Distinguishing Trifolium dubium from Medicago lupulina.
The lesser trefoil, T. dubium and black medick, M. lupulina are both sprawling clover-like plants with bright yellow heads of tiny pea flowers. They are members of the pea family, the Fabaceae. Both species are common in recently disturbed bare ground and waste places, and may occur at the bases of walls and in pavement cracks, where they often grow close together.
So how can you tell them apart?
Various differences between them are reported in the literature. Some are definitive or nearly so, but others overlap considerably.
The most reliable and useful characters are:
Hairiness: The stems of T. dubium are coarsely hairy, while its other organs are almost hairless except for a few long hairs along the main vein beneath the leaf, on the stipules and on the calyx. By contrast, all organs except the corolla of M. lupulina are downy. In young shoots the developing organs appear distinctly woolly, and the downiness persists into maturity. In particular, the leaves are hairy with long silky hairs lying appressed and aligned with the veins.
In particular if the leaf laminas are hairless then your specimen will be T. dubium.
Leaf tip characters: The leaf tips of T. dubium have a couple of long hairs which may protrude beyond the end of the main vein making it look as if there is a point. Those of M. lupulina almost invariably have a tiny triangular point and are said to be ‘apiculate’. This character is visible to the naked eye, but the use of a lens helps.
This character is definitive – if your specimen has an apiculate tip it is M. lupulina
Flower and fruit characters: In T. dubium the faded flowers turn brown, drape over and enclose the developing elliptical fruit, which droop when ripe, while in M. lupulina the fruits are exposed, distinctively coiled and turn black when ripe, hence the common name “black medick”. These differences between the two species are unambiguous.
Further, less reliable differences include:
• Flower colour is a deeper, brighter yellow in M. lupulina, somewhat paler, sometimes described as ‘dull’ in T. dubium.
• It is often stated that the terminal leaflet of T. dubium has a longer stalk than the laterals, but this character is variable and unreliable, since it also applies to many samples of M. lupulina.
• The inflorescence is generally smaller in T. dubium, with fewer flowers per head than in M. lupulina, but there can be considerable overlap depending on plant size and vigour.
• The leaves on the distal parts of M. lupulina shoots are more inclined to be minutely toothed, but this is variable even within the same plant, so cannot be used on its own as a diagnostic character.
• Leaves of T. dubium are more often notched, but again there is variation within the same plant and overlap with M. lupulina, the leaves of which are less often notched, but can be so.
Text and images © Chris Jeffree