These old Cotoneaster bushes are rooted in fissures in the limestone cliff behind Ashes quarry are Stanhope, in Weardale. Cotoneaster is a genus with many species, mostly from Eastern Asia, that has long been cultivated in Britain for its attractive berries in autumn; these plants would undoubtedly been of garden origin, originally arriving in the quarry in bird droppings. Although almost all berries in the dale have now been eaten by birds, this heavy crop remains almost untouched. This happens every year, and there is no obvious reason why birds should not have taken them. Maybe a flock of redwings or fieldfares will find them soon.

 

Milk thistle

A few days ago I had a call from a landowner who had unusual, unidentified thistles flowering on the edge of one of their fields. They turned out to be two milk thistles Silybum marianum, easily identifiable by the white variegated veins on their leaves. They are about five feet tall and are native to southern Europe, but have been introduced to many countries. They turn up sporadically in Britin, usually on waste ground, though rarely up here in County Durham. 

The plants are painfully prickly, along the leaf margins and especially on the tip of the bracts that surround the flowers. Milk thistles are biennials and it looks like this is now a small, self-sustaining population. Seeds might have arrived originally in animal feed.

I was first shown one here a couple of years ago, but then it was late in the season and all the flowers had run to seed (see bottom photo). This is the first time I've seen it in flower.

A Rapidly Evolving Botanical Hooligan......

Invasive species often hit the headlines here in Britain but we have exported a few of our own too, that have created ecological chaos in other countries. One of the most notorious is purple losestrife Lythrum salicaria. Here in the UK it's a well-behaved wetland species and the Wildlife Trusts' web site describes its 'impressive spikes of magenta flowers' as a 'valuable source for long-tongued insects like bees, moths and butterflies, including brimstones, Red-Tailed bumblebees and Elephant hawk-moths' 

In our garden it's a star performer in a small, boggy area where it's particularly popular with hoverflies ...

.... that feed on its readily accessible pollen. It does have a very high potential seed output, so most gardening experts advise removing the dead flower heads before they can set seed but in the wider countryside it's a well behaved component of our native wetland flora and is sometimes deliberately introduced to increase botanical biodiversity, as it has been .....

...... here along the Ouseburn in Newcastle, bringing a splash of colour to what was once a heavily industrialised and polluted environment.

However, when it travels abroad it develops hooligan tendencies, spreading rapidly and outcompeting native vegetation. Since it was first introduced into the New World in the New England area, probably sometime around 1800 in ships' ballast (and also deliberately because of its uses in herbal medicine) it has run riot, with a very rapid period of range expansion in the 20th. century. It's now the worst invasive plant species in wetland ecosystems throughout the eastern seaboard of the US and in Canada. You can read a detailed account of the history of purple loosestrife in North America and Canada by clicking here

Several theories have been advanced to account for its invasive behaviour, one of the most important being that when it travelled across the Atlantic it left its natural pests and diseases behind, so that it could reach its full reproductive potential. A single vigorous plant can produce fifty flowering stems that dominate surrounding vegetation and produce around two million seeds - which are carried far and wide by wind and water - annually. Once established in its new environment, and free of pests and pathogens, purple loosestrife no longer needed to devote energy to defending itself and could commit all its resources to reproduction.

Recently a research paper in the journal Science has provided another insight into the invasive tendencies of this New World immigrant. Multiple introductions of purple loosestrife produced a genetically variable population that has rapidly evolved ecologically adapted populations across a very wide geographical distribution. Plants that have colonise Canada, where the growing season is short, have evolved to flower early to maximise their seed output. Those that have migrated into the warmer southern United States have evolved to flower later and devote more of their resources to tall, vegetative growth before they switch into flowering mode. The authors of the research reciprocally transplanted sample plants from the extremes of their range to confirm the evolution of this rapid local adaptation; predictably, early bloomers from the north were less competitive in southern locations, and vice-versa.

One of the most interesting wider implications of this research is that it highlights the ability of plant species like purple loosestrife, that are genetically variable, that have a vast seed output and that reproduce rapidly once established, to evolve adaptations to new environments very quickly. Now that we are in a period of rapid climate change, it does give an indication of the ability of some plants to use their pool of genetic variation to exploit changing environments by evolving new adaptations, and it underlines the importance of maintaining genetic variability in plant species that might be vulnerable to climate change.


Source: Robert I. Colautti and Spencer C. H. Barrett (2013). Rapid Adaptation to Climate Facilitates Range Expansion of an Invasive Plant. Science 342, 364

In praise of some non-natives

Wildlife gardening evangelists frequently exhort us to plant native species in our gardens - quite rightly, as there are indeed many native plants that are decorative and provide valuable resources for insects and their larvae. 

But when it gets to this point in autumn the choice of flowering natives is pretty narrow. There's ivy, which is certainly a terrific source of pollen and nectar for insects and also food plant for holly blue butterfly larvae, but that's about it  - other that a few late hogweed flowers and yarrow, which flower up until the first frosts but don't bring much colour to a garden.

So that's when Michaemas daisies Aster amellus, whose natural range extends across southern Europe into Asia, come into their own. Whenever the sun shines the Michlaelmas daisies in our garden attract a constant stream of visitors, including ....





.... hoverflies, like this Heliophilus pendulus

..........honeybees ......

.............. small tortoiseshells ............

.............and red admirals, all photographed in the space of a few minutes at the end of last week.

Marigold Calendula officinalis, which originated in southern Europe, provides similar services for butterflies throughout autumn.

Buddleia davidii, the famous butterfly bush from Central China and Japan, has - until recently - been a favourite amongst wildlife gardeners as a nectar source for butterflies, even though it usually finishes flowering long before the late autumn generation of small tortoiseshells, peacocks, red admirals and commas get into their stride. These days conservationists give B.davidii the thumbs down, on account of its invasive tendencies, but there is a much better Buddleia alternative - B x weyeriana, which is an interspecific hybrid between B.davidii and B.globosa and has very attractive pinkish-orange flowers and none of its parent's tendency to seed itself around prolifically. It's extremely hardy and continues to flower long after the first frosts, offering a 'last-chance saloon' for any insects that need to top up with nectar before going into hibernation.

A native pollinator explores an alien flower

These sand dunes, between Alnmouth and Warkworth, have long been a favourite destination for botanists. The hollows are filled with wild thyme, biting stonecrop, bird's foot trefoil, viper's bugloss and spotted, twayblade and pyramidal orchids, to name but a few. This is also one of the only places in our region where sea pea has been recorded - which I've searched for here but never found. But when we were there a couple of weeks ago, we did come across an interesting alien.

Our visit coincided with a mass emergence of five-spot burnet moths and when they were not busy producing the next generation they were swarming all over the flowers, notably ...

...... this garden iris, possibly Iris sibirica which someone must have planted here. Bearded iris, I. germanica, is recorded as an introduced alien here in George Swan's Flora of Northumberland, but this clearly isn't a bearded iris, which has a beard of hairs in the 'gullet' of the flower . It must have been here for quite a while and bore a couple of large, empty seed capsules from last year's flowers, so might seed itself around and establish a population. 

What was mos striking about the plant was what a magnet it was for the five-spot burnets. The plant is a bit of a beacon, being the tallest brightly-coloured object in the dunes, and they homed in on it from all around.

When they landed none of the moths seemed to have any idea how to feed on the plant's nectar, but they quite quickly worked out how to reach it.

Usually, like this one and the one below, they began by probing the petal surface with their proboscis ....

..

.... then, when they found that unrewarding, moved down to the petal base to reach the nectar.....

.... but after a while most seemed to align themselves with the yellow flash in the middle of the fall petal, which led them into the legitimate route to the nectar, used by the plant's natural pollinators which are bees.

This involved forcing their way into the confines of the tube between the fall petal and the standard petal above, which has the stamen concealed underneath it, so the visitor is forced to pick up pollen on its back.

Once the flower markings had pointed the burnet moth visitor in the right direction it invariably crawled all the way in - and became stuck, unable to reverse out. After a pause and a bit of a struggle the moths finally managed to force their way out through the narrow gap between fall and standard petals - and were always carrying iris pollen when they left.

The interesting aspect of this is that it's an example of a native pollinating insect responding correctly to the floral advertisements of an introduced, non-native flower species that is adapted to bee pollination and that the moths would not have encountered before or co-evolved with. The plants that the moths normally pollinate on the dunes have numerous small flowers with easily-accessible nectar. It's clear that these moths are pollinating the iris and most likely contributing to its seed set.

Evening primrose

We came across this patch of evening primroses when we were walking through the sand dunes at Cleethorpes local nature reserve at the weekend. There are about twenty very confusing evening primrose species and interspecific hybrids but I suspect this one is probably Oenothera glazioviana (formerly known as Oe.erythrosepala), which comes from North America. 

There are no native species of evening primrose in Britain but this is one very attractive alien species that no one seems to object to when it escapes into the wild. It seems to thrive in waste places, especially in poor, arid soils. Recently I've seen it flowering well in a supermarket car park and on the edge of a pavement in a city centre. The seeds are very small and I imagine they are easily distributed on birds' feet, and the pale yellow flowers, which seem almost luminous at dusk, attract moths which must very effective pollinators; Sir Edward Salisbury, former director of Kew Gardens and noted authority on weeds and alien plants, once calculated that these plants commongly produce around 33,000 seeds each.

One species of evening primrose, Oenothera biennis, is sometimes cultivated as a crop, on account of its herbal medicinal value. The oil extracted from the seeds contains high levels of linoleic acid, which has a variety of medicinal uses.

A long way from home.....

This little alien weed is gallant soldier Galinsoga parviflora. It's a native of Peru but is currently flourishing in cracks on the pavement and along the road edge outside Durham University's Elvet Riverside building near the centre of Durham city. The plant first escaped from cultivation in Kew Gardens in 1860 and became a common weed in south-east England, but didn't turn up in Durham until it was found in a field near the city in 1940. 

In his classic New Naturalist series book Weeds and Aliens (1961) Sir Edward Salisbury describes how the plant acquired its common name. Its escape into gardens around Richmond in Surrey led to a flurry of inquiries at Kew about this new invasive weed and gardeners were told that it was called Galinsoga, since at that time this Peruvian plant had no common name in Britain. The unfamiliar Latin name quickly became corrupted to the easier-to-remember gallant soldiers and has remained so ever since.

Around Kew the plant was also sometimes known locally as Joey Hooker weed, after Joseph Dalton Hooker who was director of Kew Gardens at that time.

The plant produces numerous seeds - typically about 2000 but as many as 15,000 in large plants. They are tiny and equipped with a small and not very efficient parachute of hairs, so tend to be wafted short distances and are then lodged in crevices at the base of walls where they are washed further in by rainwater. They're also easily transported on clothing.


Galinsoga has spread to many other parts of the world and showed a similar pattern of dispersal from botanical gardens in Holland and South Africa. This is a weed that's on a world tour.


Alien introductions often leave their natural pests and parasites at home when they travel, then slowly accumulate new ones in their new geographical surroundings. This one is hosting a leaf-mining larva of an insect - probably a fly or a micro-moth. You can see the silvery trail around the edge of the leaf.

Mossy Invader

The invasive alien plant species that tend to hit the headlines are the large ones - like giant hogweed and Japanese knotweed - but there are smaller species that have snuck in largely unnoticed and have become an almost ubiquitous element in our flora. This is the Cape Thread Moss Orthodontium lineare which was unknown in Britain until 1922, when a bryologist called E.V. Watson found it on some rocks in Cheshire. Since then it has spread the length and breadth of the British Isles. It comes from the southern hemisphere, from Africa, South America, Australia and New Zealand, but until Watson found it the moss was unknown in Britain. Those drooping spore capsules are a distinctive characteristic and the fact that they are produced in such profusion in spring and release so many spores goes a long way to explaining how it has spread itself around so effectively.

Plant Public Enemy no.1

Japanese knotweed Fallopia japonica tops the table of alien, introduced plants that have become invasive weeds in Britain. Its phenomenal capacity to spread via deep, tough underground rhizomes and its resistance to herbicides, coupled with its tendencies to undermine building foundations, footpaths and roads, have created thriving businesses that specialise in trying to keep it under control. Introducing the plant into the wild is an offence and although it's not illegal to cultivate it in a garden, disposing of it when it begins to overwhelm your plot is a major problem, as Japanese knotweed-contaminated soil is classified as controlled waste and has to be removed by specialist companies. In the past much of the spread of the plant has probably been due to gardeners dumping plant waste, beside roads, rivers and canals. The fact that mortgage lenders are now refusing to lend to buyers of houses with Japanese knotweed in their garden should be sufficient deterrent for anyone contemplating planting it in their plot.

Since our Victorian ancestors first introduced it into their landscaped gardens it has spread around the country via fragments of rhizome in soil, especially in urban waste ground where it often forms four metre tall forests. Apparently, 137,500 tonnes of contaminated soil had to be removed from the London Olympics site alone, before construction could begin and, nationwide, about £150 million is spent every year trying to  control Japanese knotweed. One ray of hope for those seeking to bring it under biological control is that trials of a plant psyllid bug called Aphalara itadori, recently approved for release into infested areas, indicate that this insect could weaken the plant and make other forms of control, like herbicides, more effective

So, with all its destructive tendencies, is there anything positive to say about this aggressive invader? Well - excuse me while I reach for my flak jacket - there is. It's a magnet for honeybees. This fine specimen, that I photographed last weekend growing beside a pavement in Newcastle, near the Ouseburn, was humming with them. It's an ill wind, etc., etc................. 

Spring Beauty

This unusual plant is spring beauty Claytonia perfoliata, which has flowers that appear in the centre of a leaf that surrounds the stem - what botanists call a perfoliate leaf. Some honeysuckle species have perfoliate leaves and so does the delightful native yellow wort Blackstonia perfoliata ............ but spring beauty is an exotic introduction, hailing from North America, all the way from Alaska down to Mexico.

With such a broad geographical range it's perhaps not surprising that it has escaped from British gardens and settled into the countryside, establishing itself in areas of disturbed sandy soil. We found a lot of it in bloom along the landward edge of Holkham NNR last week - an alien species that doesn't seem to present any kind of threat to our native flora, merely adding a little unconventional botanical interest to the countryside. 

Botanical Xenophobia?

There’s a long-standing belief that dense riverbank stands of Himalayan balsam Impatiens glandulifera – like these plants flowering beside the river Tyne at Corbridge - represent a major threat to our native riverbank plants. The press loves a good ‘alien invader’ story that predicts the extermination of our native wildlife by some botanical ‘foreigner’, and it’s true that some introduced plant species that have escaped into the wild, like Japanese knotweed and Australian swamp stonecrop for example, have damaged natural habitats, but in the case of Himalayan balsam the threat is more imagined than real. Although it’s a tall, fast-growing and very conspicuous plant it’s an annual, and doesn’t compete very effectively with established riverbank perennial native plants that have dense root systems. Two recent research projects have demonstrated that it isn’t a dire threat to our flora and one very recent one has shown that it’s a positive asset as far as bees are concerned. It’s a rich source of nectar and pollen and bees visit it constantly. Sadly, once these 'alien invader' stories take root they tend persist, no matter how good the scientific evidence is that shows they are a myth. So I dare say well-meaning but misguided ‘conservationists’ will organise many more ‘balsam bashing’ events to tackle this imagined threat. Still, the plant is well established almost everywhere now, and its exploding seed pods disperse its seeds very effectively, so it will probably continue to provide a valuable food source for our hard-pressed bees for a long time to come.