I live in Toronto and my blackcurrants (now about 4 years old) have grown enormously, both up and out.
How is is best to prune them and when?
According to the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), it’s best to prune when the plants are dormant in late winter or early spring.
Blackcurrant bushes need constant renewal to ensure heavy crops. Older branches will bear fruit, but quantity and quality decline with age. Black currants produce the best fruit on one-year-old wood.
Strong one-year-old shoots, and two- or three-year-old shoots which have an abundance of strong one-year-old wood, are the most productive. Keep a total of 10 to 12 shoots per mature bush, with about half being one-year-old shoots. A few more shoots may be kept if plant vigor is very good. Remove all shoots which are more than 3 years old. Make pruning cuts close to the ground.
Every year after, remove about one third of the oldest stems – the bark is very dark to the point of being black – and any that are weak or very low.
For full information on growing these berries, you can read the rest of the OMAFRA publication here.