Most Spiraea shrubs should be pruned right after flowering. However, there are also some varieties that should be pruned at the beginning of the growing season (Spring).
Spring and early summer flowering varieties include Spiraea arguta, Spiraea Snowmound, S. Goldflame, S. Little Princes, S. bumalda Anthony Waterer. Spiraea japonica Shirobana (There are others). All of these should be pruned as soon as flowering is finished - See below. All of these varieties are normally finished flowering by end of June (UK)
Also described are pruning requirements for Spiraea billardii and S. douglasii
Spiraea arguta, S. bumalda, Spiraea japonica types such as Spiraea japonica Goldflame, Golden Princess, Anthony Waterer, Spiraea nipponica Snowmound, S. thunbergii, Spiraea vanhoutei, and also Spiraea veitchii.
Here a Spiraea Snowmound - just finished flowering (Late June). Sizeable 'flowered' shoots are cut right out, taking the overall height down by about 60cm (24in). It will regain this height again during the next few months, with plenty of flowering growth ready for next year. Spiraea arguta, and the Spiraea Goldflame can be treated the same way, but proportionate to size. (If desired, they can be cut right back to within a few cm of the ground! It is good practice to also cut back a proportion of the main stems - Prune back to ground level, or just a few centimetres above. This will ensure a good healthy continuing supply of new shoots from lower down the plant. These new shoots will flower in subsequent years. Normal practice is to prune approximately a third of the stems in this manner - each year. It follows that at the end of year three, there will be no 'old' stems left, after this type of pruning Spiraeas.
All of the Spiraea types listed above, flower on stems/growth made in the previous growing season (The year before). If you prune them in the spring ie before flowering - then you will be cutting off all of the flowering stems and flower buds for the current year, and will end up with no flowers!
Late flowering Spiraeas - These flower on stems/growths made earlier in the same year.
Spiraea x douglasii, S billardii, S. canescens, are among this group and they flower later in the year, on the tips of stems made in the current year.
Spiraea billardi should receive a hard pruning regime in early spring. The same applies to the others in this group of late flowering Spiraeas - Spiraea x douglasii and Spiraea canescens. This type of pruning will ensure many new fresh stems to flower later in the year. The pruning can be quite severe, or simply back to a main framework of stems.
Prune back to ground level - or just slightly above with this group of Spiraea shrubs.