Why Are My Borage Leaves Curling?

Aphid infestation

Aphids are the most common pest on borage. Both green and black aphids colonise the shoot tips and the undersides of the large, bristly leaves, where the rough surface of the leaves can make them harder to spot at a glance. Aphid feeding causes the leaves to curl, pucker, and develop a cupped, distorted shape. Heavy infestations weaken the plant and reduce flowering. Because borage is a major pollinator plant, care is needed in choosing how to treat aphids without harming visiting bees.

What to do

• Inspect the shoot tips and the undersides of the large leaves for aphid colonies.
• Knock aphids off with a strong jet of water. The bristly surface of borage leaves makes this less effective than on smooth-leaved plants, but it disrupts colonies and reduces numbers.
• Spray with insecticidal soap in the evening, after bee activity has ceased for the day. Repeat every four to five days for two to three weeks.
• Natural predators including hoverflies and ladybirds are attracted to the borage flowers and provide good biological control of aphid populations as the season progresses.

Powdery mildew

Powdery mildew is extremely common on borage and is often one of the first problems encountered as the plants mature. The large grey-green leaves develop a white powdery coating, and the affected areas curl, pucker, and look bleached. It spreads rapidly in warm, dry conditions and when plants are crowded together. While powdery mildew rarely kills borage outright, a heavily affected plant is significantly less attractive to pollinators and less productive for harvesting leaves and flowers.

What to do

• Thin borage plants to at least 45 cm apart to improve air circulation.
• Remove and bin badly affected leaves and stems. Do not compost mildewed material.
• Spray remaining healthy growth with a sulphur-based fungicide or a potassium bicarbonate solution every seven to ten days.
• Ensure adequate soil moisture: drought-stressed borage is significantly more susceptible to powdery mildew.
• If plants are badly affected late in the season, remove them entirely and rely on self-sown seedlings for the following year.

Drought stress

Although borage is reasonably drought tolerant once established in the ground, the large, bristly leaves have significant water demand and the plant can wilt quickly in dry conditions, particularly when young or when grown in containers. Drought stress makes the large leaves curl and droop and significantly increases susceptibility to powdery mildew. Borage grown in containers is especially vulnerable since the large plant quickly outgrows the water-holding capacity of a small pot.

What to do

• Water borage during prolonged dry spells, particularly for the first four to six weeks after sowing or transplanting.
• Mulch around established border plants to retain soil moisture through dry periods.
• Container-grown borage needs a large pot and regular watering: check the compost every day or two in warm weather.
• Sow borage direct in well-prepared, reasonably fertile garden soil for the most vigorous, drought-resilient plants.

Overwatering

Borage is a plant of dry, well-drained conditions and does not tolerate waterlogged soil. In persistently wet or waterlogged ground, the tap root deteriorates and the plant declines rapidly: the lower leaves yellow and curl, and the whole plant wilts despite wet soil. Overwatering is most likely in containers without adequate drainage or in heavy clay soil with poor drainage.

What to do

• Plant borage in well-drained soil. It thrives in poor, gritty conditions and does not need a rich, moisture-retentive bed.
• In containers, use a free-draining compost and ensure excellent drainage holes.
• Reduce watering during wet or overcast periods. Borage does not need watering as frequently as most herbs in containers.

Glasshouse whitefly

Borage grown under glass or in polytunnels can suffer from glasshouse whitefly, which clusters on the undersides of the large leaves and causes them to pale, yellow, and curl. The sticky honeydew produced promotes sooty mould. Under cover, whitefly populations build rapidly without the natural control provided by outdoor predators. Outdoor borage is much less frequently affected but can be troubled by whitefly in particularly warm summers.

What to do

• Use yellow sticky traps to monitor and reduce adult whitefly populations.
• Spray the undersides of leaves with insecticidal soap every five to seven days for four to six weeks.
• For plants under cover, the parasitic wasp Encarsia formosa provides highly effective biological control when introduced early before populations build up.

Frequently asked questions

Why are my borage leaves curling?

Borage leaves curl most often from aphid infestation or powdery mildew. Aphids colonise the shoot tips and the undersides of the large, bristly leaves, causing them to curl and distort. Powdery mildew is common on borage in dry conditions and causes the leaves to curl, pucker, and develop a white powdery coating.

Why are borage leaves turning white and curling?

White leaves on borage that are also curling or puckering are almost always caused by powdery mildew. Borage is particularly susceptible to this fungal disease, especially in warm, dry conditions or when plants are overcrowded. It commonly affects plants in midsummer as the season advances. Remove affected leaves and improve air circulation.

Can you eat borage leaves?

Young borage leaves are edible and have a mild cucumber flavour, though the older leaves are covered in stiff, bristly hairs that make them unpleasant to eat raw. The sky-blue flowers are widely used as an edible garnish. Borage leaves used for cooking should be collected from healthy plants showing no signs of mildew or pest damage.

Does borage self-seed?

Yes, borage self-seeds very freely and can become almost weedy in some gardens. If you want to control self-seeding, remove the plants before the seed heads mature and shatter. Self-sown plants usually produce a more vigorous generation than deliberately sown plants.