Plant problems

Why Are My Sloe / Blackthorn Leaves Curling?

Q: Why are my sloe / blackthorn leaves curling?

Sloe or blackthorn (Prunus spinosa) leaves curl most commonly in the UK because of the blackthorn aphid (Brachycaudus helichrysi), which colonises the young shoot tips in April and May and causes the young leaves to curl tightly into small, cup-like rolls. The infestation is usually very visible: dense colonies of small, black-brown aphids are present on the undersides of the curled leaves and on the young shoots. This aphid overwinters as eggs on blackthorn and other Prunus species, hatches in early spring, builds to large colonies on the young growth, then migrates to its summer hosts (various herbaceous plants in the family Compositae, including groundsel and sow thistle) in June and July. Other causes of leaf curl on blackthorn include silver leaf disease (Chondrostereum purpureum), which causes wilting, curling, and a silvery discolouration of the leaves; bacterial canker (Pseudomonas syringae pv. morsprunorum), which causes sunken dead areas on the stems with associated dieback and leaf curl; and cherry spot (Diplocarpon mespili), which causes brown spots on the leaves that may distort and curl as the spots enlarge.

Q: When is the best time to pick sloes for sloe gin?

The traditional advice is to pick sloes after the first frost of autumn (typically late October or November in the UK), when the cold is supposed to break down the cell walls of the fruit and reduce the extreme astringency of fresh sloes. The practical truth is that modern foragers and sloe gin makers often simply freeze the sloes themselves after picking (putting them in a bag in the freezer for 24 to 48 hours) to replicate the effect of frost, which allows picking earlier in October when the berries are plentiful rather than waiting for unpredictable autumn frosts. Sloes ripen from late August to late October depending on location and season; they are ripe when they are dark blue-black with a powdery bloom and the flesh has turned from green to a dark purple-red inside. When fully ripe, sloes are still extremely tart, dry, and astringent raw (the astringency comes from tannins and other compounds that are reduced but not eliminated by frost or freezing); they are very rarely eaten fresh but are transformed by the maceration process in sloe gin. For sloe gin: put approximately 450g of sloes (pricked with a needle or briefly frozen) in a 1-litre bottle with 200g of sugar and 700ml of gin; shake and leave in a dark cupboard for at least 2 months, shaking occasionally. The result is a vivid purple-red, sweet, fruity gin that retains the characteristic almond-plum flavour of the sloe berry.

Q: How do I grow blackthorn in the UK?

Blackthorn (Prunus spinosa) is one of the easiest native UK shrubs to establish because it is fully adapted to UK soils and climate; it grows in almost any soil type (chalk, clay, sand, loam) in any position from full sun to partial shade. It is often planted as a component of a mixed native hedge alongside hawthorn, field maple, and dog rose; a blackthorn hedge is extremely effective as stock-proof fencing because of the very dense, long, sharp thorns. Plant bare-root specimens in autumn or late winter (October to March) for the best establishment; potted plants can be planted at any time. Blackthorn spreads naturally by suckers from the roots, which can be a nuisance in a small garden where it can colonise an area quickly; in a larger garden or wildlife area this suckering is valuable because it provides dense thicket habitat for nesting birds. For sloe berry production, a sunny position produces more berries than a shaded position. No pruning is required for a wildlife hedge or thicket; for a more formal hedge, trim in late summer or early autumn (avoid pruning in spring when birds are nesting). The spring blossom, appearing in March and April before the leaves emerge on the bare black branches, is one of the most beautiful sights in the UK countryside and an important early-season food source for bees.

Q: How do I control blackthorn aphid?

Blackthorn aphid (Brachycaudus helichrysi) on sloe or blackthorn in a garden setting rarely requires active control because natural predators (ladybirds, lacewings, hoverfly larvae, parasitoid wasps) typically arrive and bring the aphid population under control by June, when the aphids migrate to their summer hosts in any case. On large hedgerow plants, the colonies are too extensive to treat practically and control is not necessary; the plant is fully capable of tolerating the infestation and the leaf curl damage. On young or small ornamental plants where appearance matters, apply insecticidal soap or neem oil spray to the shoot tips and curled leaves in April and May; the tight curl of the leaves makes it difficult to reach the aphids with spray, so apply under pressure to penetrate the roll. Encouraging natural predators is the most sustainable strategy: avoid broad-spectrum insecticides that kill the natural enemies of blackthorn aphid alongside the aphids. A winter oil wash applied to dormant stems in December to January kills overwintering aphid eggs and significantly reduces the following spring's infestation; this is the most effective preventive treatment for repeat infestations.

Sloe or blackthorn (Prunus spinosa) is a native UK shrub of hedgerows, downland scrub, and woodland edges, famous for its snow-white blossom in March before the leaves emerge and for its dark blue-black berries (sloes) used to make sloe gin. One of the most important native plants for UK wildlife, providing early blossom for bees and dense, thorny nesting habitat for birds. The blackthorn aphid causes a very characteristic, extremely tight leaf roll on the young shoot tips in April and May, which is the most common reason UK growers notice leaf curl on this plant.

Blackthorn aphid

The blackthorn aphid (Brachycaudus helichrysi) causes the most dramatic and characteristic leaf symptom on sloe and blackthorn: very dense colonies of small, black-brown aphids on the undersides of young leaves in April and May, with the affected leaves curling tightly into small cups. The infestation looks alarming but is almost always self-limiting; natural predators (ladybirds, lacewings, hoverfly larvae) arrive and bring the colonies under control, and the aphids migrate to their summer hosts (mainly Compositae, including groundsel and sow thistle) in June and July. The plant recovers normally and produces new growth after the infestation ends.

What to do

In most cases, do nothing; natural predators control the infestation by June. On young or ornamental plants where appearance matters, apply insecticidal soap or neem oil spray under pressure to penetrate the curled leaves; repeat weekly. Apply a winter oil wash to dormant stems in December to January to smother overwintering aphid eggs. Remove nearby groundsel and sow thistle to reduce the summer aphid reservoir. Avoid broad-spectrum insecticides that kill natural predators.

Silver leaf disease

Silver leaf disease (Chondrostereum purpureum) affects many Prunus species including blackthorn; it enters through wounds (pruning cuts, damage, splits) and spreads through the wood, causing a silvery or lead-coloured discolouration and sheen on the leaves of affected branches; the leaves on diseased branches may also wilt and curl. The internal wood of affected branches shows a brown stain when cut; this brown stain distinguishes silver leaf from false silver leaf (a physiological condition with no internal stain). Silver leaf is most likely to infect when pruning cuts are made in autumn and winter.

What to do

Prune any suspected silver leaf infection in dry weather in summer (June to August), cutting well below the stained wood and into clean, unstained tissue. Sterilise pruning tools between cuts. Dispose of affected wood by burning, not composting. Do not prune Prunus species in autumn or winter when silver leaf spores are most active. There is no chemical control; prevention through correct pruning timing is the key management strategy.

Bacterial canker

Bacterial canker (Pseudomonas syringae pv. morsprunorum) causes sunken, dead areas (cankers) on the stems and branches of blackthorn; the canker exudes amber gum and the shoot above the canker wilts, turns brown, and dies. The leaves on shoots affected by bacterial canker may curl, wilt, and die as water and nutrient supply to the shoot is interrupted. Bacterial canker is most active in autumn and winter; infected leaves may show shot-hole symptoms (small, round, brown spots that fall out, leaving a ragged hole).

What to do

Prune out infected shoots in summer, cutting well below the visible canker into clean wood; sterilise tools between cuts. Apply a copper-based fungicide spray in autumn (after leaf fall) and again at bud burst in spring; copper reduces the bacterial spread through the leaf scars left after leaf fall. Avoid pruning in wet weather in winter when infection risk is highest. On established hedgerow blackthorn, bacterial canker rarely causes serious long-term harm to the whole plant.

Cherry spot

Cherry leaf spot (Blumeriella jaapii, also called Diplocarpon mespili on some hosts) causes circular, dark brown or purple-brown spots on the leaves of blackthorn; as the spots enlarge and coalesce, the affected tissue falls out leaving a ragged, shot-hole appearance, and the surrounding leaf tissue may distort and curl. Heavy infection causes premature leaf drop in summer. Cherry spot is most severe in wet summers and when the plant is in a damp, poorly ventilated position.

What to do

Rake up and dispose of fallen leaves in autumn; the fallen leaves carry fungal overwintering structures and are the primary source of next season's infection. Improve air circulation by thinning congested growth. Apply a fungicide approved for leaf spot diseases on Prunus at bud burst and again at petal fall on severely affected plants. On established hedgerow blackthorn, cherry spot is not a serious long-term threat; intervention is rarely needed.

Frequently asked questions

Why are my sloe / blackthorn leaves curling?

Sloe (Prunus spinosa) leaves curl most commonly in the UK because of the blackthorn aphid (Brachycaudus helichrysi), which causes very tight, cup-like leaf rolling at the shoot tips in April and May. Dense colonies of small, black-brown aphids are visible on the undersides of the curled leaves. The infestation is usually self-limiting as natural predators arrive in June; the aphids then migrate to summer hosts. Silver leaf disease, bacterial canker, and cherry spot are other less common causes.

When is the best time to pick sloes for sloe gin?

Traditionally after the first frost of autumn (October to November) to reduce the berries' astringency; in practice, freeze the picked berries for 24 to 48 hours to replicate the effect. Sloes are ripe when dark blue-black with a powdery bloom. For sloe gin: macerate pricked or frozen sloes with gin and sugar for at least 2 months in a dark cupboard.

How do I grow blackthorn in the UK?

Blackthorn grows in almost any UK soil in full sun to partial shade; it is very easy to establish. Plant bare-root stock in autumn or late winter. Spreads by suckers, which can be invasive in small gardens but valuable in wildlife areas. Excellent stock-proof hedging plant alongside hawthorn and field maple. The March blossom before leaf emergence is important early-season bee forage.

How do I control blackthorn aphid?

Usually no action needed; natural predators control the infestation by June. For ornamental plants, apply insecticidal soap under pressure to penetrate the curled leaves; repeat weekly. Apply a winter oil wash to dormant stems in December to January to smother overwintering eggs. Remove nearby groundsel and sow thistle (summer aphid hosts). Avoid broad-spectrum insecticides that kill natural predators.