Monday, July 21, 2014

The aliens might be coming

Rhododendron x superponticum - beautifully strangling a lakeside at the Yorkshire Sculpture Park.

Whether it is kudzu vine strangling trees in Virginia, melaleuca throttling the Everglades or Himalayan balsam on the English river bank, invasive aliens are big news. Gardeners of course have a particular interest in them, as all too often they are responsible for the introduction and distribution of these plants in the first place, and in some regions, anything new in the garden is something of a potential Trojan horse.

A new book about invasives: plants and animals takes a nicely-balanced look. Where do camels belong? Its author, Ken Thompson is one of the best British writers on natural history. A retired member of staff at the University of Sheffield, he has worked on plant ecology for much of his life. I can't imagine a better person to keep a cool head on this topic. He is one of the key people behind the BUGS project which is looking at the relationship (or lack of it) between plant origin and wildlife in British gardens. He is particularly good at poking sacred cows with a science stick, and on occasion, as when he had a go at the fashionable pretences of permaculture, something of a cattle prod.

This is a very readable book and a balanced one. A key message is that many invasive alien stories have more bark than bite, that a species which may appear to be spreading may not be anything like as bad as it either appears to be, or the local press tell you it is. He does discuss some of the real horror stories, and does indeed recognise that aliens can be a very severe problem in some situations. Although if I lived somewhere where an alien species was causing real difficulties, I think I might feel the book didn't go quite far enough in recognising this. Some species really do destroy ecosystems. But, most don't and one of the valuable points of this book is pointing this out, and that over time many invasive plant species reduce in number, or start to get eaten by the local wildlife or infected by the local pathogens. 

One of the key points Ken raises is the cost and the impossibility of controlling many invasions. He discusses the whole new field of invasion biology. One can't help but get the feeling that there are a lot of people with a vested interest in keeping fears of invasives stoked up – a nice source of grant money for research/control etc. The implication is – don't throw money at things you can't do much about and keep it for things that either work or for battles that are really worth fighting. Invasives help keep journalists in business too, with lurid press stories often an opportunity for some covert racism – a topic which the book could have spent more time on. 

Being British and discussing invasives is a slightly odd position. Our flora has (like us) spread throughout the world, often aggressively. The turf-forming grasses of north-west Europe in particular – much of the native flora of western USA has been throttled by these aggressively spreading plants. Sounds like the effect of the European empires on global cultures. However the dense matt of growth these grasses form, and their ability to grow at low temperatures, does mean that it is very difficult to get invaded back. There isn't a single North American species which has become a problem here. The problems we have with our worst invasives (Japanese knotweed and Rhododendron x superponticum) are limited in geographical extent and pale into insignificance compared to the problems many other places face. Ken points out that some of our natives behave as invasives here, on home ground. As with my point about the nitrogen pollution fed nettles a few posts ago.

A recent trip to the US Pacific North West was an opportunity to appreciate just what a huge impact non-native and (in some eyes at any rate) invasive species make. It all depends on habitat. One of the wise points Ken makes in his book is that most invasive alien problems are in disturbed habitats. Any kind of succession process that starts taking a vegetation back towards what might be found naturally there will inevitably reduce their impact. Out in the open in the PNW you see a lot of European aliens, but few in the woods. In an area where thick conifer forest is the norm you could argue that any bit of open land is disturbed. Much grassland, in the Columbia River Gorge for example is populated by European turf grasses, and spattered with the European cornflower (Centaurea cyanus) and vetches, the native California poppy appearing in much lower quantities. It was almost uncanny how much of the open habitat was composed of 'interlopers'. But this kind of grassland is largely a human artifact, so should it surprise us that it is full of non-native species? None of which by the way was dominating anything else and incorporated many natives too. It all looked uncannily 'natural'. 
Eurasian vetch Vicia cracca and European grasses make a meadow in Oregon, but natives seem to join in too, but what was here before?

California Poppy - Eschscholzia californica in habitat with the far more common european cornflower, whose presence seems largely benign. This is south side Columbia river gorge, near Hood River.

Inevitably I think about what is escaping from my garden. Geranium x oxonianum 'Claridge Druce' is scattering itself into the hedge bottom. I have often seen this species escape. I suspect it will settle down to evolve into a true species and a widespread component of the British flora. With its parents from northern Spain and Italy, it is effectively a neighbour and can be expected to settle down decorously. Persicaria amplexicaulis has, somewhat surprisingly, taken up home in the hedge bottom too – funny as it never seeds in the garden. Its habit is much reduced compared to how it grows in the border and hardly has spread at all. Its scarlet spikes look oddly at home though. It is Senecio fuchsii which I think will inevitably take off. A tallish yellow daisy, it lights up woodland edge habitats from one of Europe to another, except that is did not get across the English Channel in time after the last ice age. A native really. In fact I think we can say that any north European plant is effectively a native for this reason, as they would almost certainly been here before the ice scraped everything off. The senecio seeds like crazy in the garden and will no doubt one day wander into the woods, where really it belongs. There is little in the wild flora that flowers in shade at this time, so pollinators will probably be really glad of it. Maybe it just wants to come back home.
Senecio fuchsii (S. nemorensis) good but slightly tatty as a garden plant, great in light shade.

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Unknown said...

I'm currently trying to rescue a garden I planted last year where half the plants are suffering from herbicide injury, most likely from something persisting in the soil after knotweed treatment 3 years ago. I'm sure the treatment was necessary, but it's still bloody depressing. Ironically, considering Ken Thompson's hatchet job on permaculture (perhaps he should have stopped at "I just don't get it"), I first came across the idea that our attitude to invasive plants would benefit from reconsidering the ecological reasons for why they establish so aggressively and what useful functions they may have, in the permaculture movement, particularly Masanobu Fukuoka. Here's a book from that side of the divide

It may alienate some because the guy has a scruffy beard and talks about gaia, but look past that and he's making a lot of the same points. I have sympathy and find a lot of use for both the scientific/ academic approach to things, and the intuitive/ mystical/ hippy tendencies. Ever hopeful that plant(and planet)loving people can get past the distractions of cultural differences.

Roger Brook - No Dig Gardener said...

Poor Jen, I guess some idiot used sodium chlorate to try and kill the knotweed. There are plenty of excellent herbicides such as glyphosate that leave NO residues.
I am ahead of you Noel I read Ken Thompson's wonderful book last month, could not put it down,.It helped to consolidate a lot of my own feelings which are ones of increasing sympathy for none native plants.
Loved the superponticum pics, almost identical to my own when I posted about this lovely plant- or weed depending on your point of view

Anonymous said...

Do you know this song?

The Return Of The Giant Hogweed
Genesis (1971)

Turn and run
Nothing can stop them
Around every river and canal their
Power is growing
Stamp them out
We must destroy them
They infiltrate each city with their thick dark warning odour
They are invincible
They seem immune to all our herbicidal battering

Long ago in the Russian hills
A Victorian explorer found the regal Hogweed by a marsh
He captured it and brought it home
Botanical creature stirs, seeking revenge
Royal beast did not forget
He came home to London
And made a present of the Hogweed to the Royal Gardens at Kew

Waste no time
They are approaching
Hurry now, we must protect ourselves and find some shelter
Strike by night
They are defenceless
They all need the sun to photosensitize their venom
Still they're invincible
Still they're immune to all our herbicidal battering

Fashionable country gentlemen had some cultivated wild gardens
In which they innocently planted the Giant Hogweed throughout the land
Botanical creature stirs, seeking revenge
Royal beast did not forget
Soon they escaped, spreading their seed
Preparing for an onslaught
Threatening the human race

Mighty Hogweed is avenged
Human bodies soon will know anger
Kill them with your Hogweed hairs
Heracleum Mantegazziani