Montana Field Guide

PLANTS: Erect, single-stemmed, herbaceous biennial (sometimes annual or perennial) from a taproot. Herbage nearly glabrous and 50 to 100 cm tall. Sources: Hitchcock et al. 2018; Lesica et al. 2012.

LEAVES: Leaves alternate and trifoliate. Leaflets oblong, 1-3 cm long, typically smooth, and serrated almost to the base. Sources: Hitchcock et al. 2018; Lesica et al. 2012.

INFLORESCENCE: Inflorescence is of many narrow, minutely bracteate (2 mm), elongated, spike-like racemes 5-16 cm long, arising in the leaf axils and supporting 20-80 nodding flowers. Flowers are pea-like, strongly sweet-scented, 3-4(-5.5) mm long, nodding, and white. Wing petals about equal in length as the keel petal. Calyx is about 2 mm long. Fruit is an ovoid legume, about 3 mm long, scarcely dehiscent, and mostly containing one seed. A single plant can produce thousands of viable seeds. Sources: Hitchcock et al. 2018; Lesica et al. 2012.

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).

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 (more squarish).
* Habitat: In Montana there is a tendency for White Sweetclover to grow more commonly in riparian areas and in slightly moister sites.

Yellow Sweetclover – Melilotus officinalis
* 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.

Alfalfa – Medicago sativa
* 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.

NOMENCLATURE
The International Code of Nomenclature sets the rules and recommendations that govern the scientific naming of plants, fungi, and algae (https://www.iapt-taxon.org/nomen/main.php; formerly called the International Code of Botanical Nomenclature). According to the rules Melilotus is treated as masculine, and therefore the specific epithet must be 'albus. Melilotus has incorrectly been treated as feminine resulting in the scientific name of Melilotus alba.

Open roadsides, fields, and streambanks in the plains and valley zones of Montana (Lesica et al. 2012).

White Sweetclover tends to be more common along streams and in moister habitats than Yellow Sweetclover (Lesica et al. 2012).

POPULATION CONTROLS
White Sweetclover has been shown to increase with disturbance while some native species declined (Parker et al. 1993). In one study, White Sweetclover grew to a height of 1.5 meters in disturbed sites, shading native species, thus potentially reducing light to lower growing natives (Parker et al. 1993). Changes to soil conditions (e.g. compaction, change in mineral composition) in disturbed areas may also be a reason for why White Sweetclover proliferates over native species. In undisturbed native areas White Sweetclover was shown to not compete as well.

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 White 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, white, and 3-4(-5.5) mm long, with 10 stamens (9 united and 1 separate).

FRUITS
Ovoid legume, 3-4 mm long, scarcely dehiscent, and mostly containing one seed. A single plant can produce thousands of viable seeds.

LIFE CYCLE
White Sweetclover is primarily a biennial, however, it has been observed as annual, making it a facultative (rather than obligate) biennial. Plants germinate in spring. During the first season of growth, a single, branched, vegetative stem emerges while below ground a taproot develops. In the second year, numerous branched stems develop. Flowering commences in early summer and may continue throughout the growing season. Seeds develop from mid-summer to fall. 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 (Gucker 2009).

As is the case with many annual and biennial species, population sizes vary greatly from year to year and depend upon climatic conditions.

In White Sweetclover, bolting is positively correlated with the diameter of basal rosette (Baskin and Baskin 1979; Klemow and Raynal 1981).

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/roadside stabilization and reclamation, production of high-protein hay and 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).

FIRE MANAGEMENT AND MOWING
White Sweetclover has become a serious problem in native grassland communities. Prescribed burning of infested areas to control sweetclovers produces variable results (Heitlinger 1975; Kline 1984). Spring burning may promote or inhibit seed germination, depending on the phenologic stage of plants at the time of burn. One study suggested that the follow tactics should suppress sweetclover: burning annually after second-year shoots are clearly visible; burning every other year before second-year plants ripen seed; and possibly by burning annually in late summer near the beginning of the critical growth period (Heitlinger 1975). Fall burns can have no effect or may even stimulate germination (Kline 1984). Mowing plants after bolting and before seed set was shown to be somewhat effective and could be used as a management tool when burning is not practical (Kline 1984). These efforts are aimed at reducing flowering and the subsequent prolific seed crop of this biennial species. Although these treatments may be effective, control of the species is problematic due to the high number and longevity of seeds in the seed bank (Kline 1984).

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 White 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).