Digitalis purpurea L., Foxglove

Account Summary

Growth form and preferred habitats

A large, usually biennial, rarely perennial, monoecious, tap-rooted, pubescent to lanate, rarely subglabrous, rosette-forming herb up to 200 cm tall, D. purpurea is winter-green and occasionally flowers in its first, rather than second, year of growth (Grime et al. 1988, 2007; Sell & Murrell 2007). The stem is simple or sparsely branched, round in section and grows erect from a rosette of oblong-lanceolate, acute, crenate-toothed, softly pubescent, grey-green leaves. The radical leaves have a long-winged petiole, while the stem leaves are sessile.

It is widespread and common to abundant on dry or well-drained acid soils, particularly after they are disturbed, such as by woodland or scrub clearing, fire, or more intermittently as the result of animal scrapes or trampling. Foxglove also occurs, although much more rarely, in limestone districts. Typical habitats include wood, open heathland, scrub margins and clearings, hedgebanks, roadside verges, rough, occasionally mown or grazed grasslands and waste ground, especially when it is near habitation. D. purpurea is tolerant of considerable levels of shade but grows most rapidly in fully-lit conditions. It can also survive and develop to the flowering stage on stony, rocky, rather dry, shallow soil conditions, including on mountain- and sea-cliffs and on walls. It is widespread and common on ground up to around 885 m in B & I (Sell & Murrell 2007).

The established strategy of D. purpurea is categorised by Grime et al. (1988, 2007) as SR/CSR, ie it is intermediate between a Stress-tolerant Ruderal and a more balanced mixture of Competitive, Stress-tolerant and Ruderal strategies altogether.

Variation

This extremely variable, polymorphic species originated in the W Mediterranean and taxonomically is at its most complex in the SW part of that basin, there being up to five different subspecies named, plus a number of local variants based largely on plant height, leaf size and shape, degree of hairiness (indumentum), calyx, corolla colour and spotting. Only the most widespread form, subsp. purpurea, occurs in B & I.

Some local populations demonstrate combinations of these variable characters, but the pattern of variation is too complex and changeable (fluctuating) to allow them to be regarded as subspecies. In addition, dwarf alpine ecotypes occur and regional differences exist between populations on the Iberian peninsula and the rest of Europe (V.H. Heywood, in: Tutin et al. 1972; Hultén & Fries 1986).

Garden cultivars 'Alba', 'Apricot', 'Campanula' and 'Dwarf Sensations' occur, together with a range of named garden hybrids, including 'Excelsior Hybrids' and 'Foxy Hybrids' (Griffiths 1994).

Fermanagh occurrence

When precipitation is heavy and frequent, as is very much the case in Fermanagh, lime is gradually leached from soil surface layers and they become increasingly acidic and peaty, even when borne directly over limestone rock. Thus, D. purpurea is not localised in Fermanagh, but rather it is scattered widely across 238 tetrads, representing 45.1% of the VC total.

Flowering and vegetative reproduction

Reproduction is almost exclusively by seed although damaged plants can asexually develop daughter rosettes. Flowering takes place from June to September. The average plant bears a terminal, unilateral, spike-like raceme inflorescence with around 60 irregular, downward-directed (pendulous), pale to deep pinkish-purple, bell-shaped flowers with deeper purple spots on a ciliate white ground inside. The flowers, with a corolla 40-55 mm long, produce nectar and attract bumble bees that pollinate them, but if this fails, then the flowers are self-compatible and fertilise themselves (Hickey & King 1981).

Seed production and dispersal

An average plant of D. purpurea is capable of producing around 85,000 tiny seeds to add to the soil seed bank and large plants can generate between 100,00-500,000 (Salisbury 1942; Richards 1997b). The fruit capsule, 11-13 mm in length and longer than the calyx, contains a mean of around 1,030 seeds and germination in light is 95% (Salisbury 1942). Seed dispersal is by wind. "The capsule, at first conic, splits into halves, each half dehiscing again on the inner face, but remaining so closely connected with the other half at the base that the seeds collect near the bottom and are shaken out of the cup so formed in small quantities at each puff of wind." (Ridley 1930).

Seed longevity

The seed of the species has a reputation of being long-persistent in soil (Grime et al. 1988, 2007), but the survey of soil seed banks of NW Europe found a range of values: three estimates suggested the seed was transient (surviving less than one year), eight studies considered it short-term persistent (less than five years), four reckoned it was long-term persistent (more than five years) and three estimates could not assign their samples to one of the three seed bank types (Thompson et al. 1997).

A complicated reproductive strategy

Being monocarpic, the parent plant dies after flowering and although often described as a biennial, in reality in many less-favourable habitats D. purpurea may take more than two years growth to reach a size and have accumulated the photosynthetic energy resources to trigger successful flowering and fruiting. Perhaps it is best considered as a short-lived, once-flowering (ie technically speaking 'hapaxanthic') perennial [from the Greek 'hapax' = 'once' and 'anthos' = 'flower'] (Holmes 1979).

Even this, however, does not fully cover the potential reproductive strategy of D. purpurea, since very occasionally individual Digitalis plants (particularly those regularly monitored in a cultivated garden setting) are known to 'forget to die' after their first seeding. These individuals may continue to grow and flower one or two additional times (Cragg-Barber 1996). One such specimen in Bergen flowered three times and it also developed a complicated caudex (ie a thickened, woody, truck-like appendage) at the base of the stem, which bore many adventitious leaf rosettes, apparently making it capable of continuing life indefinitely (Faegri 1980). Faegri (1980) even went as far as suggesting that the early demise of hapaxanthic specimens of species like D. purpurea might be more due to external competition than to internal factors.

Empirical observations indicate that mechanical damage to the flowering stem, sufficient to prevent it from setting seed, can lead in about 20% of instances, to specimens producing basal side rosettes and overwintering to flower the following year. This is particularly the case when considering well-grown plants in sheltered, rich, damp soil situations, Richards (1997b), who made these observations, agrees with Faegri that the capacity for perennation in D. purpurea certainly seems to be under environmental control. However, further observation of related European species suggests that perennation may be under genetic control as well (Richards 1997b).

Toxicity

D. purpurea is a very poisonous plant, all parts of it being toxic, especially the seeds (Hutchinson 1972). The active toxic principles, digitoxin, digoxin and gitoxin (plus many other biochemically similarly substances with names derived from the botanical name of the genus (Cooper & Johnson 1998, p. 197)), are cardiac glycosides that remain widely prescribed in orthodox medicine as heart muscle stimulants.

The toxicity of the plant is unaffected by drying, storage or boiling and, as with Senecio jacobaea (Common Ragwort), stock fed hay containing Foxglove may be poisoned. Animals usually know to avoid the plant, but if alternative grazing is scarce, they may eat it and can develop a craving for the plant after being poisoned (Cooper & Johnson 1998, p. 63).

British and Irish occurrence

D. purpurea is common and widespread, and locally abundant on suitable soils throughout most of Britain and most of Ireland, although it is purely an introduction on Shetland (VC 112). In Ireland, it appears scarce, rare or absent in a band across the island from Dublin to Co Clare (H21-H9), which represents the underlying region of most limestone geology, although many large local peat bogs also occur in this area of the country (New Atlas).

In B & I, decorative forms of D. purpurea are also widely cultivated in gardens, sometimes involving imported foreign seeds from major horticultural suppliers; some of these forms are believed to escape and may become naturalised (Reynolds 2002).

European and world occurrence

Native in W, SW & WC Europe, D. purpurea is also found in Morocco. It belongs to the suboceanic southern-temperate phytogeographical element and has been widely introduced and cultivated elsewhere for ornamental and medicinal drug purposes (Clapham et al. 1987).

Subsp. purpurea covers most of the European range of the species; subsp. mariana (Boiss.) Rivas Goday occurs in S Spain, subsp. dubia (Rodr.) on the Balearic Islands and subsp. mauretanica Maire & Emb. in the Atlas Mountains.

As a medicinal and ornamental plant, D. purpurea has been spread well beyond its original range in W Europe and is now naturalised in parts of C & N Europe, N & S America, S Australia and New Zealand (Hultén & Fries 1986, Map 1638).

Medicinal uses

Foxglove was used by old herbalists for various purposes, most of them entirely remote from the valuable properties used in modern scientific medicine. Gerard (1597, 1633) recommended it for falls from high places, and the expressed juice of the leaves was used in the form of an ointment to treat scrofulous swellings. Bruised leaves were applied to clean old sores and ulcers. The plant was also boiled in wine and used as an expectorant (Dodoens 1554). The leaves alone are used to manufacture the drugs for which the plant is now used in the treatment of heart conditions.

The plant contains important glucosides, three of which are cardiac stimulants. The most powerful, digitoxin (also referred to as 'digitalis'), is an extremely poisonous and cumulative drug, insoluble in water; digitalin is crystalline and also insoluble in water; and digitalein, which is amorphous but readily water soluble and therefore can be administered subcutaneously in very minute doses. Dosage is extremely important in this latter case, as too little of the drug is ineffective, and a slight excess can prove fatal. Another ingredient, digitonin, is a cardiac depressant ± identical to saponin, the chief constituent of the Senga root (Grieve 1931). Digitoxin is used as a medication for heart failure. Its clinical use was pioneered by William Withering who in his Account of the foxglove and some of its medicinal uses etc. (1785) recognized that it reduced dropsy, increased urine flow and had a powerful effect on the heart.

Names

There are 91 English common names listed for D. purpurea by Grigson (1955, 1987), ranging from the familiar 'Foxglove' to 'Dead Man's Bells', and 'Hedge Poppy' to 'Poppers' and 'Scotch Mercury' to 'Witch's Thimble'. Many of the names are variations on the idea of the flower corolla being like a finger-stall and they were often playthings of children in this way, despite the extremely poisonous nature of the species. The fact that some names referred to 'Deadmen's fingers' and Deadmen's Thimbles' suggests an awareness of the toxic content of the plant tissues.

There are numerous references to flowers that 'pop', such as 'Flop-a-dock', 'Flop Poppy' and 'Flops'. The connection with foxes is also frequent, including 'Fox-and-leaves', 'Fox Docken', 'Fox Fingers', 'Foxter' and 'Foxy', possibly derived from the notion that the plant grew from fox droppings (Grigson 1955, 1987).

Foxglove was also considered a fairy plant, or a goblin's plant with magical connections that had to be respected. There were also supposed to be dangers that had to be evaded, including avoiding cutting the plant, and connections with changlings and missing children.

Threats

None.

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