The first year I grew flowers, I had no idea why it would be required to support my snapdragons. Then, I saw them snapping at the base when the wind picked up and realized why support was crucial. Ever see tomato cages? Well, that’s basically the same thing.
If you have just a few dahlia plants, you can put a wood stake in the ground and tie the plant to it when it’s about a foot tall. This works for a hobby gardener. As the plant gets taller, you may need to tie the stems more than once.
The option I typically use is netting. It has plastic squares that I use t-posts to stretch and encourage the plants to grow through them. The flowers will grow up through the netting and stay straight.
I also use the corral method for dahlias. Basically, there are posts on the outside, and I run two levels of string around the perimeter to keep them “fenced in” in a row. I’ve seen certain varieties succeed with only one level, and some have needed two to three levels.
Most seed packets will tell you if “support” is needed.