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Melilotus albus, known as honey clover, white melilot (UK), Bokhara clover (Australia), white sweetclover (USA), and sweet clover, is a nitrogen-fixing legume in the family Fabaceae. Melilotus albus is considered a valuable honey plant and source of nectar and is often grown for forage. Its characteristic sweet odor, intensified by drying, is derived from coumarin. Melilotus albus is of Eurasian origin but can now be found throughout the subtropical to temperate zones, especially in North America, and is common in sand dune, prairie, bunchgrass, meadow, and riparian habitats. This species is listed as an “exotic pest“ in Tennessee, “ecologically invasive“ in Wisconsin, and a “weed“ in Kentucky and Quebec. Melilotus albus is an annual or biennial legume that can reach 2.5 m (8 ft 2 in) in height. The light green stems are round or slightly terete (furrowed on all sides), glabrous (smooth), and often branched. The 1/2 - 2 inch long alternate leaves are trifoliate and sparsely distributed along the stem and alternate in arrangement. The upper stems terminate in narrow racemes of white flowers about 2-6 inches long that have a tendency to hang downward from the central stalk of the raceme. Each flower is about 1/3 inch (8mm) long, consisting of 5 white petals and a light green calyx with 5 teeth. Flowers are small, floppy, tubular at the base and become broader toward the outer edges and wing and keel petals are about equal in length. The ovoid seedpods are black to dark grey, smooth or slightly reticulated and about 0.3 inches long (3-4mm). Each pod usually contains one, but sometimes two, yellow seeds that are ovate to kidney-shaped. Plants generally flower and die during the second year of growth. The blossoming period occurs from early summer to fall, reaching its peak during mid-summer, and lasts for about 1–2 months for a colony of plants. An extensive root system of deep taproots imparts resistance to summer drought. The roots of M. albus are associated with nitrogen-fixing bacteria. Like white sweetclover, yellow sweetclover (Melilotus officinalis) is erect, tall, and branching, but is distinguished by yellow rather than white flowers. Though they share most botanical characteristics, yellow sweetclover is typically found in drier habitats and has a tendency to bloom about 2–4 weeks earlier than white sweetclover. Some authorities regard white and yellow sweetclover as the same species, in which case white sweetclover is referred to as Melilotus officinalis alba; however, most consider them to be separate species. Sweetclover seedlings closely resemble those of alfalfa, but may be distinguished from alfalfa by the absence of pubescence on the underside of the leaves and by their bitter taste. White sweetclover is native to much of Asia and southern Europe, from France and Spain to Kazakhstan and Myanmar, and also northeastern Africa. M. albus has been introduced from Eurasia into South Africa, North and South America, New Zealand, Australia, and Tasmania, as well as many parts of Europe outside its native range. In the US, the species is considered most common in the upper Midwest and Great Plains regions, including Kentucky, Nebraska, Tennessee and Wisconsin. White sweetclover was brought to North America as early as 1664 as a forage crop. Since then, it has spread from cultivation and thrives in waste places and roadsides. White sweetclover is found in every state in the United States and all but 2 Canadian provinces. It establishes in aspen woodlands and prairies in Canada and the lower 48 states and riparian communities in Alaska. The early spread of M. albus was probably facilitated by beekeepers and agriculturalists, as it can be an important fodder crop and food source for honey bees, and it was brought to Alaska in 1913 as a potential forage and nitrogen (N) fixing crop. While it has a preference for full sun, slightly moist to dry conditions, and calcareous clay-loam or gravelly soil, M. albus is adapted to a wide range of climatic conditions and will grow in practically all soil types, including soils with very low nutrient levels, fine, and course-textured soils (though its size varies considerably with the moisture and fertility of the soil). Sweetclover is adapted to pH levels ranging from 5.0 to 8.0, is salt-tolerant, drought resistant, and winter-hardy. With a range that extends as far north as the Yukon and North West Territories in Canada and Alaska in the United States, M. albus occupies habitats with extreme weather, with rainfall being below 170 mm in some places and almost 4000 mm in others, and with the annual temperature in inland Alaska as low as -3.3 degrees Celsius. M. albus is drought tolerant due to long taproots and only requires adequate moisture for germination. It is then capable of development under extremely dry conditions. However, M. albus is poorly suited for acid soils, is shade-intolerant, and cannot withstand prolonged flooding, t
