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Those other yellow flowers by the side of the road: Black medic and hop clover

Craig Sheaffer, Extension forage agronomist, and Roger Becker, Extension weed scientist

This spring’s abundant rainfall has stimulated the emergence of many small, yellowed flower legume plants in permanent pastures, on roadsides and in lawns. These are birdsfoot trefoil, black medic, and hop clover, all introduced from Europe in colonial times through contamination with sown forage seeds. Although ecologists, gardeners and turf managers consider these plants weedy, each can contribute to pasture productivity by providing quality forage and nitrogen from biological N2 fixation in grazed permanent pastures where inputs are typically limited.  We discussed the merits of birdsfoot trefoil in a previous article,  Those pretty flowers by the side of the road: Birdsfoot trefoil. Here we will discuss black medic and hop clover.

Black medic (Medicago lupulina)

Black medic growing with Kentucky bluegrass
in a pasture.
Black medic, also called yellow trefoil, is related to alfalfa (Medicago sativa L.). It is widely distributed throughout Minnesota often in association with perennial grasses. Black medic is a prolific seed producer and stands regenerate in spring from seed produced in previous years. It primarily grows as an annual and is killed by freezing temperatures in the fall, but sometimes plants can overwinter.


  • Black medic has low-growing, spreading stems that can be up to 2 feet long radiating from a small central taproot.
  • The leaves are pinnately trifoliolate (3 leaflets), with individual leaflets rarely over ¾ inch long, oval shaped with a projecting tip at the apex. The leaf stem (petiolule) of the center leaflet is slightly longer than petiolule of the two lateral leaflets.
  • Black medic produces a very tight, compressed cluster of 10 to 30 tiny flowers. Plants flower over a long period of time from spring into the fall with mature seed pods developing while flowers are still present. Once the flowers mature, they form a very tightly coiled black seedpod, hence the name ‘black medic’. A pound of seed will contain over 275,000 seeds, compared with about 200,000 seeds per pound of alfalfa.


Black medic is not economically important as a forage crop but can provide some high-quality forage when it volunteers in pastures. Our research showed that with a spring or late summer seeding in pure stands, an improved selection of black medic (George) had high forage protein and RFV but low yields of from 0.6 to 1.0 ton/acre at a single July or October harvest (Tables 1 and 2). Like alfalfa, forage quality decreases with maturity from vegetative to seed production stages. Black medic will biologically fix nitrogen in association with the same Rhizobium that normally nodulates alfalfa. It can fix from 25 to 45 lbs nitrogen per acre when grown in pure stands. A portion of that fixed nitrogen is available to grass growing in association with black medic. Black medic’s flowers are visited by pollinators including honeybees and other insects.

Table 1. Forage yield and quality of annual medics planted in early May and harvested in July.  
Medic Maturity Forage
tons/a %
George black medic flowering 1.0 24 206
Santiago burr medic seed 1.6 21 174
Sava snail medic seed 1.7 20 140
Nitro alfalfa* bud 1.5 20 169

*Nitro is a non-dormant alfalfa providing increased N contribution for use in short term crop rotations.

Table 2. Forage yield and quality of annual medics and alfalfa planted in August and harvested in mid-October.

Medic Forage

tons/a %
George black medic 0.6 26 290
Santiago burr medic 1.5 20 225
Sava snail medic 1.6 25 213
Nitro alfalfa* 0.9 25 228

*Nitro is a non-dormant alfalfa providing increased N contribution for use in short term crop rotations.

Other annual medics

There are at least 35 other medic species (genetically distinct types) that are all related to alfalfa. In earlier research, we looked at the yield and forage quality of spring and fall seeded annual medics that had been developed in Australia. Some of these medics had yields of nearly 1.5 tons/acre and have potential for emergency forage or as a cover crop. When established after a spring seeded oat crop and incorporated in the fall, medics supplied a fertilizer nitrogen equivalency of 150 lb/acre to a following corn crop. Seed of medics is generally not marketed by seed dealers within Minnesota but is available to purchase on-line.

Hop clover (Trifolium campestre)

Low hop clover growing with smooth bromegrass
in a pasture.
Hop clover is another yellow flowered summer annual clover that volunteers each year. It is more frequently found in northern and eastern portions of Minnesota where rainfall amounts are higher. There are three forms of hop clover, the type most frequently found in Minnesota is low hop clover, Trifolium campestre. Its leaves are also pinnately trifoliolate like black medic, but its flowering head is larger with larger florets. It produces seeds even smaller than black medic , containing about 1 million seeds per pound. Hop clovers grow only from 5 to 10 inches tall and produce a limited amount of forage that at immature stages is high in forage quality. As a legume, hop clover also produces biologically fixed N but quantities are unknown.

Ecological Insights

Black medic and hop clover have high seed production capacities and can quickly produce large numbers of seed creating a seed bank in the soil. As annuals, both black medic and hop clover rely on constantly reestablishing from this seed bank and as such, require disturbance and an open niche within existing vegetation stands. These legumes may show up in healthy ecosystems here and there, but rarely develop into infestations requiring control. Well managed, resilient prairies, pastures and lawns do not provide the disturbed sites necessary and give considerable competition resulting in high mortality for the few volunteering seedlings that may try to gain a foothold.

In the aforementioned article, we have discussed the potential of black medic and hop clover to benefit forage systems; however, they are typically not the most productive species and their presence should be an indication of less than optimum forage production conditions. Recommended management strategies to improve productivity include selection of productive adapted species, fertilization, adjusting grazing and harvesting schedules, and irrigation in drought cycles. In some situations, herbicides may be required to kill black medic. Penn State Extension has a thorough herbicide guide for control of black medic: Lawn and turfgrass weeds: Black medic.

Hop clover is less frequently on herbicide labels, but most herbicides that control black medic will also control hop clover. An excellent site to search for products with specific active ingredients is CDMS:

Additional sources of information

Black Medic. University of Minnesota extension.

Black Medic. Minnesota wildflowers.

Hop clover. Minnesota Wildflowers.

Zhu, Y., et al., 1996. Forage yield and quality of six annual Medicago species in the north-central USA. Agron. J. 88:955-960.

Zhu, Y. et al., 1998. Inoculation and nitrogen affect herbage and symbiotic properties of annual medico species. Agron. J. 90:781-786.
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