When to plant clover
 

I'm pretty sure I want to plant clover in my bunnies' play area. When
is the best time to plant clover seed? Can I plant it in the spring?
Should I wait for the last frost? I live in the northern part of zone five
(Iowa City), and it's been warm here this week but last frost isn't until
early April. My rabbits and i appreciate the help!

Thanks,
Adam