Why Are My Blackberry Leaves Turning Yellow Between the Veins?

Interveinal chlorosis — leaves that are yellow or pale green between the veins while the veins themselves remain dark green — is the classic symptom of iron or manganese deficiency. On blackberries, this is almost always caused by the soil pH being too high (too alkaline) rather than by the absence of these minerals from the soil. At a pH above 7.0, iron and manganese become chemically locked in the soil and unavailable to plant roots even when the soil is well-fertilised. Blackberries prefer a soil pH of 5.5 to 6.8.

Testing and correcting soil pH

Use a simple soil pH test kit (widely available) to check the pH of the soil around the plant. If the pH is above 7.0 and the plant is in open ground, work sulphur chips or granules into the soil around the plant (follow the rate instructions on the packaging). Sulphur reduces soil pH over several months. Ericaceous (acid) compost or pine bark mulch worked into the surface also gradually acidifies the top soil layer. Correcting pH in very chalky soil is difficult — consider growing in containers with acidic compost instead.

Chelated iron treatment

While pH correction is the long-term solution, chelated iron (sequestrene) gives a faster improvement. Chelated iron is in a form that remains available to the plant over a wider pH range than standard iron compounds. Water a chelated iron solution into the root zone of affected plants in spring. Visible improvement in leaf colour often occurs within two to four weeks. This is a temporary fix in high-pH soil unless pH is also corrected.

Container growing as a solution

If the garden soil is very alkaline (chalk downland, mortared rubble, post-construction fill), growing blackberries in large containers filled with ericaceous compost resolves the chlorosis permanently. Use a container of at least 45 cm diameter, and refresh or top-dress with fresh ericaceous compost every two years to maintain the acid conditions.