Hi Laurie,

It sounds like you have experience already growing Dahlias, but if not, I have a couple of resources for you:

Here's one on planting dahlia tubers: https://planttalk.colostate.edu/topics/annuals-perennials/1094-planting-dahlia-tubers/

Here's one on summer blooming tubers and corms: https://extension.colostate.edu/resource/spring-planted-bulbs-corms-and-roots/

I also read over what several Colorado area flower growers reported online on experimenting with planting in greenhouses.  One said it was a lot less work than transplanting in the busy spring season, but that it seemed like that shifted the colors and the dahlias grew super tall, top-heavy stems, and they wouldn't be doing it again.  Meanwhile, a big flower farm in Mancos wrote in their dahlia growing guide that they've grown them in the greenhouse and could put them in about a month before field grown dahlias:  https://mountainrootsproduce.com/dahlia-growing-guide

Dahlias can withstand partial shade, so you could also experiment with putting shadecloth on your greenhouse to keep the temperature more in check. 

I hope that helps, and thank you for reaching out to Extension!

Best,

Monica