Why Won't My Blueberries Turn Blue and Ripen?

Watching blueberries stay stubbornly green or turn reddish and then stall is one of the most common frustrations with growing this fruit. The berries look formed and healthy but refuse to develop the deep blue colour and sweet flavour that mark a ripe blueberry. Ripening in blueberries is driven by a combination of heat accumulation, sunlight and the internal maturity of the berry, and when one of these factors is missing the berries stall at the pink or reddish stage for weeks.

Not enough sun or warmth

Blueberries need full sun — a minimum of six hours of direct sunlight daily — to ripen properly. Plants in partial shade may flower and set fruit but the berries stall at the pink stage, lacking the warmth needed to complete ripening. Shaded berries also tend to be less sweet and more tart, even if they do eventually colour up. If your bush is in less than full sun, consider moving it during winter dormancy or pruning back overhanging plants that shade it. A south- or southwest-facing position is ideal in temperate climates.

Wrong variety for your climate

Blueberries vary enormously in their chilling hour requirements — the number of hours below seven degrees Celsius needed during winter to break dormancy and ripen fruit properly. Highbush varieties bred for southern or mild maritime climates may not accumulate enough chill hours in colder areas, resulting in poor flowering and fruit development. Conversely, varieties requiring high chill hours grown in mild climates may break dormancy poorly and produce delayed, patchy ripening. Check the chill hour requirements of your variety against your local climate, and consult a specialist nursery if the mismatch is significant.

Picking too early

A berry that has turned fully blue is not yet necessarily ripe. Blueberries continue to sweeten and develop flavour for several days after they turn blue. A ripe blueberry drops into your palm with the lightest touch — if you have to pull it, leave it for another two to five days. The flesh should feel slightly soft rather than firm, and the flavour should be sweet rather than tart. Patience at harvest time is rewarded with significantly better flavour.

High potassium shortage limiting ripening

Potassium plays a central role in fruit ripening, colour development and sugar accumulation. A plant that is short of potassium may flower and set fruit normally but the berries stay pale and do not develop full colour or sweetness. If the growing conditions are otherwise good — adequate sun, correct pH, consistent watering — a potassium-rich feed applied as the berries begin to colour can help complete the ripening process. Use a feed formulated for fruiting plants or ericaceous plants.