Prune broom in time
This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences
As I mentioned a few days back, it is always best to prune a shrub from an early age, for various reasons. Formative pruning can help encourage low-down bushiness and although that means the plant is slower to gain height, and may delay flowering, the advantage makes it worthwhile.
A few plants need pruning from early on in their life. These tend to be quick-growing, short-lived shrubs and include artemisia, cistus, cytisus, ceanothus, genista, lavender, santolina and sage. They tend to be shrubs that like well-drained soil, full sun and could loosely be called ‘Mediterranean’ though not all are strictly Mediterranean. These all grow quickly and, without pruning, can become loose in habit and bare at the base. Some can be pruned hard if they get overgrown and you can rejuvenate most artemisias and santolina with a hard prune in spring, but lavenders, famously, will not sprout if cut back into bare wood.
The same is true of cytisus, which was the subject of a question that asked ‘Can I cut an overgrown broom bush back hard after flowering and will it sprout from the old wood?’
The answer includes yes and no. The right time to prune a broom (cytisus) is immediately after flowering, before the seed pods form. They usually set lots of seed and this reduces the summer growth and then the flowers the following year. So they need a shear immediately the flowers fade. Every year – from year one. Unfortunately, if you leave them several years, so they get straggly and bare at the base, they cannot be cut back into the old stems because they rarely sprout from these bare stems. If there is no alternative you may have to risk it but it is best to start the pruning early.
The Biking Gardener 
Oh my! Some species of the genus are among the most aggressively invasive exotic species here! Now that it is raining, we pull what we can. I suppose that if some are actually desirable, timely pruning to minimize seed dispersion would be helpful.
I know that ‘scot’s broom’ is a terrible weed around you and further north but it is not that invasive here though it is common in acid, light soils. The coloured forms are popular as garden plants though not as common as they used to be.
Scots’ broom is famously problematic, but may not be the worst of the problem. At least three species have aggressively naturalized here, and none of them seem to be reliably identified. To me, the worst is Scot’s broom, which we call ‘Scotch’ broom. (I have no idea why ‘Scotch’ means ‘Scots” or ‘Scottish’ here.) However, some consider that particular species to be Spanish or French broom! I have no idea which is which.
We just call Cytisus scoparius ‘broom’ though I undertsood that you call it scot’s broom – it does grow in Scotland. Spanish broom is Spartium junceum and is not native here and although it does self seed I am not aware that it is a problem in this part of the world.
Cytisus monspessulanus is supposedly the worst of the three aggressively invasive brooms, and is commonly known as Scotch broom here, although it may be French broom elsewhere. The local common name may be derived from the Cytisus scoparius, which likely naturalized here first. Cytisus scoparius is more prolific than Cytisus monspessulanus in some regions.
Cytisus monspessulanus is native to around the Med and is to tender to survive in most gardens in the UK and Ireland.
This is a Mediterranean climate here, so it is right at home. Many of the most aggressively invasive exotic species are such because they lack the pathogens and wildlife that eats them where they came from. Broom seems like it would be like this anywhere it is happy, even if it has pathogens at home.
I don’t think they suffer from many problems here either though there is a gall and they are naturally short-lived.
Lack of problems is a problem! I know that is must serve a purpose within its own ecosystem, but would not miss it at all here.