Botanical Illustration: Invasive Plant Species

Story posted: Thursday, 4. June 2015 by Lizzie Harper

Natural history illustrators have to do natural science and botanical illustrations of both beneficial and of problematic species.

Many times, the reason a plant or animal is causing problems within an ecosystem is because it doesn’t belong in that system; it’s not evolved in equilibrium with the other species there, and thus can out-compete them.  In many cases this is because it’s been introduced from another part of the world and subsequently invades; hence the terms “invasive” and “introduced “ species.

This blog will look at three of the most problematic invasive plant species currently threatening British water ways; namely the Giant hog weed, Japanese Knot weed, and Himalyan balsam.

These plants create problems for canals and rivers (and other habitats) by out-competing native species for sunlight, water, and space; and can choke the waterways with their rapid growth.  They also kill other plants that colonise and stablise canal and river banks; this can lead to big increases in erosion.

The Giant hogweed (Heracleum mantegazzianum) is a monster amongst umbellifer plants, growing up to 5m tall.  It bears an umbrella of tiny white flowers in summer and has a hollow, ridged green stem, sometimes flushed or spotted with purple.  It has large divided leaves.

Its height shades out other plants, killing those which help bind the banks of waterways together and help slow erosion.  Originally introduced to ornamental gardens in the 19th C, it soon proved an opportunistic coloniser, spreading along streams and rivers with its water-borne seeds.  In fact, it is now illegal to plant or transplant Giant hogweed outside of gardens, and has been since the 1980s (Wildlife & Countryside Act 1981).

Lizzie harper natural history illustrator botanical illustration of Giant Hogweed

Gaint Hogweed Heracleum mantegazzianum

Despite its beautiful froth of white flowers, like cow-parsley on steroids, the Giant hog weed has sap which can burn the skin if it gets onto you and then is exposed to sunlight.  Before it was generally known that it had toxic sap, children would get blisters as they used the hollow stems to make pea-shooters or telescopes.

You can get rid of Giant hogweed by digging it out, or, at a push with weedkiller, but all skin must be carefully covered if you’re handling it to avoid the threat of blistering.  The RHS advises  only cutting it down on overcast days to reduce the reaction the sap has with sunlight.

Japanese Knotweed (Fallopia japonica) can grow up to 2m tall.  It has alternate smooth green leaves, and a climbing habit.  Flowers are little, fluffy and white and can be seen in autumn.  Stems are crimson red and hollow.

As before, the problems posed by this plant mainly relate to it out-competing native species for space, light, and water.  With the ability to grow up to 2cm each day it’s a veritable rocket of a plant, and rapidly shadows then kills other species whose roots are involved in binding river and canal banks together, thus increasing the risk of erosion.

Lizzie harper natural history illustrator botanical illustration of Japanese Knotweed

Japanese Knotweed Fallopia japonica

Again, it began life in Europe as an introduced garden plant, taken from Japan, Taiwan, and China in the 19th C.  It now smothers wasteland, waterways, and the edges of roads across the country.

It is particularly difficult to eradicate as it can regrow from fragments of as little as1cm in length, so cutting it down is advised against.  If you do try to eradicate it from your garden, or at least to control it, as you are now forced to do by law (the Crime and Policing Act 2014 insists on it) then any part dug up mustn’t be put out with other garden or household waste; it’s “controlled waste” and must be disposed of at designated landfill sites.  Glyphospahte weedkiller is recommended by the RHS.

The last of this terrible trio is actually a very beautiful plant.  Himalayan balsam (Impatiens glandulifera) can be identified by its pink spotty slipper-shaped flowers, tall scarlet ridged stems, and slightly toothed lance-shaped leaves (sometimes edged with crimson).  In autumn they produce amazing seed pods which fire the seeds long distances (up to 7m!) when touched; and it’s almost impossible for a child to avoid walking along setting off as many as possible.

Lizzie Harper natural history illustrator botanical illustration of Himalyan balsam

Sketchbook study of Himalayan Balsam Impatiens glandulifera

It’s yet another example of a plant first introduced to make a garden pretty (in 1830 – Wlidlife Trusts UK), which escaped and has now colonized much of the UK

If you’ve walked along a river bank near a town in much of Britain recently you’ll have seen the thickets formed by Himalayan balsam.  They out-compete most other bankside species, including grasses, which results in erosion once the balsam dies back in autumn.  It has a prodigal spread as each plant can produce 800 seeds; however,  it can safely be uprooted and left to decompose on site, and if you get the plant before it seeds then there’s some chance of success as Cornish trials suggest seeds can only be viable in the soil for 1 year.

Lizzie Harper natural history illustrator botanical illustration of Himalyan balsam

Himalayan Balsam Impatiens glandulifera

These plants were illustrated for an article in Gridline, the magazine of the National Grid (electricity) who have to maintain power cables and supplies across land and water which is often choked by these three invasives.

For more on these plants, please follow the links to the Wildlife Trusts pages: Giant hogweed, Japanese KnotweedHimalayan balsam; and for information on invasive species of the UKs waterways, please check out the Canals and Rivers Trust link.

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