Why Are My Blackberry Canes Thin and Weak?

Healthy blackberry canes should be thick, stiff and able to hold themselves upright (or climb a support) without flopping. Thin, spindly canes — pencil-thin rather than finger-thick — that flop over at the tips or collapse under their own weight indicate that the plant is not allocating enough resources to each individual cane. The most common causes are too many canes competing for the same root system, insufficient sunlight reaching the crown, or poor soil fertility.

Too many canes: the primary cause

A blackberry plant will produce as many new primocanes as the root system has energy to support — but if 15 or 20 canes all emerge at once, the available resources are spread thinly across all of them, and each one is weak. The correct approach is to select only the strongest 6 to 8 new canes in early summer and rub out or cut off the rest at the base while they are still young. The remaining canes receive all the plant's vigour and grow strongly. This is one of the most impactful management interventions available to the home blackberry grower.

Shade and light

Blackberry primocanes growing in significant shade — from a fence, wall, building or overhanging tree — are consistently weaker than those in full sun. The plant needs direct sunlight for photosynthesis to drive the vigorous growth that produces thick canes. If shade from a specific source is the problem, consider relocating the plant or removing the shade source where possible.

Feeding

A blackberry plant growing in impoverished soil or not fed for several years will produce thin canes as it lacks the nutrient resources for vigorous growth. Apply a balanced granular fertiliser (e.g. Growmore) in early spring at the manufacturer's recommended rate, and mulch with a 5 cm layer of well-rotted manure or compost. The improvement in cane thickness is typically visible within the same season.