Furano/Biel farms and where to stay

I'm planning a 2 weeks trip to Tohoku and Hokkaido. We want to visit and view flower farms in full bloom understand farms in Hokkaido have different varieties of flowers blooming from May - Oct. When is the best time to visit if we want to avoid hot and humid weather?

We are thinking either in August when lavender will be in full bloom or Sept. But we are not sure if it's worth visiting in August since it's peak summer and might be too hot?

Places to visit in Furano/Biel

Shirogane Blue Pond

Farm Tomita

Shirahige Waterfall

Ningle Terrence

Where should we be based for a night Furano or Biel since it is a long way from Sapporo.