When should you apply fertilizer to your marigolds?

Try a fertilizer that has a higher middle NPK number, like 10-30-10.

These are often sold under the names like "Bloom Booster"

The middle number is generally for increased bloom production, which for flowering annuals (like marigolds) and vegetables is a good thing. Always follow manufacturer's directions for mixing fertilizers - it's okay to use a weaker solution, but do notuse a stronger mix, for it can burn your plants and/or kill them.