Why Are My Blackberries Sour and Not Sweet?

Sour blackberries are one of the most common complaints from home growers — and in the vast majority of cases, the berries were simply picked too early. Blackberries do not continue to ripen after picking and a berry that looks black on the outside can still be unripe, sharp and lacking in the sugars that develop only in the final few days on the plant. The other leading cause of persistent sourness across the whole crop is insufficient sun reaching the fruiting canes.

The colour deception

Blackberries turn from red to deep black several days before they are fully ripe and sweet. During this period the berry looks fully ripe by colour but is still firm, and the sugars are still developing. A berry picked at peak visual colour will often be sour. The transition to true ripeness is marked by the berry becoming slightly soft — gently plump and yielding to pressure — and releasing from the plant with the faintest pull. Early fruit in a cluster ripens first; fruit at the base of the cluster may need several more days.

Shade and sugar development

Blackberry fruit ripened in deep shade is consistently less sweet than fruit on the same plant that receives direct sun. The photosynthesis-driven sugar production that takes place in the leaves adjacent to the fruit is directly proportional to light exposure. In a shaded site, or when fruiting canes are trained in a congested layer where outer canes shade inner ones, the fruit will be reliably less sweet. Moving the plant to a sunnier position, or training canes so that fruiting laterals all receive direct sun, improves sweetness the following season.

A cold or wet summer

In a poor summer — overcast, cool and wet — blackberry fruit across the season will generally be less sweet than in a warm, sunny year. This is beyond the grower's control for outdoor plants. Growing against a south-facing wall significantly improves sun exposure and warmth and produces noticeably sweeter fruit than open-garden training in the same season.

Variety selection

Some wild-type or hedge blackberry varieties simply have more acid in the fruit than named garden varieties bred for sweetness (e.g. Loch Ness, Loch Tay, Chester Thornless). If the plant is a cutting taken from a wild hedgerow, the fruit may always be sharper than named varieties regardless of growing conditions.