Nodding Bur-Marigold, Bidens cernua. Rayed form (var. radiata), 2nd September 2019.

At the beginning of September Carl, my older brother-in-law, told me about an interesting plant he had seen at Whitlingham Country Park, on the outskirts of Norwich. It was the rayed form of Nodding Bur-Marigold, Bidens cernua (var. radiata), a plant I hadn’t seen, so the next day I cycled out to take a look. I found the plants very easily, just off the path on the southern edge of the Great Broad (a former gravel pit).

Nodding Bur-Marigold, Bidens cernua, is a member of the Asteraceae, the Daisy family, and I have already written about some other members of the family on this blog, including Oxeye Daisy, Pineapple Weed, Gallant Soldier, Dandelion, Mexican Fleabane, and Bristly Oxtongue. The Asteraceae is probably the largest family of flowering plants, with more than 32,000 species worldwide (note 1).

Members of the Asteraceae have composite flowers – what appears to be a single flower is actually a composite flowerhead known as a capitulum (note 2). Taking just three examples:

• Dandelion flowerheads are made up only of ray florets.
• Thistle and cornflower flowerheads are made up of only disc florets.
• Flowerheads of the Oxeye Daisy consist of an inner cluster of yellow disc florets, surrounded by a ring of white ray florets which look like petals.

The rayed form of Nodding Bur-Marigold (var. radiata) has flowers structured rather like an Oxeye Daisy. The inner disc florets are a dull yellow, and these are surrounded by a ring of bright yellow, petal-like ray florets. But this very attractive form of Nodding Bur-Marigold is not particularly common and you are far more likely to see plants that only have disc florets. Both forms were growing by Whitlingham Great Broad.

In contrast to the rayed form of Nodding Bur-Marigold, the rayless form of the plant is not very spectacular, and it blends into its surroundings. However, both forms of the plant have characteristic undivided leaves with saw-tooth edges.

Nodding Bur-Marigold, Bidens cernua. (Rayless form.) 2nd September 2019.

Bidens cernua is an annual, and when I visited the site again later in September the plants were already running to seed and many were developing mildew and looking past their best.

Nodding Bur-Marigold grows in damp places, such as on the margins of slow-flowing rivers and streams, in ditches and marshes and by broads, ponds and meres. The sites where it grows are often places subject to flooding in winter. It is widely distributed in the British Isles, particularly in England, Wales and Ireland. However, it is declining in the south-east of England as its habitats are destroyed. It is also found through many temperate parts of Europe, Asia and North America. There are good pictures of the plant on the Wild Flower Finder (rayless form only) and Illinois Wildflowers websites. (The latter lists some of the insects that are attracted to the plant in the United States.)

Bidens cernua contains the compound Phenylheptatriyne (PHT). This chemical, a type of polyacetylene, is phototoxic to bacteria and also acts as a fungicide. PHT may also have allelopathic properties: it appears to inhibit the growth of seedlings from other plants. Quantities of the chemical in Bidens cernua vary throughout the year, reaching their highest levels in October.

The closely related Trifid Bur-Marigold (Bidens tripartita) was growing alongside (and amongst) the Nodding Bur-Marigold at Whitlingham. Its flowers were similar to the rayless form of Bidens cernua but the leaves were distinctively divided into threes – hence trifid (but not triffid) (note 3). Although Trifid Bur-Marigold grows in similar areas to Nodding Bur-Marigold, it prefers slightly drier ground. It is also an annual. The leaves can be eaten when cooked and the plant has various possible uses in herbal medicine and was used medicinally to used to staunch blood flow.

Trifid Bur-Marigold, Bidens tripartita, growing next to Nodding Bur-Marigold at Whitlingham Great Broad (22nd September 2019).

The teeth on Bur-Marigold seeds attach to passing animals, including plant photographers and canoeists, and this aids the plant’s spread. This is known as zoochorous seed dispersal. (The Wild Flower Finder website has some good pictures of the seeds.) They also give the plant its generic name: Bidens comes from the Latin bis (“two”) and dens (“tooth”).

Notes

Note 1 – With around 32,000 species, the Asteraceae is usually considered to be the largest family of flowering plants. The next largest is the Orchid family (Orchidaceae), which has around 28,000 species. The exact numbers are difficult to determine as new species are discovered or reclassified, hence the “probably”.

Note 2 – The Asteraceae is also known as the Compositae, because of its composite flowerheads.

There is a useful glossary on the Kew Gardens ‘Compositae of Bolivia’ web pages.

Strictly, there are six possible kinds of florets, as explained in the Flowers of Asteraceae. (on the website of the South African National Biodiversity Institute).

Note 3 – Bidens tripartita can be quite variable, and there is rare form, var. integra, that doesn’t have trifid leaves. The Wildflower Finder website gives more details.