Why Are My Blueberry Leaves Turning Red in Summer?

Red autumn colour on blueberry leaves is one of the most beautiful sights in the autumn garden. But when the leaves go red in midsummer — while the bush should be green and fruiting — it is a signal the plant is under stress. Red pigments called anthocyanins build up when the plant is struggling to move sugars normally, and several distinct problems trigger this response. Identifying which one is at work is the key to responding correctly without making things worse.

Phosphorus deficiency

Red or purplish leaves that develop on younger growth in spring or early summer, before temperatures get truly hot, are the classic sign of phosphorus deficiency. Phosphorus is essential for energy transfer in the plant, and when it is short the leaves accumulate those anthocyanin red pigments as a side effect. This happens most often in cold soils in early spring — the roots cannot take up phosphorus efficiently when the ground is cold even if there is plenty available — and it usually clears up as temperatures rise. If it persists into warm weather, a single application of a low-pH phosphorus feed or fish meal worked into the surface typically corrects it within a few weeks.

Drought and heat stress

Blueberries have shallow, fibrous roots concentrated in the top thirty centimetres of soil, which makes them very vulnerable to surface drought. When the roots dry out, sugar transport breaks down and the leaves redden, often starting at the margins and tips. This is especially common for container-grown plants in summer, for bushes growing in light sandy soil, or for plants that have missed a week of watering during a hot spell. If the soil feels dry to a depth of several centimetres, the plant needs water immediately. Mulching thickly — ten to fifteen centimetres of pine bark or wood chip — is the single most effective way to hold moisture through summer heat.

Root damage

Physical damage to the root system from digging, from voles or root weevils feeding underground, or from planting too deeply can cause the same reddening by disrupting water and sugar movement. If the leaves redden suddenly after cultivation or transplanting, root disturbance is the likely cause. Keep the plant well watered and avoid disturbing the soil around the base. The bush should recover over several weeks if the damage was not too severe.

Natural autumn colour is not a problem

If the reddening appears in late August or September, the leaves are simply heading into their spectacular natural autumn display. Blueberry leaves turn vivid shades of red, orange and crimson before dropping, and this is completely normal. The plant is not in trouble — it is preparing for dormancy. If berries have been produced normally through summer, the autumn colour is a reward, not a warning.

Matching colour to cause

Time of year and which leaves are affected gives you the diagnosis. Whole-plant reddening in cold spring on young growth: phosphorus lockout in cold soil. Tips and margins reddening in a dry hot spell: drought stress, mulch and water immediately. Sudden widespread reddening after root disturbance: root damage, rest the plant. Mid-season reddening with no obvious stress: check pH and consider a light acid feed. Reddening appearing in late summer on otherwise healthy plants: normal, enjoy the colour.