Hi Matt ,
I live in the midlands of South Carolina.
I didn't dig up my Dahlias for the winter.
I just cover them with pine straw.Now that it is spring I am taking some up to check or divide them .
Yesterday I already plant some of my Dahlias .
I plan to cover my dahlias with clear plastic containers this week as our forecast is for a low one night is near 39 as well.
The Dahlias I left in the ground over the winter are now coming up , here is photo .
I know i should have divided this plant but I don't really like this kind.
Larisa.

{{gwi:638673}}

can't help with the temperature thing

but Dahlias stay sturdy if they get full sun, and little additional watering if the soil is good enough.

I water mine only if their leaves droop and still hang in the evening

flower size:
if you want giant flower heads, don't top the plants. fewer stems produce larger flowers. Disbudding would help to give larger flowers.

But then you should probably stake each stem individually.
Another part of sturdiness is genetics, cut flower types produce long stems, so the plants are rather leggy than compact.

If you have several plants of the same cultivar, you could experiment and try topping and not topping them.

bye, Lin