Rosemary shrubs are relatively trouble-free. However, they do require a bit of pruning from time to time. In recent years we have seen the re-introduction of Rosemary Topiary specimens.
Left to their own devices, Rosemary generally look after themselves, but pruning after flowering is sometimes beneficial. Be aware that like the Lavenders - with which they are often confused - Rosemary are not particularly fond of being hacked back. That having been said, hard pruning at the right time - late spring - is an option for well overgrown and neglected plants.
In keeping with most evergreens, the best time to prune is when in active growth. Rosmarinus are best pruned immediately after flowering in late spring. This will encourage good flowering growth for the following year, and also keep the shrub from getting untidy - or leggy.
The same time of pruning is required for all types of Rosemary - whether upright, bush or spreading types. If grown in containers the growth is sometimes a little bit woody - as a result of too little feed (not a problem) - so the active growth timing is particularly important.
Rosemary flower in late winter - early spring, and often further into the year. Whilst waiting for the flowering to finish is tempting, the shrubs should not be trimmed or pruned too late in the year. End of July being the latest to ensure flower the following winter/spring.
Hedge-grown Rosemary should be clipped as soon as flowering has finished. This prompts soft attractive growth, and does not leave the unsightly bare patches that are often the result of unclipped hedge Rosemary plants. Do not cut back too far into the mature wood, for the hedge will take a little time to re-generate, and you may even lose a few of the plants - not good for an attractive hedge.
As has been intimated above, it may well be that your Rosemary plants do NOT require pruning. Don't prune them for the sake of it - unless as a hedge or you are trying Topiary!
General pruning of Rosemary simply entails clipping off the faded flower - you can save them for drying and potpourri. This can be done with a paid of hedge clippers if you want a formal shape to your finished shrub/herb, but is best carried out with a pair of secateurs. Snip back to just below the faded flower section, but not too far into the old wood. If you have a valuable plant (sentimental etc) then if you need to reduce its size dramatically, do it in two or even three stages. Starting in late spring and finishing by the end of August.
For hard or rejuvenation pruning, cut out a third of the plant - NO MORE than a third of the way back. That is to say, cut back/prune one out of every three branches. Then six to eight weeks later - as new growth starts on your pruned section, you can cut half of the remaining branches (You have now pruned two thirds of your old Rosemary shrub. The remaining third can be cut back finally after your last pruned sections start to send out new shoots.
I have always found that Rosmarinus prostratus (R. lavenduloides) - the creeping one, responds well to a good clip after the first flowers have faded. They often flower again early winter if the weather is not to hard!
Don't waste the old Rosemary clippings. Either put them in a paper bag and allow to dry out, when they will give many months of heady aroma, or chop them up into small sections and place them into a linen bag for immediate use. DON'T use these old clipping for the kitchen. The lamb deserves the best. After your new growth has started, that is the time for a few sprigs for the kitchen!