You could cut them back at any time of the season if size or shape is a consideration.
Basicly, for established plants cutting back to 6-8" in a spring is a common practice. When you do that, remove the oldest, thickest and therefore unproductive stems. In no time your spirea will be the same size as a season before.
You could shear them right after the bloom and they'll rebloom for you in a August-September.
For yellow leaved cultivars (Goldflame, Goldmound etc) second pruning/shearing is even more desirable since new yellow/copper/red growth is the main attraction of the plant, not a flowers.
During the second pruning you may cut shrub in a half with no ill effect. Cutting deeper will not give plant enough time to gain former size by the fall.
Note of caution, be sure to collect all remnants (leaves, branches, twigs) after pruning. Spireas roots easily and you may end up with unwanted plants in unwanted places. While they could be easily removed while young, 2-3 years old seedlings have a good size root system and might be not a good companions for not so agressive perennials.

OK -- I'll cut back by 50% after they bloom, will clean up well, and will reevaluate size later in the year. Thank you!

My spireas were planted under the canopy of a horse chestnut. Over the last 7 + years the tree has grown and the spireas are now in quite a deep shade. They have gotten very leggy and don't produce as many blossoms as they are capable of doing. I guess I should transplant them to an area with more light. My questions are: should I prune first or transplant first? What time of the year is best to take these actions? How much light do spireas need to do their best? I have a light shade spot under the very high canopy of a few trees - no direct sunlight, but light shade. Still too dark?

All advice is very much appreciated! Barbara

I cut S. bumalda way back too, like Ego45. It is vigorous and will send fresh foliage back. If it's just to clean up after flowing, I sometimes only snip off the spent flowers, nipping back to just above the next lowest branch.

For transplanting, you may want to transplant first and then prune. This said, because you don't know how much of the rootball you'll end up with, and it might be best to base your pruning on how much root there is to support foliar growth. The amount of top growth should mirror the size of the rootball you have. The basic rule, though, is to take off 1/3rd of the topgrowth.

Spireas really are sunlovers, but I do have Goldmounds that are in part shade. They gets some direct sun, though. Maybe a couple hours. The rest of the day they're in light shade. They aren't leggy, but they don't get the wonderful eye-popping color that Goldmound is popular for. They need full sun for that effect.

If your spirea is flowering now, even in shade, perhaps moving it to a "lighter" shade will be enough of an improvement to make it less leggy and maybe get more flowers. It's worth a try, particularly if you have no better sun opportunities.

If left unpruned for several years spireas become leggy in any location, sunny or shady.
Pruning to 6-8" AND removing completely older thicker stems in a spring will rejuvenate plant. Strong existing root system will produce a lot of new growth and in no time shrub will be of the same statue as before, only better looking and more floriferous.
You could transplant it at any time of the year when grounds are workable. This is very forgiving and tough plant.
To transplant it in a spring will be just more convinient, it will be leafless and you'll see a branch structure more clearly. Transplant and then prune.
If you want to transplant it now, take it out of the ground, remove thickest stems to the base (will be easier to do when out of the ground), but leave at least 1/2 of the greenery on, plant, water and re-prune in a spring.

Re: sun-shade situation.
Spireas will live and may even flower in light shade, but they are really sun lovers and perform best in close to full sun locations. Why would you want to plant it in a shade knowing beforehand that it will perform at maximum of 1/3 of its capacity? There are many shade loving plants to take that place and be happy there. Moving from one shady place to another one wouldn't do anything good to YOU.

Hi - new gardener here. I understand how to prune my Spiraea Anthony Waterer, but I don't know how to deadhead it properly. Do I just cut the spent bloom off, or cut it lower down on the stem?

Thanks for your help, Jan

I think what the above advice is telling you is that you could do either, but if you give it a decent "haircut" after blooming, it will put out attractive new growth and you'll get some rebloom in fall. There are a lot of blooms on mature well grown spireas, so have patience. Do not leave the trimmed branches as "stubs" of wood. Prune back to a set of leaves, so there remains a leafy outline to the plant.

>you don't know how much of the rootball you'll end up with, and it might be best to base your pruning on how much root there is to support foliar growth. The amount of top growth should mirror the size of the rootball you have. The basic rule, though, is to take off 1/3rd of the topgrowth.

Actually, the guys on the shrub forum laughed at me when I said I was planning something like this, prune after transplant. They claim that it's best not to prune after you move a shrub. I was skeptical, especially since I'd just bare-rooted a nice crape myrtle, but they convinced me to leave it alone... despite the size of the top compared to the tiny root ball - which, frankly wasn't even a ball at all by the time it was re-planted.

The idea of balancing the top and bottom seems very sensible, however there have been field experiments done that have shown that it's not only unnecessary but harmful. If I still had the links, I'd post them - maybe I'll go search the shrubs forum for that thread.

For spirea, I'm sure it would not hurt the plant to prune after moving; but it's no longer "what's done," apparently.

Over the winter, which was much colder and for a longer time period than usual, our spirea hedge died out in places. Some have leaves coming in slowly, others look like they died over winter. We have lived here 11 years and this has never happened before. Should we cut all back to a low height, as all plants seem to be growing at the bottom. Or what should we do. Thanks.

I came to this board for the exact same reason as Waneta D. I hope someone responds! Thanks.

Most Spirea only have a 7-10 year Lifespan.

The species of the section Chamaedryon, and also S. canescens and S. bella, should be pruned as little as possible�only thinned out and the weak wood removed� while those of the sections Spiraria and Calospira can be pruned more severely if necessary, since they produce their flowers at the ends of the young shoots.

Spireas grow in almost any moderately moist soil and do not stand drought well, the spiraria species being generally more moisture-loving; S. tomentosa thrives well only in a peaty or sandy soil.

Help! I trimmed back my goldflame spirea in the Spring (leafed out but before flowering). It has regrown back to the original size but every stem I trimmed remains with no foilage on it and is just stubs amongst the new growth. How do I "fix" this? Do I trim out the "dead" stems or cut back the whole thing and start over? And when? I suspect the stubs are supporting the new growth somehow, but if I trim again (flowering done now), I will have a lot of old stems sticking out.