Sweet clover, a plant belonging to the genus Melilotus, is a widely recognized legume found across the globe. Its classification is highly debated, making it one of the most controversial plants encountered by land managers, farmers, and ecologists. This duality stems from its potent agricultural benefits on one hand and its aggressive, weedy characteristics on the other. Understanding when sweet clover is considered a weed requires examining its biology and the contexts in which it appears.
Identifying Sweet Clover Species and Lifecycle
The sweet clovers most often involved in agricultural and ecological debates are the white sweet clover (Melilotus albus) and the yellow sweet clover (Melilotus officinalis). Both species are native to Eurasia but are now prevalent throughout North America and other temperate regions. The white variety typically grows taller and has a more erect form than the yellow species, which tends to bloom a few weeks earlier.
Sweet clover is generally an obligate biennial, meaning its life cycle spans two years before the plant dies. The first year is primarily dedicated to establishing a deep, extensive taproot system and a small vegetative crown, with little above-ground growth. In the second year, the plant uses the stored energy to rapidly grow tall stems, flower profusely, set seed, and then complete its life cycle. This deep taproot is a characteristic feature that enables it to thrive in nutrient-poor and compacted soils.
Negative Impacts Why It Is Considered a Weed
Sweet clover is primarily considered a weed because of its invasive characteristics and its potential toxicity to livestock. The plant produces a high volume of seeds, with a single plant capable of generating up to 350,000 seeds. These seeds are known as hard seeds, meaning they can remain viable in the soil for up to 30 years, making eradication extremely difficult once a seed bank is established.
Its rapid, competitive growth in the second year allows it to quickly dominate disturbed areas, roadsides, and open habitats. In natural ecosystems, particularly native prairies, its tall, dense growth shades out and displaces shorter, sun-loving native species. This competitive advantage is compounded by its ability to flourish in poor soil conditions where many native plants struggle to grow.
A significant concern for farmers is the potential for sweet clover poisoning in grazing animals. The plant naturally contains a chemical called coumarin, which is generally harmless when the plant is fresh. However, when sweet clover hay or silage is improperly cured and becomes moldy, fungal metabolism converts the coumarin into dicoumarol. Dicoumarol is a potent anti-coagulant that inhibits the metabolism of Vitamin K, leading to a blood clotting disorder in livestock. Ingesting hay with dicoumarol concentrations of 20 to 30 milligrams per kilogram over several weeks can cause fatal internal hemorrhaging, a condition sometimes called “sweet clover disease.”
Ecological and Agricultural Benefits
Despite its weedy nature, sweet clover is highly valued for its ecological and agricultural contributions. As a member of the legume family, Melilotus species perform nitrogen fixation through a symbiotic relationship with soil bacteria. This process converts atmospheric nitrogen into a usable form, significantly enhancing soil fertility and reducing the need for synthetic fertilizers. An established stand can contribute an average of 100 to 200 pounds of nitrogen per acre.
The plant’s deep taproot provides physical benefits to the soil structure. These roots penetrate dense layers of soil, effectively breaking up hardpan and improving water infiltration and soil aeration. This action helps to scavenge nutrients like phosphorus and potassium from deeper soil horizons, making them available for subsequent crops.
Sweet clover is also a major resource for pollinator populations, particularly honeybees. The flowers produce abundant nectar, which is used to create high-quality, light-colored clover honey. For beekeepers, the extensive bloom period from early summer to fall makes sweet clover a dependable and valuable honey plant.
Management and Control Strategies
Effective control of sweet clover relies on exploiting its two-year life cycle. The primary goal of any management strategy is to prevent the plant from setting seed in its second year. Mechanical methods, such as mowing or cutting, are most effective when timed just before or during the initial flowering stage of second-year plants.
Cutting the stem close to the ground, typically at 2.5 centimeters or less, prevents regrowth and seed production. First-year plants, which have not yet flowered, can be controlled with targeted broadleaf herbicides such as 2,4-D. Since the seeds remain viable for decades, control measures must be sustained over multiple years to deplete the existing seed bank. In natural areas, control is often implemented specifically to prevent the plant from displacing fragile native flora.