When is best time to divide perennials
If the soil isn't too wet, this is a good time to divide most perennials, but it's best to divide some in late summer or fall. Following is a list of commonly grown perennials and the best time to divide them:
Daylilies--spring or immediately after flowering
Heuchera--spring or fall
You might be able to divide the spirea by digging up the root ball and chopping it into suitable sized pieces. Each piece should have healthy roots attached to viable stems. Cut the stems back somewhat before digging and replanting.
For more certain positive results, follow the propagation methods recommended in this North Dakota State University bulletin:
Go here to read the complete bulletin: