|Adapted from North Carolina State University, Department of Horticultural Science,
Horticulture Information Leaflet 8206 by E. B. Poling, Extension Horticultural Specialist
Thornless Blackberries: For two years after planting, thornless blackberry primocanes tend to grow along the ground, like a vine. Growers may have to move trailing canes in the direction of the row to allow room for cultivation. Generally, only a small crop of fruit is produced in the first season. If growth is poor during the first season, cut the canes back to several inches in late winter to force development of sturdier, more fruitful canes. In the second and succeeding years, shoot growth is more vigorous and upright. Tie these new shoots to the trellis when they reach a length of four to six feet. Some growers prefer to wait until after harvest to remove old canes before tying new shoots to the trellis in a fan shape (do not bunch them). In the spring before growth starts, prune any laterals back to twelve inches to encourage larger fruit.
Thornless blackberries have been grown successfully using a variety of trellising systems which are required to hold canes above the ground. If the tops of new canes are pruned during the summer to keep growth below three to four feet, no trellis is needed. Construct the blackberry trellis by stretching a wire between posts set twenty feet apart in the row. For erect blackberries, use one wire attached to the post about thirty inches from the ground (Figure 1B). For semitrailing blackberries, use two wires at heights of three feet and five feet from the ground (Figure 1A).
Erect blackberries such as Cherokee and Cheyenne require pruning out of the root suckers that arise from the crown. During the growing season, it is desirable to allow root suckers to develop to about a twelve-inch-wide row. Any growth beyond this should be eliminated.
Figure 1. (A) Train trailing plants to a two-wire trellis. (B) Train erect blackberry plants to a one-wire trellis.
When the new shoots of erect blackberries reach thirty to thirty-six inches in height, cut off the tips. This will force branching lower on the canes and will cause the canes to thicken, making them better able to support a heavy fruit crop. During the winter, prune the laterals to twelve to fourteen inches for convenient harvesting and larger berries. In late winter, remove any remaining dead or weak wood. Leave healthy, vigorous canes spaced at six canes per linear foot. (Figure 2).
Figure 2.. An erect blackberry plant (A) before pruning and (B) after pruning.
Pruning the old canes is critical to the prevention of disease. As soon as the last fruit has been picked in summer, cut all the old canes and burn them. Also, prune damaged or weak canes, leaving four to eight new shoots. Some growers tip primocanes when they reach a height of twenty-four inches so that low growing laterals are more easily protected during winter.