Plant of the Week: Sweetclover

Sweetclover, Melilotus officinalis, can be found growing on the Interim Walkway by the Western Rail Yards. It is an annual, sometimes biennial, ranging in height between two feet in poor, dry soils to eight feet tall in richer, moist soils.

The leaves are in alternate arrangement on the stems, with three leaflets per leaf. The flowers are bright yellow throughout spring and summer. The fruit is a pod, with typically one seed contained in each. Tap roots that maximize on moisture reserves in the soil and a long seed viability help it gain a competitive advantage over endemic species. It is a cosmopolitan plant, found abundantly in open disturbed lands, prairies and savannahs, full sun to partial shade.

Photo by Ayinde Listhrop
Before the mainstream use of synthetic fertilizers after World War II, it was introduced to North America as an agricultural cover crop and livestock forage. Like other members of the Pea and Bean family, Fabaceae, Melilotus has the ability to convert atmospheric nitrogen for use by plants in the soil through a relationship with symbiotic bacteria called Rhizobia found in the root nodules of Fabaceae members.

In a contemporary context, Melilotus is used in phytoremediation, which is the use of plant material to remove or contain toxins in environmental settings. In particular, sweetclover is utilized for the treatment of dioxin-contaminated soils. Dioxins are byproducts of fossil fuel combustion, Polychlorinated biphenyls (PCBs), metal recycling, paper manufacturing and bleaching, and chlorinated pesticides. Dioxins accumulate in soil, entering the food chain through animal and plant tissue. The river beds of the Hudson River, which flank the High Line thirty feet below, are still contaminated from the direct dumping of PCBs between 1947 and 1977 from upstream manufacturing.

Photo by Ayinde Listhrop
Melilotus officinalis is a preferred nectar and pollen source of European honey bees (Apis mellifera). Caterpillars of Reakirt's blue butterfly (Hemiargus isola) consume the flowers, buds, and leaves, caterpillars of the Sweetclover root borer moth (Walshia miscecolorella) feed on the roots and stems. Sweetclover is also host to a variety of long-tongued bees, wasps, beetles, and flies. The leaves produce a sweet, hay-like smell when bruised because of the presence of coumarin, a fragrant compound found in many plant species. Coumarin is found naturally in many garden plants. Sweet woodruff, Galium odoratum and vanilla sweet grass, Hierochloe odorata are other plants that contain naturally occurring coumarin.

When Melilotus officinalis turns moldy, often in situations of improperly cured hay for livestock, the coumarin within the plant converts to dicoumarol, which is a powerful anticoagulant that can lead to internal hemorrhaging when ingested. Dicoumarol is the basis of the Warfarin class of blood thinners used in Western medical practice, such as for the treatment of blood clots, pulmonary embolism, and various heart valve conditions. Warfarin is listed on the World Health Organization's List of Essential Medicines. Naturopathic and folk medicine practitioners have used Melilotus for a wide range of maladies, albeit cautiously: varicose veins, insomnia, anxiety, flatulence, eye inflammations, and swollen joints.

Aggressive in open fields and woodlands where it can outcompete native species, sweetclover is a prolific self-sower and it often forms colonies at favorable sites. It is commonly found in chalky, loamy and clay soils with a slightly acidic to alkaline pH level and is also cold and drought resistant. Propagation outside of an agricultural or phytoremediation setting is not advised, as it can outcompete local flora. In those applications, propagate it by collecting seeds shortly after the pods dry and scatter seeds in the planting area, or allow naturally colonies to form.

Melilotus officinalis can be found growing on the Interim Walkway, just north of the 11th Avenue park entrance.

The High Line's planting design is inspired by the self-seeded landscape that grew up between rail tracks after the trains stopped running in the 1980s. Today, the High Line includes more than 500 species of perennials, grasses, shrubs, and trees – each chosen for their hardiness, adaptability, diversity, and seasonal variation in color and texture. Some of the species that originally grew on the High Line's rail bed are reflected in the park landscape today. Every week we share one of our gardeners' current favorites with you

Our horticultural team counts on members and friends like you to help keep the High Line beautiful and thriving. Join our community of supporters who play an essential role in the High Line's most important gardening projects.

TD Bank is the Presenting Green Sponsor of the High Line.

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