I used Jobe's tomato spikes on mine as someone one the dahlia forum suggested. You could pot the tubers up in the spring to give them a headstart but with 200 plant that would take up a lot of space.

You could try reposting this on the dahlia forum for suggestions.
Hope this helps.
-Katy

Did the tomato spikes work?
Kathy

This is my first time using them, but I have blooms earlier this year than I ever have before. I have quite a few blooming or with big buds and I didn't start any early this year. So in short I would say yes. Some people on the dahlia forum swear by them but others say it turns the tubers to mush over the winter.
-Katy

Kathy -
I think everything is behind here in north central Oregon. But, hopefully Trish can answer! I just cut my first 5 stems of dahlias this weekend. I have about 600 planted!
Just keep hoping for warm nights! The days have been ok, but the cool nights really put a damper on everything, including my mood!
Good luck,
Wendy

Wendy and Katy,
Thank you for replying. Wendy- I'm sorry you're struggling to. I'm hoping for a nice fall, but it is harder to sell bouquets after kids go back to school and vacations end.
Good Luck and let me know if things pick up,
Kathy