The Value of Thistles in Nature Conservation
Thistles are friend and foe on Wilden Marsh. Thistles with their armament of long, strong, sharp thorns pointing in all direction, make travelling through the many different habitat compartments of this flagship nature reserve a painful experience at this time of year. The most effectively armed thistle is the spear thistle, aptly named I think; it gets its name from its spear-shaped lobed leaves.
Whilst thistles can be painful to humans and animals that invade their spaces, they have their uses, some of which can greatly benefit humans as well as the marsh, but as a food source for birds and invertebrates thistles excel.
The problem, of course, is that thistles are invasive: some are perennials, some are biennial, and some are both perennial and biennial. The spear thistle spreads by seed only. Creeping thistles spread by seed and root. So, cutting biennial spear and marsh thistles before they seed is an effective method of controlling their spread. Creeping thistles are not so easily controlled.
Creeping thistle (Cirsium arvense): is a herbaceous perennial and biennial species of flowering plant in the family Asteraceae, native throughout Europe and northern Asia, and widely introduced elsewhere. This thistle forms extensive clonal colonies from thickened roots that send up numerous erect shoots during the growing season. It is a ruderal species. The standard English name is creeping thistle; it produces dense spines on its leaves, but very few on its stems.
Creeping thistle is designated an “injurious weed” in the United Kingdom under the Weeds Act 1959.
The seeds are an important food for goldfinch and linnet, and to a lesser extent for other finches. Creeping thistle foliage is used as a food by over 20 species of Lepidoptera, including the painted lady butterfly, and engrailed moth.
The plant provides a great deal of nectar for pollinators. It was rated in the top 10 for most nectar production (nectar per unit cover per year) in a UK plants survey conducted by the AgriLand project, which is supported by the UK Insect Pollinators Initiative. Marsh Thistle, Cirsium palustre, was ranked in first place while creeping thistle was ranked in sixth place. It also was a top producer of nectar sugar in another study in Britain, ranked third with a production per floral unit of (2323 ± 418μg).
I have found that cutting creeping thistle is July does reduce its coverage the following year, but you need to keep on top of it in order to maintain a useful balance. This thistle propagates by root, rather than by spreading seed: most of the seeds it produces are infertile. If chemical solutions are to be avoided, then the only option is stressing the plant by frequent cutting and trampling.
Marsh thistle (Cirsium palustre): Marsh thistle is biennial, so cutting prior to seeding is a viable option to control their proliferation.
Spear thistle (Cirsium vulgare): is designated an “injurious weed” under the UK Weeds Act 1959. This thistle is spread only by seed, not by root fragments as in the related creeping thistle C. arvense. It is best cleared from land by hoeing and deep cutting of the taproot before seeds mature; regular cultivation also prevents its establishment.
Spear thistle has large, deeply-lobed leaves with large spines at the margins, as well as hairy, spiny stems.
Spear thistle provides a great deal of nectar for pollinators. It was rated in the top 10 for most nectar production (nectar per unit cover per year) in a UK plants survey conducted by the AgriLand project which is supported by the UK Insect Pollinators Initiative. Marsh Thistle, Cirsium palustre, was ranked in first place while this thistle was ranked in sixth place. It also was a top producer of nectar sugar in another study in Britain, ranked third with a production per floral unit of (2323 ± 418μg).