Black Medic (or Yellow Trefoil)

Black Medic weed, commonly known as black medick or hop clover, is a low-growing annual weed that is native to Europe and Asia. It has become a common weed in lawns, gardens, and agricultural fields throughout North America. 

Black medic weed is a member of the legume family and is often used as a cover crop to improve soil health. This weed has small yellow flowers that bloom from May to September. 

The leaves are trifoliate and have a toothed edge. The plant can grow up to 2 feet tall but is usually much shorter. Black medic weed has a deep taproot that can reach up to 3 feet deep, making it a resilient and difficult weed to control. 

Read on to learn more about the identification and control of the black medic weed.

Black Medic Weed Identification

Black Medic Weed: A Quick Overview


Black Medic Weed, also known as Medicago lupulina, is a common weed in the legume family. It is native to Europe and Asia but has become widespread in North America and other parts of the world.

It is an annual plant that grows up to 50 cm tall and has small yellow flowers. The flowers are followed by small, curved seed pods that contain one or two seeds.

Black Medic Weed is often found in lawns, gardens, and other disturbed areas. It has a taproot that can grow up to 30 cm deep, which allows it to tolerate drought and poor soil conditions.

The plant has a spreading growth habit and can quickly form dense mats that can crowd out other plants.


The leaves of Black Medic Weed are divided into three leaflets and have a distinctive crescent shape. The plant produces small, yellow flowers that are arranged in clusters.

Black Medic Weed is not considered a serious weed in most situations, but it can be a problem in lawns and gardens. It is tolerant of many herbicides, but selective herbicides that target broadleaf weeds can be effective.

Cultural practices such as mowing, watering, and fertilizing can also help control the weeds.

Weed Photos

Black Medic
Black Medic

Yellow trefoil spreading growth habit

Black Medic Seeds

Medicago lupulina L.
Image courtesy of Forest and Kim Starr

Illustration of Black Medic

Identification and description

The plant has a shallow taproot and can grow up to 20 inches tall. Its leaves are trifoliate, meaning they have three leaflets, and are arranged alternately on the stem. The leaflets are oval-shaped and have toothed margins.

The plant’s stems are slender and hairy, and they can branch out from the base.

During the summer months, black medic produces small, yellow flowers that are arranged in clusters at the tips of the stems. The flowers are self-fertile and can produce seeds without the need for cross-pollination.

The seeds are small, round, and have a hard outer coating. Black medic weed is often confused with clover because of its trifoliate leaves and yellow flowers.

However, black medic’s flowers are smaller and more clustered than those of clover. Additionally, black medic’s leaves are darker green and have a more serrated edge than clover’s leaves.

Habitat and distribution

Habitat and distribution

Black medic weed can grow in a wide range of habitats, from fields and meadows to roadsides and disturbed areas. It prefers well-drained soils and can tolerate drought conditions.

Black medic weed is also found in disturbed areas such as roadsides, parking lots, and construction sites. Black medic weed is an annual or biennial plant that can grow up to 60 cm tall. It has a taproot that can reach up to 1 m deep.

The stems of black medic weed are hairy and can be green or reddish in color. The leaves are composed of three leaflets and are arranged alternately on the stem.

The flowers of black medic weed are small and yellow and are arranged in clusters on the stem.

Life cycle and reproduction

Black medic weed, also known as Medicago lupulina, is an annual plant that grows up to 50 cm tall. The life cycle of black medic weed is relatively short, as it completes its life cycle in a single growing season.


The plant starts its life cycle as a seed that germinates in the spring when the soil temperature reaches around 10°C. The seeds of black medic weed can remain viable in the soil for up to five years.

Once the seed germinates, the plant starts to grow rapidly, and within a few weeks, it produces its first set of leaves.


Black medic weed reproduces through self-pollination. The plant produces small yellow flowers that are self-fertile, meaning that they can pollinate themselves without the need for external pollinators.

The flowers of the black medic weed bloom from June to September. After the flowers are pollinated, they develop into small, curled pods that contain the seeds.

The seeds of the black medic weed are small and round, with a diameter of about 1 mm. The seeds are dark brown in color and have a hard seed coat that protects the embryo inside.

The seeds are dispersed by several means, including wind, water, and animals. The seeds can remain viable in the soil for up to five years, allowing for the plant to reseed itself and continue to grow in subsequent years.

Impact on agriculture

black medic enviro impact

Black Medic Weed is a common weed found in agricultural fields and can have both positive and negative impacts on crop production. On one hand, Black Medic Weed can provide benefits to the soil by fixing nitrogen.

This means that the plant takes nitrogen from the air and converts it into a form that can be used by other plants. This can lead to increased soil fertility and improved crop yields.

Impacts of Black Medic Weed

However, Black Medic Weed can also have negative impacts on agriculture. It can compete with crops for nutrients, water, and sunlight, which can lead to reduced crop yields.

Additionally, the weed can serve as a host for pests and diseases, which can then spread to crops. Farmers can manage Black Medic Weed through various methods, including cultural, mechanical, and chemical control.

Control methods

Cultural control methods include crop rotation and planting cover crops, which can help to suppress weed growth. Mechanical control methods involve physically removing the weed, such as through hand weeding or mowing.

Chemical control methods involve the use of herbicides, which can be effective but should be used judiciously to avoid negative impacts on the environment and human health.

Hand weeding

Hand pulling helps reduce infestations. This is best done before the weed has had a chance to seed. If not, you will just be spreading seeds for future germination.

Chemical control

Use a pre-emergent herbicide in the spring to prevent weed seed germination. Pre-emergents are especially effective in areas where the black medic was a problem the previous year.

Broadleaf herbicides – Broadleaf herbicides work best when applied in early spring or fall when the plant is young and actively growing. Products that contain 2,4-D, MCPP, dicamba, clopyralid or triclopyr as an active ingredient can successfully control the plant and work better than 2,4-D applied alone.

Glyphosate (roundup) – Roundup is non-selective and can also be used to kill yellow trefoil in landscape beds and along curbs, sidewalks and driveways.

Vinegar-based (20% acetic acid) and Citric Acid

Herbicides are considered a natural organic weed killer. They can be used as a non-selective herbicide in place of glyphosate and will kill annual weeds.

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