Unless you have an abundance of castings, it would be a waste to use them like that as they too would dry out and lose some of their microbial value.
Perhaps it's better to lightly dig some compost, lawn clippings, dried leaves, etc into the patch and then mulch.
Keep the castings as a side dressing when you plant seedlings. Once they're well established, use some more to boost growth - it can also be used to make worm tea - simply wrap the castings in a cloth and soak in a bucket (like a tea bag). The resulting liquid fertiliser is gentle enough to use on all plants. The used casting can also be used as above.
'Every time I learn something new, it pushes some old stuff out of my brain.'