To prevent your dahlias being blown over in windy conditions, use a garden cane, tied to the stem of the bloom with ‘rubberised’ wire, close to the bloom head, and near the other end of the cane. Do not tie it too tightly.
The length of the cane should be such that when the bloom is cut, at the bottom of the cane, it is a good length for staging in a vase, or arranging in a floral art or flower arrangement (as well as the correct length to fit in your vehicle when transporting to a show). This will make it easier when you are selecting and cutting blooms.
In the picture below two blooms of Eastwood Moonlight are supported, the one on the right has grown since the cane was added, and the support now needs to be raised closer to the flower head, as can be seen with the bloom on the left.