Along with many soft fruit bushes, pruning is important for two main reasons. Firstly it helps to encourage good cropping year on year, and secondly, it can also help reduce the incidence of fungal attacks.
As blackcurrants produce fruit on stems grown the previous year it’s important to achieve a regular supply of fresh new growth year on year. to replace the older wood as it becomes less viable for cropping.
To begin with all new blackcurrant bushes - normally grown from hardwood cuttings - would be just a single stem, and this is cut back to a couple of inches above soil level to encourage 'stooling' (this is the traditional method of growing edible berrying bushes). These are now left for a couple of years before more work is needed. Modern varieties that may have been grown as a multi-stem can also have all their growth cut back to a couple of inches above the soil level.
Once they have been in the ground for their first two years they would have formed their basic bush shape, but as this wood becomes older it will also become less productive. This production issue is dealt with by the removal of any stems that are four years or older. This selective pruning is carried out while the bush is dormant, after leaf-drop in the autumn. Each year – as a general rule of thumb - remove about one quarter to one third of the oldest stems. These can be recognised from their darker bark which is a deep purple in colour - almost to the point of being black. If the bush is a mass of thick congested growth you can also use this time to remove any weak stems helping to open up the bush allowing in light and more importantly air circulation. Also remove any stems that are rubbing together, as well as dead, diseased or dying stems that you may also find.
If you are growing mildew resistant blackcurrant varieties like’ Ben Garin’, and ‘Ben Connan’ then you may wish to try an alternative method of planting. Instead of cutting back to a couple of inches in the first year in order to create a ‘stooled’ bush, you can plant your blackcurrant bush about 6 inches deeper than the actual soil line. That way the plants will produces vigorous its new stems from below ground. These are then used to replace older ones that are removed as they loose productivity.
If you have taken on patch of land or garden with old or neglected blackcurrant bushes then again, come the autumn, all of the wood can be cut back down to just above soil level and give it a good mulch of well rotted farm manure.