Q.When Do I Start Fertilizing My Flower Seedlings?
I have started Celosia from seed and they are about 2 inches tall. When should I start fertilizing them and what do you recommend I use?

Certified GKH Gardening Expert
If these have been started into small cells, then I would not recommend feeding, at all! Just plant into some very rich soil, and that will feed for the entire, annual, plant's life. If you choose to use a weaker soil, then feeding a very weak feed solution about about 1/4 strength of your preferred all-purpose fertilizer of choice will do. Just be careful not to overfeed, as they are sensitive.
This article will help you with their care:
https://www.gardeningknowhow.com/ornamental/flowers/cockscomb/growing-flamingo-cockscomb.htm