Pest number 1: Himalayan Balsam (Impatiens balsamifera) Pest number 2: Japanese Knotweed (Fallopia japonica) Pest number 3: Parrot's Feather (Myriophyllum aquaticum)
Pest number 4: Giant Hogweed (Heracleum mantegazzianum) Purple Loosestrife (Lythrum salicaria) and Meadowsweet (Filipendula ulmaria) Yellow-Loosestrife (Lysimachia vulgaris)
Fringed Water-lily (Nymphoides peltatus) with damselfly Yellow Flag (Iris pseudacorus) can become rampant Bogbean (Manyanthes trifoliata) is from the Periwinkle family
Small Teasel (Dipsacus pillosus) Ragged Robin (Lychnis flos-cuculi) is in decline Great Willowherb (Epilobium hirsutum) is a significant native coloniser of damp areas
Southern Marsh Orchid (Dactylorhiza praetermissa) is the subject of a Biodiversity Action Plan Greater Dodder (Cuscuta europaea), a nationally scarce parasiteMegachile willughbiella male covered in Himalayan Balsam pollen

One of the greatest threats to our wetlands is escaped non-native species, of which there are a host. Across all habitats, English Nature has found more than 2,000 non-native species and hybrids in the wild in Britain, of which almost 1,800 are flowering plants, most of them garden escapes.

The negative effect of these on biodiversity can be immense, with Giant Hogweed (Heracleum mantegazzianum), Japanese Knotweed (Fallopia japonica), Himalayan Balsam (Impatiens balsamifera) and Australian Swamp Stonecrop (Crassula helmsii) among the worst culprits in flora.

All four grow fast and have spread far and wide - the whip-spring method Himalayan Balsam employs to propel its seeds anything up to seven metres from the plant is one explanation of why it has travelled so fast down our rivers. Moreover, all four aliens squeeze out native plants, and none is easily eradicated. Giant Hogweed in particular is dangerous to humans since brushing against this giant, which can reach five metres, damages the skin. The only advantage Himalayan Balsam offers is to provide excellent foraging for certain invertebrates,Alder (Alnus glutinosa) showing fruits and male catkins such as the ghostly male Megachile willughbiella bee pictured alongside.

They can be found in or by water, which indicates the level of the threat to our waterways. Parrot's Feather (Myriophyllum aquaticum), a water milfoil, is not much better, and is particularly hard to eliminate once it has turned up in a pond.

In passing, it remains a mystery why plants and animals, such as Terrapins and Bullfrogs, which are known to pose a direct threat to native flora and fauna if escaping or released, are permitted to be sold in pet shops in the UK. Perhaps there is no mystery - if money is to be made, and commercial lobbyists can exert influence, the chances of prohibiting sale via legislation are next to nil.

On to something brighter - native wetland plants, though like the aliens Great Willowherb (Epilobium hirsutum) can have a tendency to grow fast and dominate. It often reaches 1.5 metres, which gives it an edge. Of similar stately size are Purple Loosestrife (Lythrum salicaria) and Meadowsweet (Filipendula ulmaria), which are common on riverbanks and in ditches, though Himalayan Balsam has been having an effect on the former. They flower from June to August, though not often together.

Purple Loosestrife has become something of a pest in the USA, just as their dashing Monkeyflower (Mimulus guttatus) is having an impact on native species in parts of the UK. Meadowsweet, with its peculiar almost sickly smell, is a member of the Bramble family. Despite sharing the name, Yellow Loosestrife (Lysimachia vulgaris) is not from the same family as Purple Loosestrife, coming instead from the Primrose family, but is equally found in wet areas. It provides sustenance to a scarceAlder carr is a valuable habitat found in wet or waterlogged areas bee, Macropis europaea.

Small Teasel (Dipsacus pillosus) is related to Teasel but smaller, much less showy and found almost entirely in damp areas, including by watercourses.

Drainage has destroyed many colonies of Southern Marsh Orchid (Dactylorhiza praetermissa) in lowland England and the species is the subject of a Biodiversity Action Plan. This is one of our largest orchids, to 70cm in extreme instances, and large stands make an impressive sight in June and July. It can tolerate a wide range of soils but they have to be moist or damp, including marshes, wet meadows, dune slacks and fens.

Bogbean (Manyanthes trifoliata) is a member of the Periwinkle family and is found only in shallow wet areas where it can spread fast. The flowers attract damselflies and also Elephant Hawk-moths (Deilephila elpenor), which are generalists in habitat and also use Great Willowherb as a foodplant.

Fringed Water-lily (Nymphoides peltatus) is increasing in part because it is popular with aquarists, which leads to escapes. It, too, can provide very dense cover and, like all water-lilies, it needs managing vigorously.

The same applies to the commonly found Yellow Flag (Iris pseudacorus), which can carpet an area of marginal wetland to the exclusion of other plants.

Greater Dodder (Cuscuta europaea) is a nationally scarce plant which parasitises principally Common or Stinging Nettles (Urtica dioica). It seems at home mainly by rivers, canals and other fresh water, where these days, thanks to over-enrichment of the water with nitrates, there tends to be no shortage ofAlder provides winter feeding for mixed flocks of finches including the dashing Lesser Redpoll nettles.

Ironically, Greater Dodder looks much less impressive to the layman than, say Himalayan Balsam or Japanese Knotweed, both of which are upstanding with pretty flowers, and as a result it is sometimes subject to unintentional cutting.

Ragged Robin (Lychnis flos-cuculi), with its distinctive 'human' petals, is a plant of wet meadows and marshes. Unsurprisingly, it has suffered a decline thanks to the removal and/or drainage of these habitats in modern times.

One of the commonest trees found in wet areas is the Alder (Alnus glutinosa), which is abundant and provides food for good numbers of finches most winters, including Siskins (Carduelis spinus) and Lesser Redpolls (Acanthis cabaret). The latter is handsome in the extreme with bright red colouring on the male and, to a lesser degree, the female. Unfortunately many Alders are being affected by the aggressive fungus Phytophthora alni, which tends to kill the trees it infects and is causing trouble in most of northern and central Europe.

Alder carr is the type of woodland associated with the eponymous tree and used to be common wherever there was regular seasonal flooding or spring-fed soil. There are not so many now, thanks in part to 'improvement' via drainage, and this loss is significant.

The fact that the ground is wet or waterlogged means there has often been little management, allowing a wealth of invertebrates to thrive on any dead wood. Moreover, the habitat suits various flora, including grasses, ferns and mosses.

Images © Jeremy Early. All rights reserved.

In 2013 I published My Side of the Fence - the Natural History of a Surrey Garden. Details may be found, and orders placed, via this hyperlink My Side of the Fence. In November 2015 Surrey Wildlife Trust published the atlas Soldierflies, their allies and Conopidae of Surrey, jointly written by David Baldock and me. Details are on this web page: Atlas.