If you live in the English County of Kent and especially the part called Thanet, you will be familiar with this invasive alien. Its alternative name is Thanet Cress. It forms dense colonies at the exclusion of other species. At this time of year it looks attractive with its tiny white flowers crowded together to form a white foam, somewhat like lace. The reason it occurs especially in Kent is linked to a British military debacle in the Napoleonic Wars. We’ll return to that later.
The flowers, each with four petals (2.5-4.5 mm), four sepals and five stamens. You can just see the short white hairs on the leaves and stalk. Photo: Chris Jeffree
It belongs to the Brassicaceae, the family of mustard and cabbages. It has the characteristic flower of the Family, with four petals arranged like a cross. The Family name used to be Cruciferae – meaning ‘cross-bearing – and today people often call its members ‘crucifers’. The fruits of this Family are quite variable, and used in identification. For Hoary Cress, the fruit is like a flattened heart. The genus Lepidium has about 200 members worldwide, but in Britain and Ireland there are only 14. One of them, L. sativa, is commonly eaten as the Cress in the salad ‘Mustard and Cress’.
The inflorescence, very crowded and flat-topped. Note also the clasping stem-leaves. Photo: Chris Jeffree
The plant Hoary Cress has an extensive root system, forming first a tap root and then rhizomes which spread in the soil. Thus, a single colony like those shown in the images here, is really one single plant. Abundant small seeds are produced and their mucilaginous coating (when wetted) helps them to cling to feet of birds and humans, and the tyres of vehicles. Wind dispersal may also occur then the dry seeds are shed from the fruit. However, the way the plant usually spreads as a weed in arable fields is through cultivation. When ploughed, rotavated or dug, fragments of rhizome are scattered and thus in the following year the shoots appear in locations well beyond their original location. These days, soil is often transported to building sites and from road constructions – and a many species therefore hitch-hike across the country.
It’s called ‘Hoary’ because the leaves (both sides) usually have numerous very short hairs, which scatter light and make the leaves look greyish. The upper leaves are rather distinctive; they have no stalks and they clasp the stem.
L. draba, the fruits. Quite distinctive, heart shaped, 3-5 mm, there are two chambers per fruit, each containing 1 or 2 seeds. There is no special dispersal mechanism. Photo: Chris Jeffree.
The plant is native throughout southern Europe, north Africa, parts of southern Asia extending to northwest China. It has spread to all parts of the world where Europeans have settled. It came to Britain in the early 1800s; it was recorded from the United States in 1862 (New York) and there are early records from Canada in 1878 (Barrie, Ontario) and 1893 (Niagara Falls). According to the excellent review by Francis and Warwick (2008) it was likely introduced to North America by European settlers either with grass and alfalfa seed or in ships’ ballast.
One of its names is Thanet Cress, a botanical name connected to an event in history. The story of its introduction to Britain is told in Salisbury’s still-very-useful but somewhat out-of-date book Weeds and Aliens. In 1809 Britain sent 39,000 soldiers and 15,000 horses to the Netherlands on their way to surround and possibly capture Antwerp, all part of the Napoleonic Wars. They went first to Walcheren, which was then a swampy island. Many fell ill with ‘Walcheren fever’ (a combination of malaria, typhus and dysentery is blamed). About 8,000 died, and the entire army, with its many illness-stricken soldiers, was brought home to Ramsgate in what historians have called the Walcheren Debacle. But their sick-bed mattresses were filled with Dutch hay that contained Lepidium draba. Once in Kent, the hay was stored in a gravel pit and eventually given to a farmer in Thanet who incorporated it into a compost to fertilise his fields. Hence, the species found a foothold in Kent and earned the name Thanet Cress. Its spread throughout Britain from this introduction is well documented (Scurfield, 1962).
Image showing the Walcheren Debacle. ’Evacuation de Walcheren par les Anglais – 30 août 1809’ – Henri Félix Emmanuel Philippoteaux – (1870). Public domain from WIkipedia.
However, there is an earlier record (1802) from the banks of the River Tawe in Swansea, Wales. The plant persisted but is not especially common there at present.
Hoary Cress is a plant of roadsides, railways, docks, waste ground, saltmarshes, arable fields and pasture (Rich 1991). It grows best in soils with a high pH, and its current British distribution does reflect the geological pattern of limestone rocks. Its tendency to hug the coastline may be because coastal soils are often enriched with calcium carbonate in the form of ground-up sea-shells.
I was surprised to find this species at Loch Ryan near Stranraer. This sea-loch has plenty of crushed sea-shells, but the location is an ‘outlier’ well outside the normal Scottish distribution of the species. The site is next to a car park used by ornithologists who go there to spot Black-throated and Red-throated Divers and Slavonian grebes. It is also frequented by dog-walkers, who arrive in motor vehicles. Formerly, it was a site for seaplane testing in the First World War. I wonder, did rhizomes/seeds come via paws of dog, tyres of car, seaplane or feet of bird1?
Habitats of L. draba in Scotland. Top left, Wig Bay on the west shore of Loch Ryan near Stranraer, SW Scotland. Top right, by a railway at Inveresk, East Lothian. Lower: Firth of Forth, Prestongrange, East Lothian. Photos: top-left John Grace, others Chris Jeffree.
The invasive nature is achieved mainly by colonisation of the soil by the extensive root system, but in addition there is evidence that the plant may reduce germination of other species by compounds produced by leaves and roots (Kaya et al 2015). In parts of north America and Australia it is classed as a noxious weed, but it can be controlled with herbicide treatment (in former times, the go-to herbicides included the plant-hormone- mimic known as 2,4-D but in recent decades 2% glyphosate is favoured). On the positive side, in some parts of the world it has served as a traditional medicine and it does seems to have useful pharmaceutical properties (Chyad 2017). Plants for the Future finds uses for it, as a salad plant, for seasoning, and they say that the seeds can be a substitute for pepper. It can be grazed by sheep, but when grazed by cattle it is said to taint the milk.
It has been given various names. Formerly it was called Cardaria draba. The recommended vernacular name is Hoary Cress but in other English-speaking territories it is called Globed-podded hoarycress, Peppergrass, Whitetop and Whiteweed.
L. draba, distribution in Britain and Ireland, from BSBI/Maps. Left, distribution to the year 2000, Right, records since 2000.
In Britain it is no longer increasing, and it may even be receding in some parts, perhaps because of increasing herbicide control of weeds in arable fields since about 1960. In the world as a whole it may still be increasing, and in its introduced range it tends to be more vigorous than when in its natural range (Hinz et al 2012).
L. draba, world distribution according to GBIF.
References
Chyad AH (2017) Evaluation of anticancer, analgesic and anti-inflammatory activities of the ethanolic extract of Lepidium draba Linn. leaves. Advances in Animal Veterinary Science. 5, 7-13.
Hinz HL et al. (2012) Biogeographical comparison of the invasive Lepidium draba in its native, expanded and introduced ranges. Biological Invasions 14, 1999-2016.
Kaya Y et al. (2015) Phytotoxical effect of Lepidium draba L. extracts on the germination and growth of monocot (Zea mays L.) and dicot (Amaranthus retroflexus L.) seeds. Toxicology and industrial health 31, 247-254.
Mulligan GA & Findlay JN (1974) The biology of Canadian weeds. 3. Cardaria draba, C. chalepensis and C. pubescens. Canadian Journal of Plant Science 54, 149–160. doi:10.4141/cjps74-024
Rich TCG (1991) Crucifers of Great Britain and Ireland. BSBI Handbook No. 6. Botanical Society of the British Isles, London
Scurfield G (1962) Biological Flora of the British Isles. No. 84. Cardaria draba (L.) Desv. (Lepidium draba L.). Journal of Ecology 50, 489–499. doi:10.2307/2257459
1after writing this speculative comment I looked up the records on the BSBI database. The first BSBI record in Wigtownshire is from 1960, and the first record specifically from Wig Bay is 2012. The sea-plane hypothesis is therefore not supported.
©John Grace
Botany in Scotland 