Plant problems

Why Are My Isoplexis Leaves Curling?

Q: Why are my isoplexis leaves curling?

Isoplexis (Canary Island foxglove or Tenerife foxglove) leaves curl and drop most commonly in UK cultivation because of cold damage (isoplexis canariensis and I. isabelliana are frost-tender, generally only reliable outdoors in UK USDA hardiness zone 9+ conditions, approximately the very mildest coastal areas of south-west England, Wales, and Ireland; the large, lance-shaped leaves curl, yellow, and drop after exposure to frost below about 0 to -2°C), drought stress (the large leaves curl downward and develop brown edges when the roots dry out; isoplexis has large, transpiring leaves and needs consistent moisture, particularly in summer in pot-grown specimens), or insufficient light causing etiolated, floppy growth with curling leaves in UK winter conditions (isoplexis is an evergreen subshrub that needs bright light year-round; in a dark UK winter position, the stems become elongated, the leaves droop and curl, and flowering is suppressed). Isoplexis is a small genus of three to four species of evergreen shrubs and subshrubs in the plantain family (Plantaginaceae, the same family as Digitalis, to which isoplexis is closely related) from the Canary Islands and Madeira; they are grown in UK gardens and cool greenhouses for their spectacular, upright spikes of densely packed, tubular, orange-amber, rust, or orange-red flowers from June to September, and for their architectural, foxglove-like foliage. Isoplexis canariensis (Canary Island foxglove) grows to about 1 to 1.5 m with lance-shaped, glossy, dark green leaves and produces extraordinary upright spikes of densely packed, vivid orange-amber flowers.

Q: How do I grow isoplexis in the UK?

Isoplexis is grown in the UK as a container plant in a frost-free, bright greenhouse or conservatory through winter, moved outdoors to a very sheltered, sunny patio or courtyard position from late May to October. Use a large pot (at least 30 to 40 cm diameter) with freely draining, loam-based compost (John Innes No. 3 with 15 to 20% additional grit); the plants grow to 1 to 1.5 m and need a substantial container to maintain stability and an adequate root volume. Position in maximum available light both indoors in winter and outdoors in summer; isoplexis is from the laurel forest and rocky hillside habitats of the Canary Islands where light intensity is high, and it flowers most freely in full sun or very bright conditions. Water consistently throughout the growing season from spring to autumn; allow the compost to approach dryness between waterings but do not allow complete desiccation as the large leaves lose moisture rapidly and drought stress causes rapid leaf curl and drop. Feed with a high-potash liquid fertiliser monthly from April to August to encourage strong stem growth and a generous flower display. The spectacular orange flower spikes (to 60 cm) typically appear from June to September in the UK; deadhead spent spikes to encourage further flowering. Reduce watering from October as the plant is moved indoors; water only when the compost is almost dry through the winter. Prune back by about one-third in late winter or early spring to maintain a compact, well-branched form.

Isoplexis canariensis (Canary Island foxglove) is a frost-tender, evergreen subshrub from the Canary Islands with lance-shaped, foxglove-like leaves and spectacular upright spikes of densely packed, vivid orange-amber flowers from June to September. Grown as a container plant in the UK, moved outdoors from May to October and overwintered in a frost-free greenhouse or conservatory. Cold and drought are the main causes of leaf curl.

Cold damage

Frost is the most common cause of isoplexis leaf curl and drop in UK cultivation; the large, lance-shaped leaves curl downward, yellow, and drop after exposure to temperatures below about 0 to -2°C. Even a brief cold night below 0°C in a poorly insulated greenhouse or conservatory can cause significant leaf loss. The stems are somewhat harder than the leaves and may survive a light frost that kills all the foliage; new shoots from surviving stems appear in spring if the root system is intact.

What to do

Move indoors to a frost-free position (minimum 5°C) by late September to early October before the first autumn frosts; do not wait until after frost damage has occurred. In the greenhouse, move pots to the warmest available position (not on the cold floor) on very cold nights and cover with a layer of fleece if temperatures are forecast to fall near 0°C. Frosted leaves do not recover; remove them once the plant is in a frost-free environment and new growth begins from the stems.

Drought stress

Drought stress causes isoplexis leaves to curl downward, develop brown edges, and eventually drop; the large leaf surface transpires water rapidly and the plant dries out quickly in hot, sunny conditions. Container-grown plants in a patio or courtyard position in summer are particularly vulnerable; the pot can dry out completely within a day or two in a hot July. Drought stress during the flower spike development period also reduces the number and size of flowers produced.

What to do

Water consistently from spring to autumn; check container moisture levels daily in hot weather and water whenever the compost is approaching dryness. A saucer under the pot in summer can provide a temporary water reservoir in very hot conditions; empty the saucer in autumn to prevent waterlogging. A larger pot dries out more slowly than a small one; repot into the next size up if the plant is drying out too rapidly between waterings.

Insufficient light

Insufficient light causes isoplexis stems to become elongated, the leaves to grow large and floppy, and the overall plant to develop a drooping, leaf-curling habit as growth etiolates toward available light sources. In a dark UK winter position (such as a poorly lit conservatory or a greenhouse with dirty glass), isoplexis grows weakly and may lose leaves progressively through winter. Low light also reduces the quality and quantity of the flower display in the following season.

What to do

Position in the brightest available indoor light from October to May; clean greenhouse glass maximises winter light levels significantly. In summer, move outdoors to full sun or very bright conditions from late May. Avoid positioning against a north-facing wall or in deep shade from neighbouring plants; isoplexis is from high-light Canary Island habitats and never fully adjusts to the low winter light of northern Europe.

Frequently asked questions

Why are my isoplexis leaves curling?

Isoplexis leaves curl most commonly because of cold damage (leaves curl, yellow, and drop at or below 0°C; move indoors by October; minimum temperature 5°C; frost-free greenhouse or conservatory required), drought stress (large leaves transpire rapidly; check container moisture daily in summer; water consistently from spring to autumn), or insufficient light in winter (stems etiolate; leaves droop and curl; maximum available winter light essential; clean greenhouse glass). All three factors are manageable with the right growing strategy.

Is isoplexis hardy in the UK?

Isoplexis is frost-tender and is not reliably hardy outdoors in most of the UK; it is killed by any significant frost below about -2°C. In the very mildest UK areas (Scilly Isles, coastal Cornwall, coastal west Ireland) outdoor growing in a sheltered position is possible, but elsewhere it requires frost-free greenhouse or conservatory overwintering from October to May. The minimum temperature is 3 to 5°C; brief dips to 0°C cause leaf damage but rarely kill the plant.

How do I grow isoplexis in the UK?

Grow in a large container (minimum 30 cm) in John Innes No. 3 plus 20% grit; place outdoors in full sun from late May to early October, then overwinter in a frost-free, bright greenhouse or conservatory (minimum 5°C). Water consistently spring to autumn; water sparingly in winter. Feed monthly April to August with a high-potash fertiliser. Prune back by one-third in late winter to maintain compact, well-branched form. The orange flower spikes appear June to September.

How is isoplexis related to foxglove?

Isoplexis is closely related to Digitalis (foxglove) and was formerly classified within it; both are in the plantain family (Plantaginaceae). They share the foxglove-like tubular flower spikes and lance-shaped leaves. The key differences: isoplexis is an evergreen subshrub (foxgloves are usually biennial or short-lived perennial), frost-tender (foxgloves are fully hardy), and flowers in vivid orange-amber (a bird-pollinated colour from the Canary Islands). As a companion to foxgloves in a summer display, isoplexis extends the season and adds exotic, orange contrast.