Do you know your yellow clovers?

So you’re walking through the grass and at the edge, where it’s all trampled, there’s this yellow clover, or is it, and is it only one?

Well in Britain we have at least four common yellow flowered small to medium clover-like plants. Although they all have trifoliate leaves (three leaflets) they are not called clover, at least the commonest common names are not clover they are trefoils or medick and here they are listed in size order smallest to biggest:

• Sender trefoil – Trifolium micranthum
• Lesser trefoil – Trifoilium dubium
• Hop Trefoil – Trifolium campestre
• Black Medick – Medicago lupulina

These four all look rather similar, they’re all quite small plants, usually creeping along the ground (though not rooting at nodes) and they all have leaves with three leaflets and small yellow flowers, so how to tell them apart?

Well if the plants are flowering they all have the familiar pea-flower shape of the family Fabaceae (as it is in Britain at least) but there are differences in flower colour and in the petals.

Black Medick has fluorescent yellow flowers, while the three Trefoils have paler yellow flowers, turning brownish with age. Of the three trefoils, Hop Trefoil is the simplest to tell apart as it has a broad flat standard petal (the one at the top) while the other two have this petal ridged like a roof, and in Slender Trefoil this petal is notched while in Lesser Trefoil it is un-notched.

In all these species the flowers are grouped together in heads and Lesser Trefoil has 5-20 flowers and slender trefoil 2-6 flowers in each head.

Even if you have no flowers on the plant it is still not too difficult to tell them apart, Black Medick has a little mucro (pointy tip) on the leaflets which are sparely to densely hairy, while all the Trefoils have no such mucro and the leaflets are sparsely hairy at most.

Then for the Trefoils there are differences in the size of the leaflets and in the length of the leaflet stalks (petiolule), so for Slender Trefoil the central leaflet has a very short stalk 0-1 mm long, for the other two this stalk is at least 1.5 mm long (often more in Hop Trefoil). Finally, in Hop Trefoil the terminal leaflet is at least 8 mm long while in Lesser Trefoil it is less than 8 mm.

The table below shows these characters so next time you encounter a little yellow clover in the grass, have a go and see if you can sort your Trefoils from your Medicks!

The BSBI Plant Crib gives a useful guide to the common clovers (Trifolium species), check it out here.