Plant of the week – October 18th 2021 – Hedge Bindweed (Calystegia sepium)

It’s a native perennial climber, and a beautiful one. If it wasn’t known as a ‘weed’ I believe that keen gardeners would queue up to buy it. It has wonderful white (or sometimes pinkish) trumpets, it grows strongly in all garden soils and most climates, and it’s good at covering over untidy corners of backyards, decrepit fences and abandoned ground. So, what’s not to like about it? Well, it’s a truly terrible weed in the garden, hard to eradicate. More about that later.

It belongs to the Convolvulaceae, and formerly it was called Convolvulus sepium. It’s still called that by French botanists (Muratet et al 2017). It’s one of several Bindweeds in Britain, very like its foreign cousins the Morning Glories (Ipomoea) which are sometimes found in southern England after escaping from gardens (they are tropical, Scotland is too cold for them).  The Morning Glories contain hallucinogenic substances and have sometimes been grown for that purpose. In addition, it is said that S. American Indians invented rubber using Ipomoea sap. But Calystegia sepium doesn’t appear to be hallucinogenic or to be a source of rubber. However, there is much research on the chemical constitution of C. sepium and its close relatives; many completely new chemical compounds have been found.

I liked the very first description of the plant, from 1548, reproduced in Pearman (2017): “the withwynde or byndeweed … windeth itself about herbs and bushes”. It is a plant I remember from infant-hood; we played with the flowers as parachutes. But we didn’t know the childhood game I’ve read about called Granny, Jump out of Bed. The calyx is squeezed and the corolla pops out. Is it still played? I suspect children find better things to do these days.

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The characteristic trumpet-shape of the flower, 3-6 cm wide, attracts insects (Paulk and Corbet 2000) but may have a thermoregulatory role also, as shown in Ipomoea by Patino and Grace (2002). The mechanism is complex, involving transpiration and orientation of the flower in relation to the sun. What I’ve noticed in Calystegia as well as in Ipomoea is that the trumpet does not point directly at the sun, but at a slightly divergent angle, so that the gynoecium is not exposed to direct solar radiation.

The ‘trumpet’ is formed as a corolla of five fused petals. The calyx is 5-lobed, subtended by two leafy structures (‘bracteoles’, see them at label 2 in Lindman’s artwork above) almost covering the calyx (calyx is revealed when the bracteoles drop off, see label 3). There are five stamens, and the pistil consists of two fused carpels. The flowers are solitary and axillary. They appear from May to October.

Let’s talk about habitat. Stace (2019) says it is found in  “hedges, ditches, fens, marshes, by water and on rough and waste ground”. Yes, but what about its urban environment? For me, it is a good example of an urban plant. It’s tough and it grows on fence posts and all sorts of abandoned structures: anywhere where it can find vertical elements around which it will twine. It likes hedges (see the pink flowers in my photo) but not woodlands (which are too dark). It twines ferociously up branches but does not attach physiologically. It’s a very efficient system, no woody tissues of its own are needed.  

By the way, it’s a myth that plants twine in different directions in north and south hemispheres, like the myth about water vortices in the bath. Coriolis forces are pretty weak at a small scale. All the bindweeds I’ve seen twine anti-clockwise when viewed from above (clockwise from below). Beans are the same but Hops and Honeysuckle twine clockwise. Early botanists were fascinated by twiners. Both Charles Darwin and his American friend Asa Gray wrote much about them in the 1870s. You can see Gray’s work at https://www.biodiversitylibrary.org/page/34071946#page/15/mode/1up.

I’ve recently been looking at the BSBI botanical records in Scotland’s central belt, going from Glasgow to Edinburgh in 10 km steps. The records show the number of times each species was recorded in successive 10 km x 10 km squares. Truly urban species should be related to the size of human populations and I wanted to test this hypothesis. I discovered that the counts of Calystegia sepium are related to the natural logarithm of how many people live in the 10 km x 10 km square. The correlation is very strong. Actually, there was an exception: the square containing the town of Livingstone had fewer records than expected – I think that’s an ‘outlier’ because it’s a new town, and too tidy for plants like bindweed.

The plant does produce seeds, but the individuals I’ve looked at are not very prolific seed producers. The species excels in vegetative spread via rhizomes and roots. These subterranean organs are white, several mm thick and often a few metres in length (see the picture). Hence, they will rapidly colonise any garden and a small fragment will be sufficient to make a new population. There is much advice on control measures in the horticultural literature. Some say just ‘dig it out’, and others point out that the systemic herbicide glyphosate works well (although banned in many parts of the world because of fears for human health – however, still in use in the UK as ‘Round-up’). One ‘expert’ advised covering the ground with tarpaulin for five years (but I suspect the plant would grow out at the edges).

Calystegia has 22 (2n) chromosomes. C sepium rarely hybridises with the less common C. silvatica (silvatica is larger-flowered and has ‘pouched’ bracteoles that strongly overlap and hide the sepals). C. sepium is recorded by Ellenberg as being slightly salt tolerant, but even more salt tolerant is its rather shy seaside relative, C. soldanella, with flowers that are pink with white stripes.  Like seaside-lovers everywhere, it is fleshy and it scrambles and trails between dune and beach.

References

Gray A (1872) How Plants Behave. Accessed with kind permission from https://www.biodiversitylibrary.org/page/34071946#page/15/mode/1up

Muratet A,  Muratet M and Pellaton M (2017) Flora des Friches Urbaines. Xavier Barrel, Paris

Patino S and Grace J (2002). The cooling of convolvulaceous flowers in a tropical environment. Plant, Cell and Environment 25, 41-51. DOI 10.1046/j.0016-8025.2001.00801.x

Paulk A and Corbet SA (2000) Arthropods in flowers of hedge bindweed Calystegia sepium (L.)/ Entomologists Monthly Magazine 136, 77-83.

Pearman D (2017) The Discovery of the Native Flora of Britain and Ireland. BSBI, Bristol.

John Grace