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.
that does seem strange to me. I had dahlias that were similar in color and size and didn't have that problem. Have you googled to see if there are suggestions for a cure. The stems really look almost limp, rather than simply bending.
Found this online. Not recommending it because it is an Alaskan product. They seemed to feel it would add potassium and give stem strength.
You can use Mor-Bloom 0-10-10 which is an Alaska Corporation product at the rate of one quart a week per plant as a drench and this will stiffen your stems without giving the plants any nitrogen. The mixture rate is one tablespoon per gallon. You can also use it as a spray. You can find Mor-Bloom on the net or Lowe's and Home Depot now carry some Alaska products.