Coreopsis Perennial Fertilizer

[odSteve]

Hi,
About a month ago I bought the last two Coreopsis, Solanna Golden Sphere plants from the local nursery. I was told they don't produce viable seeds so I took a chance and ripped apart the crown of one of the plants. It was very root-bound. I tore it apart into 10 pieces that had some roots. I cut the stems back, removing the flower buds regardless if they had opened or not. I also put them into individual pots to better control the soil composition and watering and now have them mostly in the shade of a tree. I'll gradually move them into more sun. Their final destination will be full sun which can be intense here. I also put 6 stalks into a glass of water on my window sill that had no roots at all.

It appears there could be two of the ten that don't make it - including the one that had the most roots. The others are holding their own but have shown no new growth at all. Is it possible they don't grow in summer when temps are in the high 90's to 102*F? What fertilizer (and don't be afraid to say brand names) do you recommend that I could sprinkle in the pots to promote vegetative and root growth? I fear later they won't survive the long, freezing winter here if they don't start to show some growth now.

Surprisingly some of the plants in the glass of water have started to grow roots too.
Thank you.