The growth and fruiting characteristics of blackberries are similar to raspberries. The blackberry plant's roots and crown are perennial, while its stems or canes are biennial. Blackberry canes are strictly vegetative during the first growing season. These first year canes are referred to as primocanes. The following year, these same canes (now called floricanes) flower, produce fruit, and then die.
In Iowa, the canes of most blackberry varieties suffer extensive winter injury. As a result of this damage, plants produce little or no fruit. However, there are two hardy varieties that can be successfully grown in the southern half of the state. 'Darrow' produces large fruit on vigorous, erect, thorny canes. 'Illini Hardy' bears medium-sized fruit. The fruit are produced on vigorous, erect, thorny canes.
Primocane-bearing blackberry varieties (Prime-Jim™ and Prime-Jan™) are a new option for gardeners in Iowa. Prime-Jim™ and Prime-Jan™ produce fruit in late summer/early fall on the current year's growth. Fruit are medium-sized, conical, soft, good flavored, and glossy black in color.
To obtain maximum yields, blackberries must be pruned properly. Pruning procedures for summer-bearing and primocane-bearing blackberries are provided below.
Summer-Bearing Blackberries ('Darrow' and 'Illini Hardy')
In late winter or early spring, prune out canes that are diseased, damaged or crowded, leaving four to six healthy canes per plant. Also, prune back the lateral or side branches to a length of 12 to 15 inches to encourage larger fruit.
In summer, pinch out or cut off the tips of the new canes when they reach a height of 36 inches. Pinching encourages side branch growth and increases the fruiting surface area, resulting in higher yields. After the last harvest, cut off the old fruiting canes at the soil surface. Remove the pruned material from the garden and destroy it.
Primocane-Bearing Blackberries (Prime-Jim™ and Prime-Jan™)
Prune all canes back to ground level in late winter or early spring. No additional pruning is necessary during the remainder of the year.
Blackberry plant showing primocanes (first year growth) and floricanes (second year, fruit-producing canes). Drawing from University of Illinois.