Cuscuta epithymum (clover dodder)
Clover dodder is one of the most rapidly growing parasitic plants.
About this species
Cuscuta epithymum is a rootless, wholly parasitic plant that occurs most commonly on heather (Calluna vulgaris), gorse (Ulex europaeus) and clover (Trifolium) species. Clover dodder is native to Europe, but occurs almost worldwide. It is a member of the Convolvulaceae family, which includes Ipomoea and Convolvulus species, many of which are commonly known as bindweed and morning glory. Clover dodder is normally insect-pollinated, but can also self-pollinate. Studies have shown that flowers are visited by many different insects, including ants, bees and wasps.
Geography & Distribution
Although clover dodder is native to Europe, it has since spread across six continents, giving it an almost worldwide occurrence. However, the genus Cuscuta does not occur at high latitudes in the Northern Hemisphere or in desert areas of Africa and Asia. Small, remote islands such as the Galapagos Islands also lack species of this genus. Introduction of clover dodder to non-native areas can follow unintentional import of seeds of C. epithymum in forage crops.
Clover dodder is a rootless, parasitic plant in which leaves are reduced to minute scales. Its stems are hairless, thread-like (0.25–0.40 mm in thickness) and can be yellow, red or purplish in colour. Clusters of 7–25 stalkless, purplish flowers are arranged in a compact, sphere. Flowers are 2.5–3.0 mm long and have five sepals and petals. The sepals come together to form a cup shape, each individual sepal being longer than it is wide. The corolla tube (a tubular structure formed when all petals in a flower are united) is longer than the cup-shaped sepal assemblage. Stamens (male parts) protrude from the flowers and have oval shaped anthers (pollen-bearing parts). Within flowers, the style (female structure) extends from the top of the ovary and is usually as long as the ovary. The fruit is a spherical capsule, which opens along its circumference and is topped by the withered petals. Each capsule contains four small seeds about 1 mm in diameter.
There are six varieties of Cuscuta epithymum, which are distinguished by differences in their flower structure as well as the number of flowers and how they are grouped.
Illustration, left: From Joseph Jacob von Plenck, Icones plantarum medicinalium, Vienna, Apud Rudolphum Graeffer et Soc., 1788. Centuria 1. Plate 71.
Illustration, right: ‘Cuscuta’ by Thomas Duncanson in Royal Botanic Gardens, Kew. 1824.
Invasion of the host plant
Clover dodder seeds germinate April–July in response to seasonal temperature changes. A swollen, root-like organ emerges from the seed but can only survive for a few days and hence must find a host plant quickly. When a host is found, clover dodder grows a sucker that attaches to it. The root-like organ then withers and clover dodder continues to grow using nutrients stolen from the host plant through the sucker.
Cells of the sucker produce an adhesive substance that works as an effective glue. Within the tissue of the sucker, an intrusive organ (haustorium) develops and directly penetrates the host tissue to create a bridge between host and parasite. This direct contact enables rapid growth and is one of the reasons why Cuscuta species are the most rapidly growing parasitic plants.
Clover dodder stems wind counter-clockwise around the host plant as it grows. New haustoria (organs that penetrate the host tissue) develop close together wherever the dodder stem produces tight coils as it climbs up the host plant. These tight coils alternate with looser coils that search for new host branches or new hosts to parasitize. Extent of growth depends on the clover dodder’s compatibility with the host and vigour of the host plant.
When growing on perennial hosts, Cuscuta epithymum induces the formation of galls (growths of host tissue) where parasitic tissue is able to overwinter. The following spring, new clover dodder plants develop from these galls.
Threats & Conservation
In north west Europe, Cuscuta epithymum flourishes on heathland that are 0–3 years old. Disturbance resulting from turf-cutting, burning or mowing of heathland aids the germination of clover dodder seeds. European dry heath is usually managed on a rotational basis, which maintains a high level of biodiversity. Clover dodder seeds may lie dormant, which extends germination potential over several years, and this is crucial for the persistence of this species. In this way it can build up a ‘seed bank’ within the soil to cope with the dynamic nature of managed heathland. However, if a heathland remains unmanaged for a long period of time or if the seed banks are too small or not viable for long enough for successful reestablishment, local extinction of clover dodder may occur.
Clover dodder is used in traditional medicine as a purgative and to treat disorders of the liver, spleen and urinary tract. However, there has been little scientific investigation into the actual effectiveness of its use.
Millennium Seed Bank: Seed storage
The Millennium Seed Bank Partnership aims to save plant life worldwide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
See Kew’s Seed Information Database for further information on C. epithymum seeds.
This species at Kew
Herbarium specimen of Cuscuta epithymum
Clover dodder is not currently recorded at Kew, but the related species Cuscuta europaea (greater dodder) can be seen growing on Scutellaria altissima (tall skullcap) in the Order Beds at Kew.
Dried and spirit-preserved specimens of C. epithymum are held in Kew’s Herbarium where they are available to researchers by appointment. The details of some of these specimens, including some images, can be seen online in Kew’s Herbarium Catalogue.
Specimens of the flowers and seeds of C. epithymum, as well specimens of it growing on heather, are held in Kew’s Economic Botany Collection in the Sir Joseph Banks Building, where they are available to researchers by appointment.
Cuscuta epithymum is one of 50 species on the Botanical Society of the British Isles (BSBI) Threatened Plants Project list. The herbarium specimens of these species are being digitised as part of the UK Virtual Herbarium Project. Find out how you can help in this project by visiting herbaria@home.
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