When should I prune my Spirea?
A general rule of thumb when pruning flowering shrubs is to do so just after they have finished blooming. Once the flowers start to fade and/or go to seed you can prune the plant back to achieve a desirable size or shape.
Most flowering shrubs begin to set the growth that will produce next year's flowers on the new shoots that begin to grow after the current year's flowers have died off. If you wait too long you will cut off the parts of the plant that will bear next year's flowers.
So...in this case...if you want to see lots of flowers next spring/early summer...it's too late to prune this plant. On the other hand, if it's gotten too big, or it's become overgrown and woody and needs to be shaped you can prune it late this year. Wait until late fall, or very early next spring (March). If you prune it now the plant will respond by sending out new growth that will be too tender to survive our long, cold Minnesota winter. Just be aware that it won't have many flowers next spring, but it will probably produce a better showing of flowers in subsequent years.
Here is a link about pruning shrubs and trees:
I hope this is helpful. Please contact AaE again if you have further questions.