Yellow sweet clover (Melilotus officinalis)

Synonyms: Melilotus arvensis, Melilotus leucanthus, Melilotus lutea, Melilotus officinalis var. micranthus

Common Names: Yellow melilot, yellow sweetclover, common melilot, ribbed melilot

Description:

Yellow sweet clover (Melilotus officinalis) is a tall, fragrant, yellow-flowered legume from Eurasia that was widely planted across North America for forage, honey production, and soil improvement starting in the 1700s [1]. MISIN tracks it because it can take over open, sunny ground in dense stands, and because moldy sweet clover hay is a well-documented cause of fatal internal bleeding in livestock [1][3].

Habit:

Yellow sweet clover spends its first year as a single, many-branched stem with clover-like leaves, storing energy in its taproot; it's easiest to overlook at this stage since it hasn't flowered yet. In its second year, it sends up several tall, leafy, hollow stems up to 3 feet or more, giving the whole plant a bushy look, and produces long, slender spikes crowded with small, fragrant yellow, pea-shaped flowers from May through September [1]. Look for it in open, sunny, disturbed ground ? roadsides, old fields, and pastures ? where it's most conspicuous while blooming in June and July.

At-a-glance cues:

1. Clover-like leaves with three finely toothed leaflets, alternating up the stem [1]
2. Tall, many-branched, often hollow stems that can give the plant a bushy shape near the base [1]
3. Long spikes crowded with small, fragrant, yellow pea-like flowers, blooming May through September [1]
4. A deep taproot with an extensive network of lateral roots [1]

Leaves:

1. Arrangement: alternate, compound (divided into three leaflets, like a common clover) [1]
2. Shape: leaflets oval to oblong, finely toothed along the margin [1]
3. Size: leaflets about 0.4 to 1 inch long [1]

Stems:

1. Habit: upright, freely branched, herbaceous, growing up to 10 feet tall in favorable conditions, though often shorter [1]
2. Cross-section: often hollow [1]
3. Growth form: can spread somewhat near the base, giving mature plants a bushy shape [1]
4. Life cycle: typically a biennial ? a single unbranched stem the first year, followed by multiple flowering stems the second year [1]

Flowers:

1. Color: yellow [1]
2. Shape: small, pea-like, fragrant [1]
3. Arrangement: many flowers crowded onto elongated spikes 1.5 to nearly 5 inches long [1]
4. Bloom time: April through October, roughly 1 to 3 weeks earlier in the season than white sweet clover [1]

Fruit and seeds:

1. Fruit type: a small, wrinkled seed pod that stays mostly closed at maturity, usually holding just one seed [1]
2. Seed output: large plants can produce up to around 100,000 seeds each, though numbers vary widely with growing conditions [1]
3. Dispersal: seeds have no wind-carrying structure and mostly drop near the parent plant, but they travel farther in moving water, and pass through the digestive tracts of livestock and wildlife still able to sprout [1]
4. Longevity: a large share of seeds are "hard" (water-resistant) and can remain viable in the soil for decades ? some studies report over 50 years [1]

Habitat:

Yellow sweet clover is native to Eurasia, especially the Mediterranean region, and has been established in North America since the 1700s through widespread agricultural planting [1]. It's most common across the upper Midwest and Great Plains but occurs throughout the continent, from near sea level up to over 8,000 feet in the West [1]. It grows in prairies, meadows, riparian corridors, and disturbed open ground like roadsides and old fields, and tolerates nutrient-poor, dry, and alkaline soils better than many other legumes [1].

Reproduction:

Yellow sweet clover spreads by seed, and those seeds most often arrive on a property in contaminated hay, pasture seed mixes, or on the coats, hooves, and manure of grazing animals; moving water and passing wildlife can carry seed too. Because a large share of its seeds have a hard, water-resistant coat, they can sit in the soil for decades ? some studies report viability past 50 years ? building up a seed bank that keeps producing new plants long after the parent plants are gone [1].

A single large plant can produce roughly 100,000 seeds, though most fall within a few feet of where they formed [1]. The plant spends its first year as a single stem storing energy in its taproot, then bolts into multiple flowering stems the following year ? pulling first-year plants in fall, once the root-crown buds have formed, or pulling second-year plants before they flower in late spring, are both effective ways to stop that plant from adding to the seed bank.

Impact and Damage:

The most serious hazard from yellow sweet clover is to livestock, not people: when the plant's leaves and stems get wet and moldy ? most often in improperly cured hay ? a natural compound called coumarin converts into dicoumarol, a potent blood thinner. Cattle and other livestock that eat moldy sweet clover hay for several weeks can develop uncontrollable internal bleeding, sometimes with no obvious cause other than a minor injury or giving birth [2][3]. This condition, first identified in the 1920s, was in fact how the anticoagulant drug warfarin was eventually discovered [3]. Anyone feeding hay that contains sweet clover should make sure it was cured dry and never feed hay suspected of being moldy to pregnant animals or before surgery [2].

In gardens and pastures, yellow sweet clover can spread quickly on disturbed, sunny ground and crowd out desirable plants once established. In natural areas, it forms dense, near-single-species stands that sharply reduce the cover of other plants, along with the light and soil moisture available to them, and can act as either a strong competitor or, in some sparsely vegetated sites, an unwanted "nurse plant" that helps other weedy species get established nearby [4]. As a nitrogen-fixing legume, it can also change the nutrient balance of invaded soil in ways that outlast the plants themselves, making native plant recovery harder even after sweet clover is removed [4].

Similar species:

White sweet clover (Melilotus albus) is yellow sweet clover's closest match, sharing the same tall, bushy growth habit and flower spikes ? the simplest way to tell them apart is flower color: white sweet clover's flowers are white, not yellow, and it tends to flower slightly later in the season.

Alfalfa (Medicago sativa) seedlings can look similar to young sweet clover, since both are clover-leaved legumes of disturbed ground, but alfalfa's leaf undersides are covered in fine hairs (pubescent) while sweet clover's are smooth.

Monitoring and rapid response:

Signs to watch for:

1. Tall, bushy, many-branched plants with clover-like leaves on open, sunny, disturbed ground
2. Long spikes crowded with small, fragrant yellow flowers, easiest to spot while blooming in June and July [EDRR-note]
3. If cutting hay that contains sweet clover, watch for mold during curing and storage ? that's the real hazard, not the growing plant itself [2]

What you can do:

1. Pull first-year plants (single, unbranched stems) in fall, once the root-crown buds have developed at the base
2. Pull second-year plants by hand before they flower in late spring ? this is the most effective single-plant timing, since a plant that has already gone to seed will keep contributing to the soil seed bank
3. Cut or remove flowering plants and dispose of them off-site so seed doesn't mature and drop
4. Because seeds can persist in the soil for decades, plan on monitoring and repeating removal for several years, not just one season
5. Natural-area crews: prioritize small, outlying patches before they seed, and consider that a single mowing or burn is rarely enough ? repeated treatments timed to the population's age structure (first-year vs. second-year plants) generally outperform a one-time effort

When to call a professional:

1. Large infestations across pasture, prairie, or natural areas where hand removal can't keep pace
2. Sites where prescribed fire is being considered as a control tool ? timing is critical and a poorly planned burn can make an infestation worse rather than better
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA Forest Service, Fire Effects Information System (FEIS). Melilotus alba, M. officinalis. fs.usda.gov
2. South Dakota State University Extension. Sweet Clover Poisoning. extension.sdstate.edu
3. Yamini B, Poppenga RH, Braselton WE, Judge LJ (1995). "Dicoumarol (Moldy Sweet Clover) Toxicosis in a Group of Holstein Calves." Journal of Veterinary Diagnostic Investigation 7(3):420?422. semanticscholar.org
4. Van Riper LC, Larson DL (2008). "Role of invasive Melilotus officinalis in two native plant communities." Plant Ecology 200(1):129?139. semanticscholar.org

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