I have been growing 'Purple Prince' zinnia cultivar for a couple years now. Primarily because it attracts so many butterflies. My soil is mostly clay.

I would like to know what the ideal soil mixture is for maximizing the speed of growing these zinnias, to attract as many mid summer butterflies as possible. I think the species is (Zinnia elegans).

I've tried starting these indoors, and it does seem to help.

This year I've been imagining using small pots to sink into the ground. I could get the seedlings started indoors, transplant them to the pots that have the ideal soil mix, then drop them in the pre-dug holes. Another benefit to the pot is being able to set them out earlier in the season and being able to bring them into the garage when we are still getting late night frosts.

Would some type of fertilizer help speed it's growth?