Plant of the Month: August 2017

Quasi-familial motives led the Hedgehog ménage to Thessaloniki recently, and a jolly time was had by all, in spite of 40 degrees C, 98% humidity, and the overwhelming nature of Greek hospitality. We had time for a little light sightseeing; and on all our travels to and from the leafy and lofty suburb where we were staying, as well as driving from the airport and back again, I noticed one particular plant which was surviving and thriving on roadsides and waste land in the extremely parched landscape.

It looked to my inexpert eye like a Solanum of some sort. Various friends didn’t know what it was called in Greek, so when I got home, I looked it up. It is in fact a Solanumelaeagnifolium (i.e. with leaves like Elaeagnus, the oleaster family, named by Linnaeus in 1753),

Elaeagnus angustifolia (L.), the Russian olive, showing the narrow leaf from which the Solanum species was named.

and was named by Antonio José Cavanilles (1745–1804), director of the Royal Botanic Garden in Madrid, who – not quite by coincidence – also named my plants of the month for the last two Augusts, Dahlia and Cosmos. Cavanilles reported in his Icones et descriptiones plantarum (vol. 3, 1796) that the plant flowered from July to October in the Madrid garden.

The reason for Cavanilles’ interest is that Solanum elaeagnifolium is, like the other two, a plant native to the south-west of the United States and Mexico. Unlike the other two, however, it has not been seized upon by hybridists to produce endless garden varieties, but is notorious as an invasive pest everywhere in Europe, America, Africa and Asia where it has spread.

Solanum elaeagnifolium growing next to an oleander on roadside.

Its local names include trompillo, white horse nettle, prairie berry, purple nightshade, silverleaf nightshade, silver-leaved nettle, silver-leaf bitter-apple, bull-nettle, horse-nettle, and satansbos – ‘the Devil’s bush’ – in South Africa, where measures are being taken to prevent a resurgence.

The plant, like so many of the Solanum family (cf. the nightshades), is toxic to livestock, especially cattle and horses. The real economic damage it does, however, is in infestation of arable crops, contaminating the harvest with poisonous berries – but this is almost as nothing by comparison with its disastrous effect on the environments it so readily colonises. It is extraordinarily hardy, thriving in minimal rainfall, and has been recorded in Pakistan growing at 1800 metres. It can survive temperatures of –23 degrees C, though it requires temperatures between 20 and 35 degrees for growth. Thessaloniki, with very cold winters as well as scorching summers, therefore seems an ideal habitat.

The (unsurprisingly tomato-like) fruits, in Arizona. (Credit: https://delange.org/)

It forms bushes up to 50cm tall, and its roots can penetrate to 3 metres deep: these factors between them both smother the typical Mediterranean natives, which tend to be low-growing, and deprive the soil of nutrients. Moreover, it spreads prolifically both by seed and through bits of root, which, if they are disturbed, develop ‘adventitious buds’ that shoot – a bit like the rhizomes of ground elder or the dreaded Oxalis pes-caprae.

The history of its spread is fascinating, and mirrors the history of colonialism. It is thought, for example, to have arrived in the Philippines in the late sixteenth century aboard Spanish galleons which traded between Acapulco in Mexico and Manila.From there it went to China, also probably in trading vessels.

The port of Acapulco in 1628, when it was the starting point of the Spanish Pacific trade. (Credit: University of Texas at Austin)

It probably travelled to California on the newly built railroads of the nineteenth century, while it may have arrived in South Africa as a contaminant in imported pig-food in 1905. Its arrival in Greece may possibly have occurred during the massive American programme of post-war aid for Greece which began in 1947 and involved, inter alia, trucks and other vehicles which may have inadvertently brought in seed.

Because of its deep root system, Solanum elaeagnifolium is almost impervious to chemical treatment. Early cutting and mowing in arable areas can keep it under control – except of course that it almost always grows back. It seems to have no natural enemies which might be used for biological control which do not also prey on other, more economically significant Solanaceae, including tomatoes and potatoes.

So, a complete pest, with no redeeming features at all – except of course the startling beauty of the flowers, in an otherwise arid late-summer landscape.

Caroline

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