.