Northern Prairie Wildlife Research Center
White sweetclover is a annual or biennial forb. It ha an erect growth form (0.5-3.5 m tall) and grows from a long taproot. Stems are highly branched, coarse, and glabrous to lightly pubescent. Leaves are alternate and pinnately trifoliolate. Leaflets are ovate, obovate, or oblong (1-4 cm long). Margins are entire except for the dentate, truncate tip of each leaflet. Each leaflet is glabrous on the upper surface and has an appressed pubescence on the lower surface. Stipules are 7-10 mm long. Stipules of the lower leaves have one to two teeth at the base. Flowers are in numerous spicate racemes (4-28 cm long). Each raceme has 40 to 120 flowers. The calyx is tubular (4-5 mm long) and has subequal, deltoid teeth. Flowers are white (4-5 mm long) and papilionaceous. The wings and standard petals of the corolla are about equal (4-5 mm long). The fruiting body is a dark brown to black pod or legume (2.5-5 mm long, 2-2.5 mm wide, and 1.5-2 mm thick). It is ovate, glabrous, and has reticular veins. The legume usually contains one seed (occasionally two or three). Seeds are oval (2-2.5 mm long and 1.5 mm wide) and yellow to greenish-yellow.
Yellow sweetclover [Melilotus officinalis (L.) Pall.] is essentially indistinguishable from white sweetclover with the exception of having a yellow flower that is 4.5-7 mm long, and a light brown legume that is cross veined or irregularly rugose. Even though yellow sweetclover had a lower significance of impact at Pipestone National Monument (PIPE) (48 vs. 51 for white sweetclover), these two species are similar in ecology, biology, and control in prairie. Both sweetclovers will be discussed as one. Exceptions will be noted. Worldwide, numerous varieties, ecotypes, and forms of these two species are recognized.
White sweetclover is native to Eurasia. It has become naturalized elsewhere in the northern hemisphere, Australia, and some parts of South America. White sweetclover is more widely distributed than yellow sweetclover. Both species, especially white sweetclover, have been planted as cash crops in Russia, Germany, Poland, Argentina, United States, and Canada. They are legumes and have the potential to fix nitrogen to improve the soil. White sweetclover was first introduced into North America in 1664 as a forage crop. Yellow sweetclover was first reported in North America in 1739.
Sweetclovers have a broad habitat range and are drought and cold tolerant. However, neither species can tolerate prolonged flooding. They are commonly found growing on calcarious soils and are most abundant in rich loams and clay loams having a pH of 6.5 or higher. Their deep taproots allow them to grow in poorer sands and gravels. Yellow sweetclover is slightly more salt tolerant than white sweetclover, and both grow well on alkaline soils. Both species grow in a wide diversity of plant communities ranging from intensive agricultural lands to roadsides and prairies. Neither species is tolerant of dense shade, but both will tolerate partial shade.
Sweetclovers are obligate biennials. During the first season, the seeds germinate from March through April. Germination is enhanced by day temperatures <15°C. Although most of the seeds germinate in the spring, some germination will continue through the summer and into the fall. After the seedlings have become well established, the above ground growth rate slows, and underground development accelerates. In early to mid-September, underground development is primarily taproot growth. The taproot may reach lengths of 120 cm. Adventitious roots also form, often reaching 30-40 cm in length. As the roots begin to develop, crown buds develop near the ground level. Contractile roots develop in the fall to pull the crown below ground level to prevent winter kill.
At the beginning of the second growing season, vegetative shoots initiate elongation and rapidly elongate until early summer. Following elongation, sweetclovers flower and set seed. Flowering is influenced by both daylength and temperature. Continuous daylengths of 17 hours have been shown to induce flowering in the first growing season. Under short daylengths, it has been shown that crown buds will remain dormant and may not be stimulated by mowing. Low temperatures retard flowering and often result in increased below ground production. Yellow sweetclover usually flowers in June or July, and white sweetclover flowers about a week later than yellow sweetclover. Although some self-pollination occurs, bees, wasps, and flies which are the primary pollinators. Legumes (pods) develop from late June through August and may remain over winter on the plant. Each pod will typically have one seed, but may contain up to three seeds. It has been estimated that one plant will produce between 14,000 and 350,000 seeds. Seeds are distributed short distances by the wind, and runoff water may move the seeds greater distances. Most of the seeds remain viable in the soil for 14 to 20 years, and some stored seed has remained viable for over 80 years.
An intermediate number of white sweetclover plants occur in patches at PIPE. If the patches were combined, they would cover less than 5 hectares. Yellow sweetclover patches cover a total of 5 to 10 hectares. Sweetclovers are found in mid-successional sites that were disturbed in the last 11 to 50 years. They have the potential to modify or retard natural secondary successional processes. The plants have only a minor visual impact on PIPE in the vegetative stage, but they may have a major impact in the flowering stage.
Chemical, cultural, and mechanical control methods have all received limited used on white and yellow sweetclovers. An important consideration in controlling these species is that the seeds have the potential to remain viable in the seed bank for more than 10 years. Many sources of new propagules surround PIPE. Most control methods may have a detrimental effect on other plant species, and they may constitute a disturbance that will favor reinvasion by sweetclovers or by other exotic species.
Fire may either control or stimulate sweetclovers. The response depends on timing of the prescribed burn. Prescribed burns conducted in March or early April often stimulate germination. Also, germination may be stimulated by fall burns. One method of controlling sweetclovers with prescribed fire is to burn in early May when second-year shoots are visible. This treatment is followed by a prescribed burn in early July of the following year. Disadvantages are that it is often difficult to get a thorough burn in July, and July burning often results in damage to desirable warm-season species. Annual prescribed burning in May will eliminate second-year plants and may stimulate germination. Thus, first-year plants will be present as long as seeds remain in the soil, but flowering by second-year plants will not occur. Another technique is to conduct a prescribed burn in the dormant season in fall or early spring to stimulate germination. This is followed by a prescribed burn or mowing in late May of the following year when plants are in their second growing season. Mowing on a schedule similar to burning has been shown to control sweetclover. It is often less effective because it does not kill the plants and a few flowers on low, uncut branches will produce seed. The key to these control methods is timing. It is important that crown shoots have began to elongate on second-year plants before applying burning or mowing treatments. Any second-year plants escaping fire due to and incomplete burn must be mowed or treated with an herbicide before producing seed.
A few chemical control options exist for sweetclover. Many herbicides are not specific to sweetclovers or may not be specifically licensed for this particular use. It is important to read and follow all label directions. The herbicide 2,4-D partially controlled sweetclover. Second-year plants were much less susceptible to 2,4-D than were first-year plants. Other herbicides that may control sweetclover include Arsenal (imazapyr), Tordon (picloram), Escort (metsulfuron methyl), Oust (sulfomethuron methyl), Banvel (dicamba), and Curtail (clopyralid and 2,4-D).
Research into the biological control of sweetclover has not received attention because of the importance of sweetclovers in agriculture. However, both species have some introduced and North American pests that may have some potential. The sweetclover weevil [Sitona cylindricollis Fahr. (Coleoptera:Curculionidae)], a native of Europe, was accidentally introduced into North America. Adults feed on the foliage of sweetclover, and larvae feed on the roots. Early spring or fall feeding by this weevil on seedlings may be severe enough to kill the plants. A sweetclover root borer [Walshia miscecolorella Chambers (Lepidoptera:Walshiidae)] is a native insect that, on rare occasions, has reached high densities and damaged sweetclover over relatively large areas. The ashgray blister beetle (Epicauta fabricii LeConte), striped blister beetle (E. vittata F.), and margined blister beetle (E. pestifera Werner) have all been documented as feeding on sweetclover, but have rarely damaged the plants. A root rot (Rhizoctonia sp.) has been reported to damage sweetclovers. Both sweetclovers are palatable plants to most grazers. Domestic livestock, deer, rabbits, and other herbivores readily feed on these plants. Heavy grazing in the spring may reduce the density of sweetclovers.
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