Now don’t get me wrong. Rosemary is an incredible plant. It grows happily in the toughest, poorest conditions, flowers in the dead of winter and instead of giving off airs of one that’s surviving with gritted teeth, has the grace to wrap itself in the rich fragrance of nana-roast.
But it’s one of those irritating plants for which there’s really no great moment to prune. The traditional wisdom with flowering plants is to prune as soon after flowering as possible (except that stuff which flowers after about mid-summer – but hang on – if I get distracted with all the exceptions to the rule now I’ll never get around to the point). The idea is to give as much time as possible for the plant to put on new growth before it’s time for next year’s flowering.
Rosemary stuffs the rule by reaching its peak of flowering in late winter, while simultaneously putting on a great burst of growth. If you wait until it has finished flowering, it feels like you’re cutting off the greater proportion of new growth that this season is likely to produce. If you don’t prune, the plant gets to tall, or too big, and starts to lose a lot of the charm that made you want to grow it in the first place.
Take the form ‘Tuscan Blue’ for example, shown in most of these pics, and grown primarily for its rigidly upright form. There are very few shrubs that do this strictly vertical thing, so I use it a lot. In order to retain those strong lines at the height you want, you’ve got to be pretty brutal with it, and reduce it to virtual stumps (at least by two thirds of its height) each year, otherwise you’ll end up with bare-legged clubs of blue flowers 2m tall.
You don’t have to be quite so strict with prostrate forms like Rosmarinus ‘Blue Lagoon’, as you’re not depending on maintaining that youthful form. But there’s still something of a dilemma about the timing of a hard cut, which is likely to be motivated by the need to reduce its spread, or to minimise the amount of dead wood that’s accumulating over the years.
There’s no easy solution. There’s absolutely nothing to be done but to cut it hard right when you’re enjoying the flowers, or to wait a few more weeks, and sacrifice a whole lot of new growth. I try and strike a balance, myself, but never feel entirely settled with the decision. Thankfully a forgiving nature is high on rosemary’s list of virtues.