The common sight of a three-leafed plant in a lawn often leads people to think of clover, but many pervasive weeds mimic this familiar look. This trifoliate structure (three leaflets per leaf) is the source of frequent misidentification between true clover (Trifolium) and unwelcome garden guests. Accurately distinguishing these plants is the necessary step before attempting any removal or management strategy. Two common impostors are Wood Sorrel and Black Medic, each possessing unique characteristics that set them apart from genuine clover.
Identifying Wood Sorrel (Oxalis)
Wood Sorrel is easily identifiable by its distinct leaf structure. Unlike the rounded leaflets of true clover, Wood Sorrel leaflets are uniformly heart-shaped, giving the compound leaf a look often mistaken for a shamrock. The leaflets are typically a bright, yellowish-green color and often fold downward along the midrib in response to stress or at night. This plant produces small, delicate flowers with five petals, which are usually bright yellow.
The growth habit of Wood Sorrel is typically low-growing, and it can spread aggressively. Some species spread via rhizomes (underground stems that generate new plants). Its unique feature is the elongated, cylindrical seed capsule that explodes upon maturity or slight touch, launching seeds several feet away. This explosive dispersal mechanism makes the weed challenging to control once it begins to set seed.
Identifying Black Medic
Black Medic (Medicago lupulina) shares the trifoliate leaf pattern, with a structure similar to true clover. Its leaflets are oval or wedge-shaped and display fine serrations near the tip. A defining characteristic is the petiole (stalk) of the middle leaflet, which is noticeably longer than the stalks of the two side leaflets. This feature is absent in true clover.
The plant grows low to the ground, forming dense mats that can reach two to three feet in diameter. Black Medic is a legume, meaning it fixes nitrogen in the soil, allowing it to thrive in nutrient-deficient areas. It produces small, dense clusters of bright yellow flowers from late spring through summer. These flowers eventually mature into tightly coiled, kidney-shaped seed pods that turn black upon ripening, giving the plant its name.
Visual Keys to Distinguish Them
A comparison of anatomical features provides the most reliable way to differentiate these three common plants. The shape of the leaflet is the easiest starting point for identification. Wood Sorrel is the only one with distinctly heart-shaped leaflets, while true clover has broadly rounded leaflets, and Black Medic has oval or wedge-shaped leaflets with small teeth at the tip.
Leaf Stalks and Flowers
Examining the leaf stalks provides another distinguishing marker for Black Medic, as its middle leaflet has a clearly extended petiole, unlike the uniform stalks found on Wood Sorrel and clover. Flower color is also an immediate indicator. Wood Sorrel and Black Medic both produce yellow flowers, but Wood Sorrel flowers are five-petaled and solitary, while Black Medic flowers are tiny and packed tightly into dense, ball-shaped clusters. True clover produces white or pinkish-white flowers in larger, puffball-like heads.
Seed Structures
The fruit provides a clear distinction between the species. Wood Sorrel forms explosive, upright, cylindrical seed capsules. Black Medic forms small, black, coiled seed pods. True clover generally produces inconspicuous seed heads that remain nestled within the flower cluster.
Management and Control Methods
Effective control of these weeds begins by fostering a dense, healthy lawn, as a strong turf canopy naturally crowds out opportunistic weeds. Proper cultural practices, such as raising the mowing height, deep and infrequent watering, and ensuring adequate soil fertility, can suppress both Wood Sorrel and Black Medic. Black Medic often indicates compacted or low-nitrogen soil, so aeration and balanced fertilization can weaken its competitive advantage.
Chemical control involves using post-emergent broadleaf herbicides, which are most effective when the weeds are young and actively growing. Black Medic is susceptible to three-way herbicide mixtures containing ingredients like 2,4-D, dicamba, and MCPP. Wood Sorrel, due to its waxy leaves and ability to spread via rhizomes and explosive seeds, often requires a more targeted approach, sometimes needing herbicides that include triclopyr or atrazine. Hand-pulling is a viable option for small infestations of both weeds, but it is necessary to remove the entire root system (the deep taproot of Black Medic and the fragile rhizomes of Wood Sorrel) to prevent regrowth.