These bushes are about 20 years old and need moved. They grow many small stems from the ground each year (cut back to 6" every spring) to a height of approximately 36"; although my husband says they have grown much taller in the past. They have small oval leaves, no flowers.
The spring blooming types of Spiraea (which usually have white flowers) bloom on last year's growth so if you prune them in late winter you are pruning off all of the buds/bud wood. The summer blooming types (which usually have pink flowers) bloom on new growth so that kind of pruning won't affect them.
I think I may do a bit of experimenting this year… Generally, I cut it back hard in the early Summer, after it has finished blooming. And when I say cut it back hard, I mean just that…I cut all the way back to the ground so that the bloom the following year is on nothing but all new growth. It then has the whole year to grow back out, and then bloom every Spring. There was one year that I did not cut it back, and it did bloom the following Spring, but quite sparingly. Hmmm. Maybe in the colder zone, they are pruning at the wrong time….
Is there some discrepancy on what's being called 'new growth? I thought that branches that grew all summer and bloomed the following spring were considered old growth, like Forsythia. Those buds formed in summer and were on the branches through winter, then bloomed.
sallyg is correct about old vs. new growth. plantfreak78 is also correct that spring-flowering Spiraea flower from old growth, so if you cut them back in the spring you will never see flowers. The best time to prune them is late spring/early summer after they've finished flowering (as tlm1 has been doing).
I have a Miss Kim lilac bush that I must prune (if it needs shaping) immediately following bloom before any new growth appears, or I would be cutting off next year's flowers...this must be the same. I thought I remembered white flowers years ago, but wasn't sure; and my husband remembered them being much taller. Evidently, we did not cut them back years ago, and they bloomed. At some point when they became too large, we cut them to the ground and have been doing that each Spring. Since I cut them down every spring, I AM CUTTING THE STEMS THAT WOULD FLOWER !!! Thanks everyone for the help; I will leave them alone and see what happens.
In my neck of the woods, it's good to transplant in the Fall. As long as great attention is given to H2O. I'm interested in hearing about your success, or lack there of in transplanting this time of year……Happy Gardening! :-)