Beginner Question-Fertilizer?

Hi all!
I am growing my first cut flower garden this year in zone 5a. I direct sowed zinnias, feverfew, cosmos, and some dahlia tubers in my raised bed about 3 weeks ago. I see everyone popping up nicely (except for the feverfew 😭)but their growth has really slowed the past week or so, even though we’ve had great sunny days. I used cow manure compost and good soil to build the bed, and I even see some tiny mushrooms popping up in the mornings so I think the soil quality is good. Do I just need to be patient or should I be adding fertilizer at all?
Thanks for any and all advice! I love seeing everyone’s flowers here and am so jealous of those in warmer climates who can plant early!