by Adriana Lopez-Villalobos and Amelie Mahrt-Smith
Clovers, medick, trefoil, lupines, vetches and beans are all members of the Fabaceae family, also known as the Pea family. These plants have a special trick that few other plants do – they can fix nitrogen, and thus help your garden grow!
By forming beneficial associations (called symbiosis) with some of the bacteria found in soil, legumes can obtain nitrogen much more easily than other plants. Root nodules containing the bacteria Rhizobium (collectively known as rhizobia) ‘fix’ free nitrogen for the plants, converting it into an easily usable form. And in return, the legumes then supply the bacteria with valuable carbon produced by photosynthesis. Nitrogen is one of three major nutrients that plants need to survive and thrive. The other two are phosphorus and potassium. When you buy a bag of fertilizer, you may notice three numbers (for example, 24-0-4); these numbers refer to the ratio of nitrogen to phosphorus to potassium. Plants usually get nitrogen through their roots directly from the soil, but by associating with ‘nitrogen-fixing’ bacteria, which can fix nitrogen from the atmosphere, legumes do not have to compete with grasses and other weeds for the nitrogen in the soil. Their deep roots store nitrogen, and when they die it becomes available to the surrounding plants. In this way, they help other plants grow by adding nutrients to the soil.
In this week’s post, we highlight some of the common species of Fabaceae growing in the streets in of Kingston, Ontario.
True Clovers – The Genus Trifolium (White and Red Clovers)
The pink and white flowers dotting lawns are a tell-tale sign that summer is on its way. These familiar weeds are just a few of the dozens of species in the genus Trifolium – “the true clovers”.
The white clovers (Trifolium repens L.) and red clovers (Trifolium pratense L.) are the most common around Kingston, but you can also find the more pinkish alsike clover (T. hybridum), which despite its scientific name it is not of hybrid origin. You can recognize clovers by their signature three-leaved pattern suggested by the genus name: Tri =three, folium=leaf. At first glance, they might just look like weeds, but there is a lot more going on just beneath the surface.
White and red clovers, like so many North American weeds, are native to Europe; in fact, the greatest diversity of species in the genus Trifolium occurs in the Mediterranean Basin (around Anatolia and Greece) where it presumably originated (Scoppola et al 2018). Two other important centres of species’ diversity are the west coast of North America, from British Columbia south to Baja California, and the alpine and subalpine highlands of central east Africa. Clovers can grow in a variety of habitats, including meadows and prairies, open woodlands, semi-deserts, mountains, alpine peaks and urban environments. However, no matter where they are, one of the most important factors determining their success is high solar radiation; few clover species tolerate shade (Ellison et al. 2006).
True clover species have some characteristics in common and can be distinguished from other similar genera because they have petals that remain on the plant after flowering; the other genera do not. Clovers also have straight pods (the structures where the seeds are contained) that rarely stick out of the calyx; the others all do stick out. In clovers, the compound leaves can have between 3 and 7 leaflets, whereas the others always have 3, and always have their flowers in variously shaped heads. The whole floral structure in clovers is adapted to insect pollination, requiring skills to push down the keel (the two lower petals forming a boat-like structure) exposing both the male (stamens) and the female parts (pistil) of the plant. In its struggle to get the nectar or the pollen, some of the pollen sticks to the insect; when it visits another flower on a new plant the pollen is transferred. In this way, cross-pollination and the mix of genetic material occurs. Some annual species, however, can set seed by self-pollination.
White clovers (T. repens), red clovers (T. pratense), and alsike clovers (T. hybridum) are all very similar except for the colour of their flowers. You can identify Trifolium species by their stalked leaflets in threes (or fours if you are lucky!). White clover leaflets are attached to a ‘creeping’ stem which runs along the ground and only grows to about 13 cm in height.
The flowers are white or pinkish and attached to a stalk and are composed of many small flowers clustered in a dense head. Red clovers, on the other hand, have upright stems that can reach up to 60 cm tall, and the signature three leaflets typically have a white V-shaped blotch. The magenta or purple flowers are similar in shape to the white clover, found in a dense head, but do not have a stalk (Newcomb 1977). The alsike clover looks like white clovers, but the flowers tend to be pink overall and the plant grows 35-75 cm high. This species does not have a white ‘V’ on the leaves.
Human uses and consumption
Because of their superior nitrogen content, clovers are a great source of ‘green manure’ – instead of using chemical fertilizers, mulch from clovers can provide your garden or crops with the nutrients they need! (Bruning and Rozema 2013). Because clovers are also high in nutrient content, these species were sown for livestock forage by colonists and have become extensively naturalized worldwide, colonizing lawns and roadsides ever since. All parts of the plant can be edible if properly prepared, and their sprouts make a tasty addition to salads, the flowers a naturally sweet tea, and the dried leaves are said to give a vanilla-like flavour to baked goods. However, caution should always be exercised when foraging – white clovers are known to produce chemicals called cyanogenic glucosides, which release the deadly toxin cyanide. While this makes white clovers dangerous for people to forage without proper knowledge of how to prepare them, it may also make them a useful pest-control agent (Bjarnholt 2008).
While herbarium specimens can provide much useful information about a plant’s history, colours often fade over time – especially in the petals – when dried. Many historical botanical collections also included beautiful illustrations of the plants pointing to important taxonomic useful traits and details such as the size and shape of the leaves, flowers, fruits, seed pods and seeds. Much like modern-day field guides, botanical illustrations can serve as a reference for identifying the species. For instance, in the illustration below shows an alsike clover, where the difference is most obvious in the colour of their flowers. You can also see more clearly than in the photos the individual flowers, the keel, the sepals, and the root.
Black Medic – Medicago lupulina
The black medic is an annual, biennial, or short-lived perennial species. The genus Medicago is also a genus of the Fabaceae family, and includes the common forage crop alfalfa, Medicago sativa. The common name ‘black medic’ is at first not an intuitive alias for Medicago lupulina. It is characterized by its small yellow flowers and leaflets in threes and is closely related to the true clovers (the Trifolium species). The name Medic is derived from an Ancient Greek word meaning ‘Median’, which was the name given to alfalfa because it was believed to have been introduced from the region of Media (which is in modern-day Iran). Today, black medic can be found throughout the world and is a common weed of lawns and waste places in Kingston, however, it is not considered to be of concern in managed agricultural systems and is not listed as a noxious weed in Canada (Weed Seeds Order, 1986).
Like the white and red clovers, black medic was introduced to North America during the era of colonization. It is used as fodder, especially for sheep, and it is commonly used for making honey. The bright yellow flowers attract honeybees, moths and butterflies, which help to pollinate the plant. As a member of the Fabaceae family (or the legumes) M. lupulina can also improve soil quality over time by fixing nitrogen through its helpful association with rhizobia (bacteria) instead of competing with other plants for nitrogen in the soil. It is less susceptible to the fungal disease ‘clover rot’ than the red clover, which makes it especially useful in agriculture (Turkington and Cavers, 1979). Chemicals produced by the black medic plant called saponins have shown antifungal properties, which has the potential for use in medicine against a broad spectrum of diseases (Zehavi and Polacheck, 1996).
Black medic bears a strong resemblance to the hop clover Trifolium dubium, which is also known as the lesser trefoil. They are similar in size, and both have small yellow flowers in a dense head, and three leaflets with a long stalk. In spring and early summer, they are especially difficult to distinguish from one another. The small, black, spirally coiled pods of the black medic fruit is the easiest way to distinguish it from similar-looking plants. Additionally, the leaves of the black medic tend to be hairier and are tipped with a small bristle. The stem is also hairy, whereas the hop clover’s stem is smooth, and the leaves have no bristle (Newcomb, 1977). Here is a great chart for differentiating yellow clovers in the field! Have you found T. dubium? Share a photo with us and we will post it here!
Annie A. Boyd, one of the very few female botanists of the late 1800’s contributed this beautiful M. lupulina specimen collected in 1897 to the Fowler Herbarium. If you are in the Kingston area you can visit Lake Ontario Park, the site where she collected this specimen. You are likely to see black medic along the trails edge – you may even be looking at a distant relative of the individual that Boyd collected herself over 100 years ago!
- Bjarnholt, N., M. Laegdsmand, H. C. B. Hansen, O. H. Jacobsen, and B. L. Møller. 2008. Leaching of cyanogenic glucosides and cyanide from white clover green manure. Chemosphere 72:897–904.
- Bruning, B., and J. Rozema. 2013. Symbiotic nitrogen fixation in legumes: Perspectives for saline agriculture. Environmental and Experimental Botany 92:134–143.
- Ellison, N. W., A. Liston, J. J. Steiner, W. M. Williams, and N. L. Taylor. 2006. Molecular phylogenetics of the clover genus (Trifolium-Leguminosae). Molecular Phylogenetics and Evolution 39:688–705.
- Gillett, J.M., Taylor, N.L., 2001. The World of Clovers. Iowa State University
- Press, Ames, Iowa, USA.
- Newcomb, L. 1977. Newcomb’s Wildflower Guide. Little, Brown and Company. New York. pp. 36; pp. 58 – 60.
- Turkington, R., and P.B. Cavers. 1979. The biology of Canadian weeds. 33. Medicago lupulina L. Can. J. Plant Sci. 59: 99 – 110.
- Weed Seeds Order. 1986. Order determining the species of plants the seeds of which are deemed to be weed seeds. Seeds Act. S-8-SOR/86-836.
- Zehavi, U., and I. Polacheck. 1996. Saponins as antimycotic agents: Glycosides of medicagenic acid. In: G.R. Waller, and K. Yamasaki, editors. Saponins Used in Traditional and Modern Medicine. Advances in Experimental Medicine and Biology 404. Springer, Boston, MA.