To obtain maximum yield, all raspberries (red, black and purple) must be pruned properly in the spring and summer. The pruning procedures for the various types of raspberries are based on the growth and fruiting characteristics of the plants.
Summer pruning of raspberries, though often overlooked by many gardeners, is vital. After the last summer harvest, promptly remove the old fruiting canes of all raspberries at the soil surface and destroy them. Removal of the old fruiting canes aids in disease control and permits more light and air to reach the new shoots, resulting in greater fruit bud development. No additional pruning of red raspberries is necessary until the following spring.
The new shoots of black and purple raspberries need to be pinched or cut off when the new shoots reach a height of 30 to 33 inches. Remove about 3 to 4 inches at the shoot tip. If the raspberries are trained to a trellis or other support, the canes can be allowed to reach a height of 33 to 36 inches for black raspberries and 36 to 40 inches for purple raspberries before being pinched. Since the new shoots reach these heights at various times throughout the summer, it is generally necessary to summer prune the planting once a week until late July. Summer shoot-tip removal on the new canes of black and purple raspberries encourages lateral shoot development and increases the fruiting surface area, resulting in higher yields. It also prevents the canes from becoming uncontrollable 8 to 10 foot long monsters. (The new shoots of red raspberries should be not pruned in this manner.)
This article originally appeared in the May 22, 1991 issue, p. 94.