Birdsfoot Trefoil Takes Over

Birdsfoot trefoil is an attractive relative of a familiar group of species of clovers known as Trifolium, which includes red, white, Swedish, Dutch and nearly 300 other species. Like clovers, it is a useful plant. While it grows on roadsides, it helps control erosion. It feeds both wild animals (geese, deer and elk) and domestic ones, without bloating the latter. The actual genus of birdsfoot trefoil is Lotus, which includes less members than Trifolium, some of which may need to be reclassified. The rest of its name which designates its species is corniculatus, and it’s very similar to other species with the birdsfoot name: slender, alpine and smallflower.

Why is it called birdsfoot? At first glance, the flower looks like that of a pea, another member of the Plant kingdom’s 3rd largest family, Leguminosae, which, of course, includes the pea-genus (Pisum) along with Lotus, Trifolium and about 750 others. But if you look at mature birdsfoot flowers, their seed pods spread out in a plane and resemble a bird’s foot.

The plant first caught my attention in the 1980s on northern stretches of the 87 interstate highway headed to New York City. In Montreal, I had never seen it. A few decades later it began to appear sporadically in small patches in parks and abandoned fields. This year in July it has become so dominant in parks and big lawns, that from a distance, one could mistake them for dandelions. Here is a field I photographed in 2011 when white clover dominated, and the same field nine years later when birdsfoot trefoil reigns.

Why the change? White clover is best suited to soils which have good moisture-holding ability. Birdsfoot trefoil does better in dry soils. Given that Montreal had unusually arid months of May and June this year ( 33.6 and 46.4 mm of rain instead of the usual 81 and 87 mm), this could have contributed to the boom in the growth of corniculatus.

Occasionally, one will notice birdsfoot trefoil flowers that are orange instead of yellow. Older flowers occasionally take on the latter color. I thought of a couple of explanations. They do contain lutein which appears yellow at low concentrations and orange to red at higher concentrations. Presumably, with age, lutein accumulates. But beta carotene is also found in the plant, so if its content builds up, it could also be responsible for the color change.

The highly conjugated structure of lutein, C40H56O2 , is responsible for the yellow color of birdsfoot trefoil flowers.
Beta carotene, C40H56, is very similar in structure to lutein. The former replaces a hydrogen on each end-ring with hydroxyl, and the ring on the right is conjugated with the rest of the structure. This means less energy is required to excite its electrons, leading to a higher blue maximum absorption peak for beta carotene. That in turn leads to a reflection of more orange than yellow. (see graph below)


Fatty acids, α-tocopherol, β-carotene, and lutein contents in forage legumes, forbs, and a grass-clover mixture.Elgersma A, Søegaard K, Jensen SK.J Agric Food Chem. 2013 Dec 11

USDA Natural Resources Conservation Service.

The Audubon Society Field Guide to North American Wildflowers. Richard Spellenberg. Knopf. New York. 1987

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