About Digitalis purpurea L.
Digitalis purpurea L. is an herbaceous biennial or short-lived perennial plant. Its leaves are spirally arranged, simple, 10–35 cm (3.9–13.8 in) long and 5–12 cm (2–5 in) broad. They are covered in gray-white pubescent and glandular hairs that give the leaves a woolly texture. In its first year of growth, the plant’s foliage forms a tight rosette at ground level. In the second year, it develops a flowering stem that is typically 1–2 m (3.3–6.6 ft) tall, and sometimes grows even longer. Flowers grow in a showy, terminal, elongated cluster; each individual flower is tubular and pendent. Most plants bear purple flowers, but cultivated plants may additionally produce pink, rose, yellow, or white blooms. The inner surface of the flower tube is heavily spotted. Flowering occurs in early summer, and sometimes additional flower stems develop later in the season. Bees frequently visit this plant, climbing inside the flower tube to access its nectar. The fruit produced by D. purpurea is a capsule that splits open when mature to release numerous tiny 0.1–0.2 mm seeds.
The native range of Digitalis purpurea covers parts of Western Europe and North Africa. In Western Europe, it is native to Belgium, Czech Republic, France, Germany, Ireland, Norway, Portugal, Spain, Sweden, and the United Kingdom. In North Africa, this species is native to Morocco. It also occurs naturally on the islands of Corsica and Sardinia. The species has been introduced to regions across the world outside of its natural range. Introductions have allowed it to expand its range further into Europe and Africa, and it has also colonized Asia, North America, South America, and Oceania. It has been widely introduced across Europe to Austria, the Baltic States, Belarus, Denmark, Hungary, the Netherlands, Poland, Réunion, Ukraine, the Azores, the Canary Islands, Central European Russia, Madeira, the Sakhalin Islands, and the Kuril Islands. In North America, it has been introduced to over twenty U.S. states: Arkansas, California, Connecticut, Idaho, Maine, Maryland, Massachusetts, Michigan, Montana, New Hampshire, New Jersey, New York, North Carolina, Ohio, Oregon, Pennsylvania, Vermont, Washington, West Virginia, Wisconsin, and Wyoming. In Canada, it has colonized multiple provinces: British Columbia, New Brunswick, Newfoundland, Nova Scotia, Ontario, and Québec. In South America, it has been introduced to Argentina (specifically the Northeast, Northwest, and South regions), Bolivia, Brazil (South and Southeast regions), Chile (Central region), Colombia, Costa Rica, Cuba, Ecuador, El Salvador, Guatemala, Jamaica, Mexico (Central, Gulf, Northeast, Southeast, and Southwest regions), Peru, Uruguay, and Venezuela. In Asia, it occurs in China (South-Central and Southeast regions), Korea, and Vietnam. In Africa, it is found in Malawi and Zimbabwe. It has been introduced to both the North and South Islands of the island nation New Zealand, and it is an established weed in multiple locations across Tasmania, Australia.
Digitalis purpurea grows in acidic soils, in conditions ranging from partial sunlight to deep shade. It can be found in a wide range of habitats including open woods, woodland clearings, moorland and heath margins, sea-cliffs, rocky mountain slopes, and hedge banks. It is commonly found and readily colonizes sites with disturbed ground, such as recently cleared woodland, areas where vegetation has been burnt, sites disturbed by clear-felling, and construction project areas. It is often one of the first wildflowers to reappear in these disturbed areas, frequently growing in large quantities. Foxgloves are eurytopic plants, and their seeds only germinate when exposed to light. For this reason, they are generally absent from permanently shaded areas such as the closed interior of woodlands. D. purpurea grows most successfully in humus-rich soil, but it can grow well in any mesic soil, and only requires a small amount of soil to survive. Bees use its nectar to make honey. Larvae of the moth Eupithecia pulchellata, the foxglove pug, consume D. purpurea flowers for food. One caterpillar crawls into a newly opening flower, then spins a silken web over the flower’s mouth to seal it closed before feeding on the flower’s stamens and developing seeds. When uninfected flowers fall from the plant, the corolla of sealed infected flowers stays attached, and the caterpillar pupates inside the flower. The foxglove pug moth is an uncommon species, with recorded occurrences in Britain, Germany, Switzerland, and Austria. Other Lepidoptera species have been recorded eating D. purpurea leaves: Mellicta athalia and Xestia ashworthii in Britain, Eurodryas aurinia in Romania, and Mellicta deione in Portugal.
All parts of Digitalis purpurea — leaves, flowers, and seeds — contain the cardiac glycoside digitoxin, making them poisonous to humans and some animals, and ingestion can be fatal. The main toxins in Digitalis species are two chemically similar cardiac glycosides: digitoxin and digoxin. Like other cardiac glycosides, these toxins work by inhibiting the ATPase activity of the sodium potassium ATPase pump (Na+/K+-ATPase), a complex of transmembrane proteins. Inhibition of Na+/K+-ATPase causes rises in both intracellular sodium and calcium, which in turn produces increased force of myocardial muscle contractions. At the correct precise dosage, Digitalis toxins cause the heart to beat more strongly. However, digitoxin, digoxin, and other related cardiac glycosides like ouabain have very narrow therapeutic windows. Because these drugs have steep dose-response curves, very small increases in dosage can make the difference between an ineffective dose and a fatal one. Modern medicine now has better treatment options with wider therapeutic windows. Symptoms of Digitalis poisoning include low pulse rate, nausea, vomiting, and uncoordinated contractions of different parts of the heart, which can lead to cardiac arrest and ultimately death.
Digitalis purpurea is a popular ornamental plant that adds height and color to gardens from late spring to early summer. Cultivated forms often produce flowers all around the central flowering spike, unlike wild forms that only grow flowers along one side of the spike. Numerous cultivars have been developed that offer a wide range of flower colors, and seeds are often sold as mixed-color blends — for example, Excelsior hybrids, which come in shades of white, pink, and purple. Some strains are easy for novice gardeners to grow, while others are more challenging to cultivate. Potted D. purpurea plants are also available for purchase in spring. This species is hardy down to −15 °C (5 °F), which corresponds to USDA hardiness zones 4–9.
The cardiac glycoside digitoxin, extracted from D. purpurea leaves, is used as a medication to treat heart failure. Clinical use of this compound was pioneered by William Withering, who recognized that it reduced dropsy, increased urine flow, and had a powerful effect on the heart by slowing the heart rate to allow it to fill with blood before pumping more efficiently. During World War II, German blockades created shortages of medicinal herbs, so County Herb Committees were established to collect wild medicinal plants including Digitalis purpurea, which was used to regulate heart rate.
