Why Are My Borage Leaves Curling?
Borage (Borago officinalis) is one of the most vigorous and trouble-free annual herbs in the UK garden. From a spring sowing it grows rapidly into a large, bristly plant covered in the vivid blue star flowers that are among the best bee plants in the British garden. Despite being generally robust, borage leaves do curl and distort in two situations: aphid attack on the growing tips in early summer, and the widespread powdery mildew that affects almost all borage plants by late summer. Both are manageable, and neither typically kills the plant before it has completed its season and shed seeds for the following year.
Aphids
Borage is colonised by black bean aphid (Aphis fabae) and related species, particularly on the soft growing tips and flower buds in May and June. Dense aphid colonies cause the leaves at the growing tips to curl tightly around the insects. As the plant matures and becomes woodier, aphid colonies typically diminish as natural predators arrive. Borage flowers are also attractive to hoverflies, whose larvae are voracious aphid predators.
What to do
- Inspect borage growing tips from May. Small colonies can be squashed by hand or knocked off with a jet of water. Borage grows so vigorously that it outgrows light aphid infestations quickly; intervention is most valuable when the plant is young and the growing tip is the only source of new growth.
- Avoid using insecticides on borage during flowering: the flowers attract large numbers of bees and other pollinators, and any insecticide residue on the plant risks harming visiting beneficial insects.
Powdery mildew
Powdery mildew on borage (Erysiphe boraginacearum) is extremely common and affects almost all UK borage plants to some degree by mid to late summer. The mildew produces white, powdery patches on the leaves, beginning on the oldest leaves and spreading upward. Affected leaves curl, yellow, and wither. The mildew is favoured by the combination of warm days, cool nights, and the moisture retained in borage's hairy leaf surface.
What to do
- For borage grown primarily for its flowers and wildlife value, powdery mildew is largely cosmetic and can be tolerated. The plant continues to flower through the mildew and the seeds ripen normally.
- For borage grown for culinary leaf harvest, remove mildewed leaves promptly and harvest young leaves from the growing tips regularly throughout the season. Removing infected material slows the spread and young growing-tip leaves remain mildew-free for longer.
- Improve airflow around densely growing borage by thinning plants to one per 45 to 60 centimetres. Water at the base of plants rather than overhead.
- A spray of diluted milk (1 part whole milk to 9 parts water) applied every 7 to 10 days from June has some evidence for reducing powdery mildew on susceptible plants; it is particularly practical for organic or wildlife gardens where synthetic fungicides are not used.
Drought
Borage tolerates dry conditions better than many garden herbs but its large, hairy leaves wilt and curl visibly in prolonged drought. Drought also accelerates the onset of powdery mildew. Seedlings and young plants are most drought-sensitive; established plants with deep taproots access soil moisture more effectively.
What to do
- Water borage during extended dry spells, particularly young plants in their first few weeks of growth. Established plants in garden soil need watering only in prolonged drought. Borage in pots needs more regular watering than garden-grown plants.
Frequently asked questions
Why are my borage leaves curling?
Borage leaves curl most often from aphid infestation, powdery mildew, or drought. Borage aphid (Aphis fabae and related species) clusters on the growing tips and young leaves, causing them to curl tightly around the feeding colonies. Powdery mildew (Erysiphe boraginacearum) is very common on borage, particularly in late summer, producing white, powdery patches on the leaves that cause them to curl and wither. Drought causes the large, hairy leaves to wilt, curl inward, and develop brown edges. Borage is generally a vigorous annual that is not easily killed by any of these problems, and the main impact is cosmetic or on the supply of young leaves for culinary use.
Can I eat borage leaves?
Yes. Young borage leaves are edible and have a distinctive mild cucumber flavour. They can be used in salads, as a garnish, in summer drinks (particularly Pimm's), or cooked briefly in the same way as spinach (cooking reduces their hairy texture significantly). Only use young, tender leaves: the large, older leaves become very hairy and rough-textured as the plant matures, making them unpleasant to eat raw. The star-shaped, bright blue borage flowers are also edible and are widely used as a garnish for desserts and drinks. Borage has been used in herbal medicine and cooking for centuries, though it contains pyrrolizidine alkaloids in small amounts; it is safe for normal culinary use but should not be consumed in very large quantities over a prolonged period.
Is borage good for the garden?
Yes. Borage is one of the most beneficial plants a UK gardener can grow for wildlife and as a companion plant. Its flowers are intensely attractive to bumblebees and honey bees: borage is consistently rated among the top plants for bee forage in UK trials and can support hundreds of bee visits per day when in full flower. The nectar is produced continuously in the base of each flower and replenishes rapidly, making borage an exceptionally reliable bee plant from June to October. As a companion plant, borage is traditionally grown near tomatoes, squash, and strawberries, where it is thought to repel tomato hornworm and cabbage worm, attract pollinators and beneficial insects, and accumulate minerals in its leaves that benefit the surrounding soil when the plant is composted. Borage self-seeds prolifically and will return to the same garden spot year after year with no intervention needed.
Why does borage get powdery mildew?
Borage is inherently susceptible to powdery mildew (Erysiphe boraginacearum) and virtually all UK borage plants develop powdery mildew to some degree by late summer. The combination of warm days and cool nights in August and September, which creates the humid conditions the mildew fungus favours, together with borage's large, hairy leaf surface which retains moisture, makes powdery mildew on borage almost inevitable. The infection typically begins on the oldest lower leaves and works upward as the season progresses. For plants primarily grown for their flowers and wildlife value, powdery mildew is not a serious problem and can be tolerated. For plants grown for their leaves in culinary use, removing mildewed leaves promptly and harvesting young leaves regularly from the plant's growing tips keeps the food-quality leaves mildew-free for most of the summer.