London’s Exotic Flora

London’s Exotic Flora

“Our flora has gone through a revolution as great as the colonization of these islands after the Ice Age. The change has been very sudden; the majority of these naturalized species have been introduced in the past 200 or 300 years. This is tremendously exciting.”

(Martin Ingrouille. Historical Ecology of the British Flora. 1995)

“The wild flowers of our towns and cities would be a drab lot if it weren’t for these immigrants. For they’re not only fascinating echoes of the human life in urban areas, but a vastly colourful collection.”

(Richard Mabey. The Unofficial Countryside. 1973)

 

Introduction

Origins

London Plane Platanus x hispanica, London rocket Sisymbrium irio and Londonpride Saxifraga x urbium itself are all, self evidently, London plants, reflecting different aspects of the Capital. They are all also, perhaps surprisingly, introductions from overseas. They join a remarkable array of other introduced plants which form a significant element of London’s flora. More than 40% of the plants occurring in the wild that are described in the latest flora of the British Isles (Stace, 1997) have originated from overseas. In London that proportion is considerably exceeded and such introductions are more varied and more significant here than anywhere else in Britain. Coming from many different parts of the world, they provide a truly international dimension to the Capital’s flora and well reflect the cosmopolitan nature of the City itself. They have arrived here by many varied routes. Some, for example, were introduced originally as birdseed, others have been brought in inadvertently as seeds traveling together with a wide range of imported materials (foodstuffs, timber, minerals, wool waste or “shoddy”, etc.). As the country’s largest trading and commercial centre, London has been particularly rich in such seed sources. These are now however, more limited as a result of modern developments, such as seed cleaning and containerization, as well as changes in world trade. In addition, a significant number of exotic plants have ‘escaped’ from gardens to become naturalized and some of these are discussed below.

The urban climate

Many introductions come from warmer parts of the world and flourish in London as a consequence of the ‘heat island effect’. In common with other large cities, the central built-up part of London experiences higher temperatures than the more peripheral areas and the countryside round about. This is accounted for by the presence of buildings and streets with their reflective surfaces, and the abundance of domestic and industrial heat sources. As a result the centre of London has a reduced number of frosty nights, a longer growing season and higher maximum temperatures in the summer. This brings the urban climate closer to that of the Mediterranean, allowing plants such as, London rocket Sisymbrium irio, Chinese mugwort Artemisia verlotiorum, Guernsey fleabaneConyza sumatrensis, hoary mustard Hirschfeldia incana, and many other warmth-demanding species to thrive. Many such exotic plants are currently scarce outside London and they add greatly to the local distinctiveness of the Capital’s flora. With global warming predicted to continue, they are likely to spread more widely outside London in the future and some are already doing so.

Garden origins and the botanical melting pot

The British are often characterized as a ‘nation of gardeners’ and nowhere is this more apparent than in London, where the number and variety of gardens is a striking feature (see Garden Habitat Statement). Many naturalized exotic London plants derive from such gardens, either as a result of seed dispersal (buddleja, michaelmas daisy, cotoneaster, antirrhinum etc.) or as ‘cast-outs’ of more vegetatively vigorous species (Japanese knotweed Fallopia japonica, ground-elder Aegopodium podagraria, Canadian golden-rod Solidago canadensis, snowberry Symphoricarpos albus, etc.). Occasionally such garden-derived species form hybrids with their native relatives, and London supports a number of these unusual plants. One example is the Highclere holly Ilex x altaclerensis which is now a feature of many London woods. This plant has arisen through hybridization between our native holly Ilex aquifolia and a species from the Canary Islands (Ilex perado). The most widespread bluebell in London is also a hybrid (Hyacinthoides x variabilis), derived from the native speciesH. non-scripta and the Spanish bluebell H. hispanica. Both these hybrids are fully fertile with their parents and ‘hybrid swarms’ result, with plants showing a wide range of intermediate characters. Concerns have been raised that, particularly in the case of the bluebell, this interbreeding might result in the eventual decline of the native parent. Intervention, however, may not always be appropriate (or practical!) in urban London where the woodland flora is simply adapting to its surroundings. Should this process spread to the wider countryside, however, where our native bluebell woods are of international importance, prompt action would need to be taken. A third example of hybridization, often found on railway land in London, involves the native sticky groundsel Senecio viscosusand the introduced Oxford ragwort S. squalidus. The cross between these two was originally named after London (Senecio x londinensis) where it was first discovered new to Britain in 1944 by the distinguished botanist J.E. Lousely. Although this particular hybrid is sterile, such events can be an important part of the evolutionary process and may sometimes lead on to the appearance of new species. Welsh Groundsel Senecio cambrensis, for example, arose in such a way through the original crossing of Oxford ragwort and our native common groundsel S. vulgaris. London may thus be seen as something of a botanical ‘melting pot’ giving rise to the origin of new plant hybrids and possible new species.

Conservation considerations

It is argued that because native plants in general support a greater variety of dependent animal life than introductions, conservation management should favour the former. While this is undoubtedly true as a generality in semi-natural habitats, it should also be remembered that there are many introduced plants that provide nectar for insects (perhaps at a time when native sources are in short supply), fruit for birds and mammals and, in the case of many coniferous trees, valuable nesting sites for birds. It is also the case that introductions can sometimes support a considerable biomass of the relatively few invertebrate species that have adapted to them. Sycamore Acer pseudoplatanus is one well known example, but there are many others as gardeners often discover to their dismay! Such large invertebrate populations can be an important source of food for insectivorous birds.

A small minority of introduced plants have caused, and continue to cause, significant problems in London through their vigorous growth and invasive tendencies. Such species will often need to be controlled where they threaten rare or declining species or their habitats. Ponds and canals are particularly susceptible to such invasions. A notorious trio of aquatic introductions have had a particularly serious impact in recent years. These are New Zealand pigmyweed Crassula helmsii, parrot’s-feather Myriophyllum aquaticum and, most recently, floating pennywort Hydrocotyle ranunculoides. All three are grown by aquarists and get discarded into ponds and other water courses where they can spread vegetatively at a remarkable rate. Their control will be addressed as a priority in relevant Habitat Action Plans. Plantlife is calling on the Government to introduce legislation to prohibit their sale and these proposals should be supported (Harper 2000). The tropical American water fern Azolla filiculoides can also cause problems, but is rarely as persistent as the above three. A few terrestrial introductions also need to be controlled in some circumstances. Japanese knotweed Fallopia japonica and giant hogweedHeracleum mantegazzianum, whose deliberate introduction is outlawed by the Wildlife and Countryside Act (Schedule 9), are particularly well known examples. However, such plants make up only a small minority of London’s introduced flora and it should be remembered that quite a few native species can also become invasive in certain circumstances (bracken Pteridium aquilinum, purple moor-grass Molinia caerulea and sea buckthorn Hippophae rhamnoides, for example). We need to balance the undoubted problems that some exotic species can create against the rich diversity, historical and cultural interest and the considerable local distinctiveness that the vast majority bring to the Capital. Most introductions are benign, and in urban areas the natural colonization of wasteland sites by native and exotic species has formed distinctive urban communities not found in the wider countryside. London would certainly be a drabber place without them!

Future Action

There are three major actions that need to be taken on behalf of London’s exotic flora. First is the urgent need to increase understanding and recognition of the ecological and cultural values of exotic plants which add so much richness to our city’s environment. Second, research into the history of London’s more significant exotic plants would be of great assistance to conservationists in communicating these values. Third, conservationists need to recognize key problem species, some of which may be exotic, as a distinct group. Their distribution and spread need to be monitored and action taken where necessary. We should do away with unhelpful native/non-native distinctions and promote management solutions to problem species whatever their origin. In addition to these three major actions for the flora, the value of London exotic fauna should be explored.

An ‘Exotic Top 20’

The following selected list of exotic plants found in the wild in London has been chosen to illustrate the variety of species present and the diversity of their origins.

 

Broad-leaved everlasting-pea Lathyrus latifolius

A colourful introduction from southern Europe whose magenta coloured pea flowers enliven railway embankments, roadsides and rough ground in many parts of London. Probably bird-sown from gardens.

Buddleja Buddleja davidii

The well known “butterfly bush” with its fragrant spikes of purple or white flowers is a native of China. Introduced to gardens at the end of the nineteenth century. it soon escaped by means of its light wind-dispersed seeds and is now a familiar sight on wastelands, old walls, banks and scrub. Can be invasive.

Canadian goldenrod Solidago canadensis

Our native goldenrod Solidago virgaurea is scarce in most of the London area but its more vigorous North American relative, escaped from local gardens, is widespread. Its numerous golden yellow flowers can provide a colourful contrast to the purple Michaelmas daisies amongst which it is often found. Both plants can provide a useful source of nectar for insects such as the common blue butterfly in the late summer. Both can also be invasive and may need to be controlled in some circumstances.

Chinese mugwort Artemisia verlotiorum

A more imposing plant than its rather scruffy native relative, common mugwortA. vulgaris, the Chinese species has tiny yellowish composite flowers that open much later, around November, so that it is rarely able to set seed in Britain. It can however, spread rapidly from detached pieces of rootstock and is locally abundant in parts of London and more widespread here than anywhere else in Britain. The details of its journey from SW and central China remain a mystery. A hybrid between the two species was first discovered in Tottenham in 1987 and is named after its finder:- “Wurzell’s wormwood” A. x wurzellii.

Cotoneasters Cotoneaster species

More than seventy species of cotoneaster are grown as ornamental trees and shrubs in gardens and parks throughout Britain. Their attractive berries are eaten by birds who thus disperse the seeds into the wild. Most come originally from China and the Himalayas.

Giant blackberry Rubus armeniacus

The commonest bramble in the wild in London has been bird-sown from gardens. Widespread in Europe and SW Asia, It has very large leaves with white undersides and much larger fruits than most native species. Sometimes known as the “Himalayan Giant”, it makes for excellent blackberry picking in the late summer.

Goat’s-rue Galega officinalis

Sometimes known as French lilac, this European member of the pea family has large, attractive bluish-mauve or white flowers. Escaped from gardens, it is now a colourful component of wasteland, railway embankments and other open habitats throughout London..

Ground-elder Aegopodium podagraria

The bane of many gardeners, the infamous gout-weed, bishop’s-weed or ground elder can be an intractable problem. It is said to have been introduced from Europe by the Romans as a medicinal and pot herb. Thrown out of gardens, it spreads most effectively into the wild by detached fragments of root. Its umbels of white flowers are a useful source of food for pollen-feeding beetles and other insects…

Guernsey fleabane Conyza sumatrensis

First recorded new to mainland Britain in London as recently as 1984, this vigorous grey-green annual plant with heads of tiny yellowish white composite flowers is now frequent throughout the capital in open habitats and disturbed ground. Coming originally from tropical South America, it has greatly benefited from the “heat island effect” of the city. Already starting to be recorded outside London, it seems likely to spread further as global warming continues.

Himalayan honeysuckle Leycesteria formosa

Grown in gardens for its handsome pendent terminal flower spikes bearing large purple bracts, this striking deciduous Asian shrub is increasing around London and its bird-sown seedlings can turn up unexpectedly in a range of habitats including ancient woodland.

Japanese Knotweed Fallopia japonica

Much admired by the Victorians for its extraordinary vigour and architectural qualities, and warmly recommended for planting in “the pleasure ground or by the waterside” by William Robinson in his classic book “The Wild Garden” (1870). Times have changed and the plant is now notorious as an invader of open habitats where its dense thickets can surpress everything beneath. However, its abundant heads of small white flowers born in the late summer are attractive to insects and its thickets can provide useful cover for birds and small mammals. It has given rise, through hybridization with its close relative the Russian vine F. baldschuanica, to a unique London plant, the Haringey knotweed found only at Railway Fields near Finsbury Park.

London plane Platanus x hispanica

This familiar London tree, so characteristic of the city’s squares and suburban streets, is the first intercontinental hybrid tree to have arisen. Although of obscure origin, it is thought to be a cross between the oriental plane from the Balkan mountains and Asia Minor and the western plane of North America. Its hybrid vigour and pollution tolerance allow it to thrive in London. Seedlings are often found though they are seldom allowed to reach maturity.

Londonpride Saxifraga x urbium

A favourite London garden plant which deserves its appellation, being also notably tolerant of pollution. The name, sentimentalised in Noel Coward’s song during the last war, is applied to a range of rosette forming Saxifrages bearing clouds of starry pink or white flowers on long stalks. The true Londonpride is of hybrid origin; one parent, St Patrick’s cabbage S. spathularis, comes from western Ireland and the other, S. umbrosa, from the Pyrenees. It often escapes from gardens and can turn up in waste places, woods and by streams in London (and elsewhere).

London rocket Sisymbrium irio

First noted by 17th century botanists as growing in great abundance following the great fire of London in 1666. It apparently became extinct in the early 19th century but re-appeared in several places around the city at the end of the Second World War. Of Mediterranean origin, this annual plant with heads of tiny yellow flowers has benefited from the warmth of central London. Scarce and sporadic in appearance elsewhere in Britain, it can still be found in abundance on disturbed ground near the Tower of London and on the old Roman wall close by.

Michaelmas daisies Aster species

The purple flowers of these familiar garden plants from North America add colour to wasteland habitats and railway embankments throughout London in the late summer. A botanically complex group, they can spread from gardens both vegetatively and through their light, wind-dispersed seeds. They provide nectar for a wide range of butterflies and other insects at the end of the summer.

Opium poppy Papaver somniferum

A frequent garden escape that bears variously coloured purple, red and white petals, each with a dark spot at the base. Its ovoid capsules are the source of an alkaloid containing latex from which morphine and codeine are extracted in warmer parts of the world. An introduction from southern Europe and Asia.

Oxford ragwort Senecio sqalidus

Originating from southern Europe (found for example on the volcanic soils of Mount Etna in Sicily) it was long grown as an exotic introduction at the Oxford Botanic Garden. First recorded as an escape on the walls of that city in 1794, the plant arrived in London at the end of the 19th century after spreading along the railway system. Today, its cheerful yellow daisy flowers can be seen throughout the capital on wall tops, pavement cracks and in other such arid places.

Russian comfrey Symphytum x uplandicum

Formerly cultivated as a fodder crop, this vigorous plant is now much the commonest comfrey in London. Russian comfrey, with its drooping flowers of purple and mauve, is a hybrid between our native plant, common comfrey S. officinale, which is found in damp places and has pale creamy-yellow (or purplish) flowers , and a species from south-west Asia, rough comfrey (S. asperum) which has sky blue flowers, is very scarce in the wild and prefers dryer sites. The hybrid thrives in both habitats and can be invasive. All these comfreys provide nectar for bumblebees.

Slender speedwell Veronica filiformis

Many suburban lawns in London now provide a home for this tiny, blue-flowered speedwell from Turkey and the Caucuses. Seldom setting seed in Britain it spreads vegetatively from fragments of broken stem and is now frequent on roadside verges, paths and in churchyards. It was first recorded in London as a weed of tennis courts in Hounslow in 1942.

Tree of heaven Ailanthus altissima

This tree is frequently planted in the streets and parks of central London and few trees thrive so well in such places. It bears attractive reddish brown keys shaped like those of the ash and sows itself about quite freely around the city. It was introduced from northern China in 1751.

 

Further Reading

Gilbert, O.L.1989. The ecology of urban habitats. Chapman and Hall

Ingrouille, M. 1995. Historical ecology of the British flora Chapman and Hall

Mabey, R.1973. The unofficial countryside. Collins.

Stace, C. 1997. New flora of the British Isles. Ed.2. Cambridge University Press

Harper, M. 2000. At war with aliens. Plantlife, London.