Plant problems

Why Are My Coronilla Leaves Curling?

Q: Why are my coronilla leaves curling?

Coronilla valentina subsp. glauca (shrubby scorpion vetch or glaucous scorpion vetch) leaves curl and drop most commonly in UK gardens because of cold damage in hard winters (the glaucous, blue-green, pinnate leaves curl and yellow in frost below about -8 to -10°C), waterlogging in wet or poorly drained conditions (coronilla is a Mediterranean plant with no tolerance of persistently wet soil; the roots rot rapidly in saturated conditions), or drought stress in very hot, dry summer conditions. Coronilla valentina subsp. glauca is a compact, evergreen or semi-evergreen Mediterranean shrub grown in UK gardens for its remarkably long and abundant flowering season: the clusters of bright, golden-yellow, pea-like flowers with a distinctive honey scent are produced from late winter (January to February in mild conditions) through spring and into early summer, with a second flush in late summer and continuing sporadically into autumn in favourable conditions; in the mildest, most sheltered UK positions coronilla can be in flower for 8 to 10 months of the year. The small, blue-green, pinnate leaves (typically with 7 to 9 small, rounded leaflets) give the plant an attractive, soft-textured foliage appearance and a grey-blue colour that associates well with other Mediterranean plants. The cultivar 'Citrina' produces pale lemon-yellow flowers and 'Variegata' has cream-margined leaves; both are attractive but slightly less vigorous than the typical glauca form. Coronilla emerus (scorpion senna) is a related, deciduous species from central and southern Europe that is hardier than C. valentina but less ornamental and less commonly grown in UK gardens.

Q: Is coronilla hardy in the UK?

Coronilla valentina subsp. glauca is marginally hardy in the UK; it is generally hardy to about -8 to -10°C in a sheltered, south or west-facing wall position with freely draining soil, and is reliably perennial in mild UK gardens (sheltered coastal and urban southern England, south Wales, mild gardens in the south-west). In colder UK positions (north of the Midlands, exposed gardens, high altitude), coronilla is not reliably hardy and may be killed in hard winters below -10°C, particularly in wet conditions. The plant's cold hardiness is strongly influenced by drainage: coronilla in lean, freely draining soil against a warm wall survives significantly lower temperatures than the same plant in moist, clay-heavy soil. The characteristic blue-grey foliage and warm Mediterranean appearance of this plant make it one of the most attractive small wall shrubs for the warmest, driest parts of a UK garden; in a gravelled, south-facing border at the base of a warm wall it combines particularly well with other grey-leaved Mediterranean plants such as santolina, lavender, and cistus, all of which share similar drainage and exposure requirements. Coronilla is short-lived in some UK garden conditions; plants may last only 5 to 10 years before becoming woody and unproductive, and they should be propagated by semi-ripe cuttings in August to ensure continuity. Young plants raised from cuttings are typically more vigorous and floriferous than old, woody specimens.

Q: How do I grow coronilla in the UK?

Coronilla valentina subsp. glauca grows best in a warm, sheltered position in full sun, in lean, freely draining, neutral to slightly alkaline soil; the gravel garden, raised bed, south-facing wall base, or rock garden are ideal positions. Like other Mediterranean plants in the pea family, coronilla is adapted to poor, dry, often alkaline soils and does not benefit from rich, moist, fertile growing conditions; excessive fertility and moisture produce soft, lush growth that is more vulnerable to cold damage and may reduce flowering. Do not add compost or organic matter to the planting hole; a gritty, lean, neutral to alkaline soil produces the best growth and the greatest cold tolerance. Plant in spring (April to May) after the last hard frost; water in well and allow to establish in the first season before the first winter. Mulch with coarse grit rather than bark to maintain drainage at the root collar and reduce the risk of crown rot in wet winters. Prune after the main spring flowering flush (May to June): trim back the flowered shoots by about one third to encourage compact new growth and a second flowering flush later in the season; do not cut into old, bare wood as this does not regenerate reliably. In winter, remove dead or frost-damaged stems in spring (April) when new growth confirms the extent of damage. Take semi-ripe cuttings in August as insurance against winter losses; the cuttings root readily in a gritty compost in a cold frame or on a warm windowsill and grow quickly into flowering-sized plants within a season.

Q: Why does coronilla flower in winter in the UK?

Coronilla valentina subsp. glauca begins its first flush of flowering in UK gardens from late January or February in mild conditions, making it one of the earliest-flowering evergreen shrubs in the UK garden; in very mild winters or sheltered positions, flowering can begin as early as December. This winter and early spring flowering is the most distinctive and valued aspect of coronilla in UK garden use, as very few yellow-flowered shrubs are reliably in bloom in mid-winter. The golden, honey-scented clusters are particularly striking on bright winter days when the rest of the garden is bare. The reason for this unusual flowering behaviour is that coronilla is adapted to a Mediterranean climate where the growing season is primarily in autumn, winter, and spring (the cool, moist season) rather than in summer (the hot, dry season); it flowers during the Mediterranean cool season, which in UK terms translates to the winter and early spring. The plant produces successive flushes of flowers over a very long period: the main winter to early spring flush (January to May) is typically the most prolific; a second flush usually follows in late summer (August to September) after the plant has made its main season's growth; and further flowers are produced sporadically between these two main flushes and often into November in mild conditions. The honey scent is strongest in warm sunshine; positioning the plant near a south-facing window or seating area that catches winter sunshine maximises the value of this distinctive fragrance.

Coronilla valentina subsp. glauca (glaucous scorpion vetch) is a compact, fragrant Mediterranean wall shrub with blue-grey pinnate leaves and golden-yellow pea flowers from January to May and again in late summer. One of the earliest-flowering shrubs in the UK garden, the honey-scented winter flowers are its signature feature. Waterlogging and cold damage are the most common threats; lean, gritty, freely draining soil in a sheltered sunny position is essential.

Cold damage

Cold damage is the most common cause of leaf curl and plant loss in coronilla in UK gardens; temperatures below about -8 to -10°C cause the blue-green pinnate leaves to curl, yellow, and drop, and kill the stems of all but the most sheltered specimens. The plant flowers from January in mild positions; late winter cold snaps in February and March can damage or kill developing flowers and young growth even in otherwise mild UK winters. In a hard UK winter, coronilla in an exposed position may be completely killed while the same plant against a warm south wall survives and resumes flowering by March.

What to do

Position against a sheltered south or west-facing wall; this is the most important single factor for winter survival. Wrap young plants in horticultural fleece from November to March in colder UK areas. Do not cut back cold-damaged growth until April; new growth from surviving buds confirms the extent of damage. Take semi-ripe cuttings in August as insurance; young plants from cuttings replace winter casualties quickly. Lean, dry soil produces harder growth that is more cold-tolerant than the lush growth of rich, moist soil.

Waterlogging

Waterlogging causes root rot and rapid decline in coronilla; the Mediterranean origins of this plant leave it entirely unequipped for persistently wet, poorly aerated soil. Symptoms develop rapidly in wet conditions: leaves yellow and drop, stems collapse, and the plant may die within a single wet autumn or winter. The combination of cold and wet is especially lethal; coronilla against a south wall in freely draining, gritty soil survives temperatures that would kill the same plant in heavy, wet clay.

What to do

Plant only in freely draining to very freely draining, lean soil; add coarse grit generously (30% by volume) to heavy soils. A gravel mulch around the base prevents crown rot at the soil surface. Do not water established plants in autumn or winter; avoid any irrigation that keeps the soil moist through the cold months. Raised beds and gravel gardens with gritty growing media are ideal winter conditions.

Drought stress

Coronilla is moderately drought-tolerant once established in lean, freely draining soil; it is adapted to dry Mediterranean summers and performs well in the hot, dry conditions at the base of a south-facing wall in a UK summer. Only the most severe and prolonged droughts cause significant stress; the small, blue-grey leaflets curl slightly at their margins in very hot conditions as a normal water-conservation response. Container-grown coronilla dries out more quickly and can suffer drought stress within a day or two of the compost becoming completely dry.

What to do

No supplementary watering is needed for established coronilla in the ground in average UK summer conditions; the plant's drought adaptation is one of its assets. Water container-grown plants when the compost is almost dry; allow the surface to dry between watering. Avoid overwatering, which is far more damaging to coronilla than drought. The main watering priority is consistent moisture in the first growing season after planting while the root system establishes.

Frequently asked questions

Why are my coronilla leaves curling?

Coronilla leaves curl most commonly because of cold damage in hard winters (the blue-grey pinnate leaves yellow and curl below -8°C; the plant often regrows from the base in spring), waterlogging in wet conditions (the roots rot rapidly in saturated soil; plant in lean, gritty, freely draining conditions), or normal leaf-margin curl in very hot dry conditions (a drought-adaptation response; the plant recovers when temperatures cool). Cold combined with wet is the most damaging combination.

Is coronilla hardy in the UK?

Coronilla valentina subsp. glauca is hardy to about -8 to -10°C in lean, freely draining soil against a sheltered south or west-facing wall, and is reliably perennial in mild southern UK gardens. Cold combined with wet kills it more readily than cold alone. Take semi-ripe cuttings in August as insurance; young cuttings are productive flowering plants within one season. The plant is short-lived (5 to 10 years); propagate regularly for continuity.

How do I grow coronilla in the UK?

Grow in a warm, sheltered, south-facing position in full sun, in lean, freely draining, neutral to alkaline soil. Do not add compost or feed heavily; lean soil produces the hardiest, most floriferous growth. Prune lightly after the main spring flush (May to June) to encourage compact growth and a second flush in late summer. Apply a grit mulch rather than bark mulch. Take semi-ripe cuttings in August annually.

Why does coronilla flower in winter in the UK?

Coronilla is adapted to the Mediterranean cool season (autumn to spring) and begins flowering in the UK from January or February in mild conditions, making it one of the most valuable winter-flowering shrubs. The golden, honey-scented flower clusters are particularly striking in winter sunshine. A second flush follows in August to September, and further flowers are produced sporadically between these two main periods; in the very mildest UK positions the plant is effectively never without some flower from January to November.