If you don't know when to fertilize your spring lawn then let me briefly explain it to you. The best time to fertilize the grass in the Spring is after the grass green-up on its own and starts growing.
The only reasonable exception to this rule of thumb is when you are applying natural and/or organic fertilizers which can be applied early so long as you understand that they won't become active in the lawn until soil temperatures climb into the upper 50s, well after your lawn has exited winter dormancy.
