Thornless blackberries are a popular addition to home gardens, offering sweet fruit without the painful harvest of their wild relatives. While easier to manage than aggressive, thorny brambles, gardeners often worry about whether these cultivars spread. Thornless blackberries do require management, as they possess natural propagation mechanisms that cause them to expand their footprint over time. Understanding these growth habits is essential for successfully containing the plants and maintaining a productive berry patch.
Defining Growth Habits: Erect, Semi-Erect, and Trailing Varieties
Blackberries are generally classified into three distinct categories based on their cane growth habit, which determines how much space and support they need. Erect varieties, such as ‘Navaho’ or ‘Arapaho,’ have stiff, self-supporting canes that stand upright and typically require minimal trellising. These types are often grown in a dense hedgerow system because their sturdy canes are less likely to arch over and touch the ground. Erect plants are generally spaced closely together, often two to three feet apart in the row, due to their vertical growth pattern.
Semi-erect blackberries, including popular thornless cultivars like ‘Triple Crown’ or ‘Chester,’ produce vigorous, thick canes that arch away from the crown. These canes are not fully self-supporting and will need some kind of trellising or support to manage their arching growth and keep the fruit clean. Semi-erect types are usually planted further apart, needing around five to six feet between plants, to accommodate their wider, more sprawling nature. Trailing varieties have long, flexible canes that are not self-supporting and will sprawl along the ground unless trained onto a trellis or wire system. Cultivars like ‘Black Satin’ are fully trailing and require a complete support system to keep the canes off the soil, which is a necessary practice for both fruit quality and spread prevention.
The Mechanisms of Spread in Thornless Cultivars
Unlike wild blackberry species that use extensive underground rhizomes, most cultivated thornless varieties spread primarily through two methods: suckering and tip layering. Suckering occurs when new canes (primocanes) emerge from the root crown or from buds on the roots near the main plant. This process is most common in erect varieties, often leading to new plants several feet away from the initial location. Tip layering is the most aggressive form of spread, especially in semi-erect and trailing types. This happens when long, arching canes touch the soil surface, prompting the cane tip to spontaneously form roots and establish a new, genetically identical plant.
Managing and Containing Blackberries in the Garden
Controlling the spread of thornless blackberries requires an approach that targets both suckering and tip layering. To manage suckering in erect varieties, gardeners can install physical root barriers made of metal or plastic sunk 12 to 18 inches deep. Any suckers that appear outside the desired area should be immediately pruned at ground level. Preventing tip layering is achieved through proper trellising and timely pruning. Training the canes onto a wire trellis keeps them elevated, and regular pruning of the primocanes encourages lateral branching, minimizing the chance of the cane arching down to the ground.