Your question is tough to answer. I start moving my dahlias into the poly house April 1. I put one or two out there to see if they can take the weather inside the polyhouse. I start planting out side May 1 doing the same thing. One or two plants to see if they will servive. The last couple of years I have had blooms in May. I start them inside between Dec, and January. I hope this helps. Every where is it's own climate. If you live on a hill things will be cooler. If down by the sea the ocean will control your weather.

Best answer I could give would be smaller shorter varieties are more likely to have more blooms at the beginning of August no matter where you live. They simply don't require the same effort as the dinner plates.

Cheers,
Russ

I plant my dahlias around the first of May here in kansas.zone 6...
the smaller bedding dahlias bloom first and thruout the summer....the tubers vary in blooming time depending on the variety
the small and medium ones from first to mid summer and continue until frost
the dinner plates are later to bloom for me but much worth waiting for.......