Joyce Goldenlily said:
I hope your young plants are not the common purple variety because it is now registered as a pernicious weed. It will grow anywhere, spreads rapidly by self sown seedlings and can put 6 feet of growth on each year once established. A nightmare in a small garden.
I'd prune each stem back to a pair of buds further down, it'll encourage a bushier plant.
They grow so quickly, that in a few years time you'll be hacking them back to keep them in the space you have them
I live in west central Scotland - not where that photo is...