If you don’t want a dense tangle of twigs and a meager harvest, sooner rather than later, you need to prune and raise your blackberries properly.
With blackberries in the spring cut off all the vines that are older than one year and have already borne fruit. So much for the theory. In practice, however, it is often difficult to distinguish between old and new in the dense tangle of twigs. This makes it all the more important to tidy things up early on, to make pruning easier for yourself, but also to achieve consistently high yields in the summer.
Blackberries require a wire trellis with at least three tension wires right from the beginning. The trellis should be about eight meters (24 ft) long, and the horizontal tension wires should be attached to wooden stakes at heights of 50, 100 and 150 centimeters (20, 40 and 60 in) above the ground. Tying the old and new shoots separately to the tension wires makes it much easier to cut the blackberries, because this way you can keep track of them at all times.
When should you prune blackberries?
Regardless of whether they are strong-growing varieties grown on a trellis or weak-growing varieties, the best time to cut blackberries is in the spring, around March. In commercial cultivation, blackberry vines are often cut in the fall after harvesting, but this is only recommended in regions with very mild winters.
Raising strong-growing blackberries by pruning
Old, thornless blackberry varieties grow very vigorously, forming fruiting vines up to three meters (10 ft) long. Generally, blackberries should be planted in the spring, as the berry bushes are somewhat sensitive to frost. They are placed in the middle of the trellis, about six meters (18 ft) long, so that the shoots on both sides have enough space to grow.
In the year of planting, even strongly growing blackberries usually sprout only weakly and form comparatively short shoots. During the spring, select the four strongest new long shoots and tie them to the upper two tension wires on the left and right. The lower tension wire remains free during the first season. Often the new long shoots of the strong-growing blackberries also form side shoots in the first year.
In March of the following year, cut back all side shoots of the four main branches to one or two buds. If they are very dense, you can also cut off individual side shoots altogether. The remaining buds will develop into fruiting wood in the spring: the new side shoots will first bear flowers in the spring and then ripe berries from August to September. The fewer fruiting shoots your blackberry bush has, the better the fruit quality usually is. If you leave all the side shoots from the previous year uncut, many very small blackberries will form in summer and ripen relatively late, which means the yield will be correspondingly lower.
While the long shoots from the previous year produce fruit, new long shoots form from the rootstock, these are reduced to four strong shoots again, as in the previous year, by simply cutting off the excess ones at ground level. Tie two of the remaining shoots in each direction to the lower tension wire of your trellis.
Next spring, cut the harvested main shoots of your blackberry just above ground level and untie them from the trellis. Now distribute the new main shoots that have been put on the lower tension wire to the two upper wires. Then cut back their side shoots as described above. The lower wire of the trellis is thus free again for four new main vines, which will sprout from the ground in the spring and bear fruit in the coming year.
How to cut weak-growing blackberries grown on a fan trellis?
Newer thornless cultivars grow relatively weak. For this reason, the shoots are usually not directed horizontally along the tension wires, but arranged in a fan-like pattern.
In the spring, cut off all harvested vines from the previous year and reduce the annual shoots of the blackberry bush down to six to ten strong healthy shoots. These annual shoots, which are now starting their second year of vegetation and will produce fruit in the summer, are then passed through the tension wires vertically in the middle and diagonally on the sides. All the shoots that will only emerge in the new season should be left to grow and the six to ten strongest shoots should be passed through the trellis in late spring or early summer in a gap between two old shoots so that the two generations of shoots do not overlap. All shoot tips of the new main vines are cut off at the end as soon as they protrude beyond the trellis. The trellis should be around 1.80 meters (6 ft) high, i.e. slightly higher than the blackberry trellis for the strong-growing varieties. For this, you can get by with a width of about three meters (10 ft) per plant.
Separating new and old, fruit-bearing vines makes it easier to care for them in the years to come. In the future, you will only cut off the older vines in each case in the spring and direct the corresponding number of new shoots back through the trellis in the same place. Other necessary pruning measures, such as pruning the side shoots in July and, last but not least, harvesting, are also much easier with clearly grown shrubs.
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