When to prune spiraeas
It is an over-simplification but there are basically two types of spiraea. It used to be more complicated when astilbes (herbaceous perennials) were included in the genus. But the two basic types are those that bloom in summer, at the ends of new shoots that grew in spring, and these are usually pink, and those that flower in clusters along the shoots that grew the previous year and these are (unless I have missed one) all white-flowered.
Spiraeas are a bit old-fashioned though a lot of breeding has been done with Spiraea japonica and there seems to an endless stream of cvs with various versions of coloured foliage which does not always augment the pink flowers. A possible big improvement are the Double Play (R) series that have a prolonged (repeating) flowering period but I have not grown any. It is still hard to beat ‘Goldflame’ in my opinion (below), since it is colourful and vigorous. Some of the new kinds are bred to be compact, which I have experienced as ‘weak’ in the garden.
Anyway, back to pruning. These pink, summer-flowering kinds are best pruned in spring. You can give a light trim after flowering is you like, to remove the old flowers, which tend to die pale brown. But a light prune in sprig is best. Every two or three years it is worth giving a hard prune, the sort of prune that happens to them in supermarket carparks so they can get to the crisp bags and lager tins. This encourages strong new growth. It is also a useful thing with ‘Goldflame’ because it promotes strong foliage and delays flowering. I don’t mind this at all because I am never quite sure if the flowers look good with the foliage anyway. Above is ‘Anthony Waterer’ and old cv. with purple-tinted deep pink flowers. Like many of this group it can get mildew in a hot, dry summer, perhaps its greatest flaw. While it is not very popular because it just has green leaves, it is hard to beat for its bloom and, like all spiraeas, it is good for pollinators.
I noticed, this summer, that the bees loved the flowers but just ran over them, at speed, so perhaps they were collecting pollen rather than searching for nectar. The spiraea above is ‘Shirobana’ which is curious for having both white and pink flowers in each cluster but the bee is sitting on the pink flowers.
The spring-flowering spiraeas, which include S. prunifolia, S. x arguta (‘Arguta’), S. thunbergii and S. betulifolia are often rather large shrubs with arching rather than upright stems. Like most spring-flowering shrubs they should be pruned immediately after flowering. Most of the flowered stems and all the small, weak twigs, should be cut out. This allows room for vigorous new shoots to grow from the base and these will bloom the next spring. This can be left for a year or two if you prefer but if not pruned at all the shrub can get dead and woody at the base and spread too far.
The Biking Gardener 
Crisp bags and larger tins?
Anyway, our summer flowering Spirea get cut to the ground annually during winter. I do not want them to get any higher than they need to. There is not trimming or grooming; just cutting back. As you say though, the bridal wreath spirea gets groomed with the ‘alternating canes’ method.
The plantings in supermarket carparks are usually full of rubbish, though not as badly here in Ireland than the UK.
You know, where I lived in town many years ago, my home was a block off the boulevard, across the road from the backsides of businesses that were on the boulevard. Some of the neighbors were restaurants. I found all sorts of nice glasses in my garden, left by patrons who took them from various restaurants. I should have returned them, but did not know where they came from.
Well at least the litter was useful!