Thursday, 8 August 2013

Taking up the cudgels for the humble thistle

Quite a lot of plants have the word thistle in their common name such as musk thistle and nodding thistle (Carduus spp.), carline thistle (Carlina spp.), sow thistle (Sonchus spp.) and globe thistle (Echinops spp.) to name just a few and all are good plants for pollinators. But the thistles you most commonly encounter in urban areas are the creeping thistle (Cirsium arvense) and the spear thistle (Cirsium vulgare) often growing on wasteland, in neglected corners, on allotment sites and in parks. Spear thistle needs bare disturbed ground to develop whereas creeping thistle can easily grow between established vegetation on grasslands and at the edge of shrubberies or woodlands.

Spear thistle growing in a park in Reading

Creeping thistle is a herbaceous perennial plant with very spiny leaves, growing up to 1.2 m tall (sometimes more) and forms extensive clonal colonies growing from an underground root system each spring. It flowers from July to August with relatively small flower heads on branched flower stems; each flower head consisting of many small pink to purple individual flowers (florets).

A bumblebee visits a creeping thistle flower head
These Small tortoiseshell butterflies love the thistle flowers

Spear thistle is (in contrast to creeping thistle) a biennial plant and forms a rosette of large spiny leaves and a long taproot up to 70 cm long in the first year and a tall branched flower stem with quite large flower heads consisting of many pink to purple florets in the second year. Flowering time is very similar to creeping thistle with flowers appearing between July and August.

Spear thistle is a favourite plant for bumblebees
These bumblebees are very occupied with collecting nectar

Both plants are exceptionally high-value food plants for pollinators with very nectar-rich flowers. One single flower head consist of many small individual flowers (florets) which makes the collection of nectar very efficient as the pollinators can move between the flowers without the need to fly. Especially bumblebees and butterflies seem to like the flowers but we have also seen hoverflies and solitary bees visiting. Often we see many pollinators visiting at the same time and so far we have not seen a thistle without any pollinators this summer.

Three bumblebees on one spear thistle flower head
A little fight breaks loose
This Volucella inanis hoverfly likes the thistle flowers as well

Thistles are also a valuable food source for other wildlife. Thistle seeds are eaten by Goldfinch, Linnet and also by some other finches and the foliage is used as food plant for caterpillars by over 20 moth and butterfly species.

So this is not an appeal to plant more thistles in your garden (they can be quite invasive plants) but for more tolerance of thistles growing in our towns and cities. So next time you see a thistle enjoy watching the countless bees, butterflies and other pollinators visiting the flowers instead of thinking "Why did the council not cut this weed down?". Later in the year you may even see birds foraging for the thistle seeds.

Another hoverfly drinking nectar from a creeping thistle
Even this wasp finds the thistle flower very interesting

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