Most shrubs in garden centres are deliberately cut back to encourage bushiness, and it is most unlikely you will find your choice already growing with a conveniently placed stem. Sort through the plants on offer to find the one with the strongest and straightest upright shoot; it can have side shoots but they should be as centrally placed in the shrub as possible. The next part is quite brutal. Having chosen your main shoot, you cut out all the other shoots as near to the base of the plant as possible. Yes, I know you have just paid a small fortune for this shrub and now you are being asked to destroy 90 per cent of it, but this will not only give you a fledgling trunk but also direct all the energy from the roots into just one shoot – so you will get rapid growth.