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Parasites galore

Two species of parasitic toothwort are currently in flower at Kew. Mike Fay explains more about their fascinating biology, history and where you can see them.

A walk through Kew at this time of year will allow you to see two root parasites in flower, both species of Lathraea (Orobanchaceae), one native and one introduced as a garden plant. Both have rhizomes and form colonies around the base of their mostly woody hosts. They lack chlorophyll and so they are totally dependent on their hosts for food. At Kew, they can both be found growing on black walnut trees (Juglans nigra).

Toothwort (Lathraea squamaria) is a native plant found in woods and hedgerows, parasitising elms (Ulmus species), hazel (Corylus avellana) and other woody plants. It is relatively widespread in Britain and Ireland, occurring as far north as central Scotland. Evidence for its status is a little confused, with some studies suggesting that the population is decreasing and others proposing that it may be increasing . Robert Gathorne-Hardy, in his characteristically dramatic style, wrote that this species has “flowers that are all the colour of raw meat”. You can find a well-established colony around the base of a young walnut tree behind the north end of the Orangery on the Little Broadwalk.

Purple toothwort (Lathraea clandestina) is altogether a more showy species native to Belgium, France, Spain and Italy. Gathorne-Hardy was much more complimentary about this species, describing it as resembling “fat bunches of violets lying about in the sylvan meadows”. It mostly parasitises willows (Salix species) and poplars (Populus species) in nature, but in cultivation it has been recorded from a much wider range of woody hosts, including angiosperms and gymnosperms, and even the non-woody Gunnera (e.g. Fay, 2010).

Purple toothwort was introduced into Britain in the 19th Century as an attractive novelty due to its relatively large purple flowers produced at ground level. At Kew, it can be seen around the foot of the large black walnut on the north side of the Woodland Garden facing the Order Beds.

It was first recorded in the wild in Britain in 1908 and it has spread widely across Britain, at least in part due to its explosive mechanism of seed dispersal. This was reported in detail by Sir Edward Salisbury (Director of Kew, 1943–1956) in his book Weeds and Aliens, including a description of the famous plantsman E. A. Bowles amusing himself by firing seeds at other members of committee that he was chairing at the Royal Horticultural Society! It also occurs as a naturalised species in Ireland.

Other aspects of the biology of purple toothwort have caught the attention of scientists over the years, notably its extremely alkaline nectar (pH 11.5), thought to be a defence against nectar robbing by ants. The high pH is due to the level of ammonia, to which bumble bees (the pollinators) are less sensitive than ants. Reports that this species is carnivorous (digesting small organisms in the air spaces in its scale leaves) have been debunked in several scientific studies, but this has not stopped such stories appearing in the scientific and popular press from time to time.

If you are in the Gardens at Kew in the next couple of weeks, why not take the opportunity to see these two closely related but contrasting species.

Further information

Braithwaite, M. E., Ellis, R. W. & Preston, C. D. (2006). Change in the British Flora 1987-2004. London: Botanical Society of the British Isles.

Fay, M. F. (2010). 663. Lathraea clandestina. Orobanchaceae. Curtis’s Botanical Magazine 26: 389-397.

Gathorne-Hardy, R. (1938). Wild Flowers in Britain. London: B. T. Batsford.

Preston, C. D., Pearman, D. A. & Dines, T. D. (2002). New Atlas of the British and Irish Flora. Oxford: Oxford University Press.

Salisbury, E. (1961). Weeds and Aliens. London: Collins.