Plant of the Week, 30th October 2023 – Trifolium pratense (Red Clover)

Trifolium pratense, Lochinver, Sutherland. Image: © Chris Jeffree

Everyone likes clover. Hence the idiomatic expression in English ‘we are in clover’, meaning we are living in luxury. Clover’s popularity is well-deserved and Red Clover has those colourful flower-heads that are admired by humans and loved by bees. It is a native species, first described by botanist William Turner in 1562. It now occupies all continents with the exception of Antarctica.  

It belongs to the pea Family, the Fabaceae. Close examination of the pale purple or pinkish flower head (preferably with a hand lens) reveals the numerous pea-flowers, up to 300 of them, packed close together, each one just like any other pea flower but appearing more prominent in the globose heads which can be up to 3-4 cm in diameter. Turner said it ‘groweth in myddoes’ and he noticed that it sometimes has a white flower (and then it might be confused with White Clover Trifolium repens). Today, it still grows in meadows and sometimes is included in seed-mixtures for making meadows of an artificial kind.

Red Clover, flower head, showing the florets and hairs on the leaf margin. Image: © Chris Jeffree

The plant produces foliage that remains green in winter, and is about 30 cm tall. The flowering stems are taller, as much as 60 cm sometimes, but they die back in winter. After pollination by bees, seeds are produced and they usually germinate in the autumn (Grime et al 1987). There are 30 species of clover in Britain and two-thirds are native. The most common by far are Red Clover and White Clover (T. repens). Red Clover is distinguished from most of the other clovers by its height (taller than most), by the chevron markings on the leaflets, and by the development of red pigmentation on the sharply-pointed stipules (see the image below from Chris Jeffree). Another clover with similar pinkish flower heads is T. medium, the Zig Zag Clover, but that one has very narrow leaflets and quite different, broader, stipules.

Red clover has pale chevron markings on the leaflets. This plant was flowering on 28th October, well beyond its normal flowering period. From an allotment garden, Edinburgh. Image: John Grace.

It isn’t just a pretty flower, it’s an important one. Clover has been grown in agricultural systems for several centuries (Kjærgaard  1994). The agricultural use of clover probably originated in Spain and was rapidly taken up by the rest of Europe, reaching England around 1620. Between 1750 and 1880 the production of European agriculture rose by 175 %, and much of this increase has been attributed to the use of clover (Kjærgaard  1994).

The agricultural form is given the name Trifolium pratense var. sativum. By a process of selection it is more robust than the wild form and has hollow stems. In Bulgaria another variety is recognised, Snow Clover (Trifolium pratense ssp. nivale, which has white flowers). These authors suggest it is a good candidate for breeding, as it is especially leafy. Grime et al (1986) mention several other varieties used for silage or hay-making. Generally, clover and grass seeds are sold as a mixture of appropriate cultivars, depending on whether the purpose is to provide green manure, grazing, silage, or hay.

Characteristic pointed stipules with red veins. Image: © Chris Jeffree

Bees of various species collect nectar from Red Clover with their long tongues, but there needs to be a match between the length of their tongues and the dimensions of the clover flower. The bee Bombus lapidarius (Red-tailed Bumblebee)  likes Red Clover cultivars with short flower tubes whilst B. hortorum  (Garden Bumblebee) selects long-tubed cultivars. But Bombus terrestris (Buff-tailed Bumblebee) is a ‘flower biter’ that gets the nectar by a more destructive means (Proctor et al 1996).

Bees expend much energy when they are foraging, and their visits to flowers must sustain their flight and also provide excess for the hive. Some of the calculations in Michael Proctor’s excellent book are intriguing. A honeybee weighing 0.1 g uses 10 mg of sugar (its  fuel) per hour. Red Clover has ample: some 0.05 mg of sugar per floret (Proctor et al 1996). The power consumption of a honey bee in flight is 45 milliwatts (similar to a single LED indicator lamp on an electrical appliance) whilst that of a bumble bee, being five times heavier, is estimated as 225 mW.

Underground view. This young plant has just started to develop N-fixing nodules. Image: John Grace.

Last week, when writing about Marsh Thistle, I became interested in how much nectar flowers produce. There is a good recent paper (it’s Open Access, so freely downloadable) by Tew et al (2022). You can also see the database on-line here. From these data I was able to discover how red clover compares with over 500 other species for nectar production. Red Clover ranks in the top 10 percent of all flowers, with over 2 mg of nectar per flower head; however last week’s Plant of the Week, Marsh Thistle, has 12 mg and ranks 4^th. Unsurprisingly, children in country districts were known to suck the nectar from Red Clover florets (now they get sweetness from chocolate bars).

How is clover good for agriculture? Like most members of the Fabaceae, its roots form a symbiotic relationship with bacteria of the genus Rhizobium which can ‘fix’ gaseous nitrogen from the atmosphere (N₂), to make a form available for plants to use (ammonia, NH₃). It does this very well:  a field of clover might fix nitrogen at a rate of 100-250 kg N per hectare per year. This is similar to the UK’s average rate of application of expensive chemical fertilizer to cereal crops (100-150 kg N per hectare). The price of fertilizers has soared in the last few years and so farmers need to consider growing clover to save money. Manufactured N-fertilizer is ‘bad’ for the global environment, being responsible for 1.4% of global carbon dioxide emissions and 1% of global energy usage.  Clover is much more friendly. But strangely, the use of red clover has diminished in recent years according the UK’s Countryside Survey (see the review by Harris and Ratnieks 2022). When sown with rye-grass for silage-making or hay-making, Red Clover persists for 2-3 years whereas the rhizomatous white clover continues. In British agriculture a grass ‘ley’ (usually Lolium perenne) is grown only for 1-5 years (with one or two ‘cuts’ per year to make silage) before being ploughed up and resown.

Trifolium pratense. Left, distribution in 1800; right, current distribution according to GBIF

Red Clover may be grown as a field crop, by itself, to provide seed. Many bees are required, as the plant produces no seed if it remains unpollinated. In one study cited by Proctor the yield of seed was 50 kg per hectare but where the clover crop was grown next to a forest it yielded 135 kg of seed. The reason given is that the forest provided adequate nesting habitat for the bees. This study is a vivid reminder of the importance of pollination in the economy of nature.

Acknowledgement

Thanks are due to Isabella Cornwell of the University of Edinburgh for directing me to the nectar data base.

References

Bowley SR et al 1984 Physiology and morphology of Red Clover. Advances in Agronomy 37,  317-347. (this is the most thorough account of the species that I have come across).

Grime JP, Hodgson JG and Hunt R (1986) Comparative Plant Ecology. Unwin Hyman, London.

Harris C and  Ratnieks FLW (2022) Clover in agriculture: combined benefits for bees, environment, and farmer. Journal of Insect Conservation 26,  339–357. https://doi.org/10.1007/s10841-021-00358-z

Hicks DM et al. (2016) Food for Pollinators: Quantifying the Nectar and Pollen Resources of Urban Flower Meadows. PLoS ONE 11(6): e0158117. doi:10.1371/journal.pone.0158117

Kjærgaard T (1994) The Danish Revolution, 1500–1800. An ecohistorical interpretation, Studies in Environment and History (Cambridge, Cambridge University Press).

Proctor M et al (1996) The Natural History of Pollination. Collins.

Tew NE et al (2021) Quantifying nectar production by flowering plants in urban and rural landscapes. Journal of Ecology 109, 1747-1757.

© John Grace