Montana Field Guide

PLANTS: Herbaceous biennial from a taproot (rarely annual or perennial). Erect, single-stemmed in the first year; multi-stemmed and more robust in the second year. Stems are glabrous to pubescent and typically 50-100 cm tall. Sources: Gucker 2009; Lesica et al. 2012; Darbyshire and Small 2018.

LEAVES: Alternately arranged on the stem and trifoliate (3-leaflets). Leaflets oblong, 1-3 cm long, typically smooth, and serrated on margins to near the base. Source: Whitson et al. 1996; Lesica et al. 2012.

INFLORESCENCE: Yellow flowers arranged in a narrow raceme, 5-16 cm long, that arises from leaf axils. Each raceme supports 20-80 flowers and is minutely bracteate (2 mm). Flowers are pea-like, strongly sweet-scented, 4-6 mm long, nodding, and yellow (turning cream to white with age). Calyx is about 2 mm long with linear lobes. Fruit is a legume, ovoid, 3-4 mm long, scarcely dehiscent, and usually one-seeded. Sources: Gucker 2009; Lesica et al. 2012; Darbyshire and Small 2018.

The genus Melilotus comes from meli, which is the Greek word for honey, and lotus which refers to a 'clover-like plant'.

Flowering in early summer through fall (Gucker 2009).

Yellow Sweetclover – Melilotus officinalis, exotic
* Flowers: yellow.
* Racemes: greater than 4 cm long. At peak flowering racemes are 6 times as long as wide.
* Leaflets: 3. Margins serrated more than half-way to the base. Leaflets usually no more than 2 times longer than broad.
* Fruits (legume): surface has elongated ridges (veins) that delimit spaces (areolae), and these spaces tend to be longer than wide.

White Sweetclover – Melilotus albus, exotic
* Flowers: white.
* Racemes: greater than 4 cm long. At peak flowering racemes are 8-15 times as long as wide.
* Leaflets: 3. Margins serrated more than half-way to the base. Leaflets 2.5-3.5 times as long as broad.
* Fruits (legume): surface has shorter ridges (veins) that delimit spaces (areolae), and these spaces tend to be as long as wide.
* Habitat: In Montana there is a tendency for White Sweetclover to grow more commonly in riparian areas and in slightly moister sites.

Alfalfa – Medicago sativa, exotic
* Flowers: purple or white.
* Racemes: less than 3 cm long.
* Fruits (legume): surface texture is veiny; coiled or curved with more than one seed.
* Leaflets: 3. Margins serrated in upper half of leaflet. Oblanceolate.

TAXONOMY
Yellow Sweetclover and White Sweetclover share similar appearances, except for their flower color, which has caused many botanists to conclude they are of the same species (conspecific) (Kartesz 2015; USDA PLANTS Database [NRCS 2022]; IT IS 2017; Bugwood 2017). However,
Darbyshire and Small (2018) report evidence that these two species are evolutionarily separate and genetically more closely related to other species in the genus than they are to each other. Breeding studies have shown that there are strong physiological barriers to fertilization, and when crossed Yellow and White Sweetclovers rarely produce fertile progeny (Darbyshire and Small 2018). Differences found in their phenology, ecology, physiology/biochemistry, seed protein profiles, chromosome structure, and DNA sequences support the theory that they evolved separately from one another (Darbyshire and Small 2018). Hence, Darbyshire and Small (2018) support the treatment that Yellow Sweetclover and White Sweetclover are separate species.

Commonly found in disturbed sites including agricultural fields, roadsides, mining spoils, borrow pits, rangelands, open slopes in badlands, prairies, floodplains, and recently burned areas. Plants are found in the plains and valley zones of Montana (Lesica et al. 2012).

ECOLOCIAL TOLERANCE
Yellow Sweetclover plants establish and grow under a wide range of temperatures and precipitation rates, and on a variety of alkaline to slightly acidic soils and parent materials (Gucker 2009). Very low nutrient levels and fine to coarse-textured soils are tolerated. Moist soil conditions are essential for seed germination; however, once established, plants are drought tolerant and capable of completing their lifecycle (Gucker 2009).

SUCCESSION
Yellow Sweetclover is a often one of the first plants to appear when ground is disturbed (Whitson et al. 1996). Plants help to stabilize soils (Whitson et al. 1996).

POLLINATORS
Bees (honeybees, bumble bees, and leaf-cutter bees) are the most common pollinators of sweetclover (Gucker 2009). Wasps, flies, and other insects are less common pollinators (Gucker 2009).

The following insect species have been reported as pollinators of Yellow Sweetclover or other Melilotus species where their geographic ranges overlap:: Bombus vagans, Bombus appositus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus huntii, Bombus nevadensis, Bombus rufocinctus, Bombus sylvicola, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus bimaculatus, Bombus griseocollis, Bombus impatiens, Bombus insularis, Bombus suckleyi, Bombus bohemicus, and Bombus flavidus (Macior 1974, Thorp et al. 1983, Johnson 1986, Colla and Dumesh 2010, Wilson et al. 2010, Colla et al. 2011, Koch and Strange 2012, Koch et al. 2012, Williams et al. 2014, Miller-Struttmann and Galen 2014).

Plants reproduce by seed.

FLOWERS
Perfect (containing male and female organs). Calyx lobes are subequal, about 2 mm long. The sweet-scented pea-like flowers are bilaterally symmetrical, yellow, and 4-6 mm long, with 10 stamens (9 united and 1 separate).

FRUITS
An ovoid legume, 3-4 mm long, scarcely dehiscent, and mostly containing one seed.

LIFE CYCLE [Adapted from Gucker 2009]
Yellow Sweetclover germinates in spring. During the first year of growth, a single, vegetative stem with many branches is produced. Near the end of the first growing season, nutrients are allocated below ground to the taproot and root crown buds are formed. In the second year of growth, the number of stems increases, the plant grows larger, and flowers and seeds are produced.

Flowering commences in early summer and may continue throughout the growing season. Seeds develop from mid-summer to fall. A single plant can produce thousands of viable seeds. Melilotus generally produces both readily germinable and water-impermeable or "hard" seeds; the latter capable of persisting in the seed bank for a decade or more and requiring scarification to germinate. Seeds are commonly dispersed by rainwater runoff and stream flow, with animals occasionally moving seeds around (especially humans).

From an agricultural perspective Yellow Sweetclover and White Sweetclover have many uses while also can be problematic for farmers, livestock management, and in restoration.

CULTIVATION
The planting and use of Melilotus species has been widely promoted for soil stabilization and reclamation, production of high-protein forage, production of honey, and a host of other uses. Yellow Sweetclover is less often used as an agricultural crop compared to White Sweetclover (Turkington et al. 1978).

Sweetclovers that grow in wheat fields can cause sweetclover taint, whereby harvested wheat absorbs the characteristic odor of sweetclover (Turkington et al. 1978). This happens when sweetclover is still green or conditions are wet during wheat harvesting.

GRAZING MANAGEMENT
Yellow and White Sweetclovers are high in coumarin which causes anticoagulation of the blood (Whitson et al. 1996). This compound can cause bloat in cattle (Whitson et al. 1996). Feeding cattle spoiled sweetclover hay can cause sweetclover disease or 'bleeding disease' (Turkington et al. 1978). Commercially available varieties of these species planted for forage have been developed to produce lower levels of coumarin and are less likely to cause bloat (Whitson et al. 1996).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides

In more recent times, efforts have begun to control the sweetclover populations because they can have negative impacts to agricultural crops (associated with viral plant diseases), can quickly and abundantly invade natural habitats, shade native vegetation, alter soil nitrogen levels, and initiate shifts in native species composition (Gucker 2009).

In roadsides Yellow Sweetclover plants can grow abundantly and very tall, often masking guard rails and road signs which creates potential problems for motorists (Turkington et al. 1978).