As long as the ground is workable you can transplant. Right now is as good a time as any. Dig the transplant hole first so the spirea roots aren't drying out waiting to be put in it's new site. I transplanted a load of shrubs I started a few years ago from cuttings. I was able to transplant right up till the end of June here. A few pieces of advice, - transplant on an overcast day, water, water, water everyday, and cut back the plant by half after transplanting (this is especially important for success).

Thanks lovetogarden. I didn't wind up transplanting yet, largely because I realized it would take me too long to prepare the transplant hole. I don't think I can transplant here until the end of June -- I'm zone 5 -- but I think I have some time. I also think the spirea will be pretty durable, and if it doesn't look fabulous this year, so be it.

End of June is awfully hot and late for transplanting. You'd be better off doing it now.

You've got most of April and about half of May you can transplant in before it starts getting a bit too warm for the shrubs. If you miss those time periods shoot for the mid to end of September, first part of October.
I'm in the Binghamton area and my ground is still frozen. Digging now is going to be tough going.

Thanks all. I should clarify, when I said I didn't think I could move shrubs until the end of June, I meant I didn't think that in my area I could transplant *through* the end of June... I would not wait that long! In fact, we moved this yesterday. It went smoothly and I'm optimistic that it will take.

This is an update.... the spirea is doing really well, much better than it did in the old spot as it has more space. Thanks to all who posted!

You have been fortunate to have cool weather and lots of rain - perfect for new transplants!