Three Weeds with Three Leaflets: Trifoliate Look-Alikes

Sometimes it is difficult to distinguish one plant from another. Yellow woodsorrel, black medic, and white clover all have leaves with three leaflets and are frequently found in home landscapes. But a closer look will reveal that you can tell them apart by examining each weed's growth habit, leaf structure, and flowers.

Yellow woodsorrel (Oxalis stricta) is a member of the Oxalidaceae family. It is classified as both an annual and a perennial. It is adaptable to many turfgrass site conditions – moist, dry, sunny, shady, nutrient-poor, and nutrient-rich. Its leaf has three leaflets with smooth margins and deep indentations at the apex. The leaflets are beautifully heart-shaped. The leaves fold up at night.

Its small, cup-shaped flowers are yellow, have five petals, and are rounded and notched at the tip. The flowers are small, at ¼ to ½ inch in diameter. They bloom from late spring to early fall.

Woodsorrel tends to start out with a prostrate growth pattern but becomes more upright. It has a taproot, but is easy to pull from the lawn, especially when the soil is moist. Improving the density of your turfgrass will help reduce the occurrence of this weed.

Black medic (Medicago lupulina) is a member of the legume family of plants. Like other legumes, it can fix nitrogen from the air to its roots. It is classified as a summer or winter annual. Black medic is most often found in dry, disturbed, nutrient-poor soils and in thin turfgrass areas. Its leaf has three oval leaflets, but the center leaflet has a longer petiole. The leaflets become slightly serrated as they reach a spur at the tip. Where the leaf meets the main stem of the plant, there is a pair of stipules, or small leaflike appendages.

The yellow flower clusters are called inflorescences. They are small, ¼ to ½ inch tall, and can be found throughout the summer months.

It is low-growing with a center crown from which long stems emerge. It does not have nodes. Although black medic has a long taproot, it can be pulled out at the crown. Improving the density of your turfgrass will decrease its numbers.

White clover (Trifolium repens) is also a member of the legume family. It is a perennial that is found most often in full sun in under-fertilized turfgrass. Its leaves have three oval leaflets, slightly serrated and with small indentations at the tips. There is also a whitish "watermark" that is shaped like an inverted "V" on the lower portion of each leaflet.

Its inflorescences are larger than black medics', at ½ to ¾ inch tall, and are white or sometimes pinkish in color. Flowering peaks in late spring to early summer. It is a favorite of many pollinators.

White clover grows low to the ground and forms mats as it produces runners that root at the nodes. You can pull it, but that can be a time-consuming task. Fertilizing and improving the density of your turfgrass will help reduce white clover.

The following chart may be useful in distinguishing the three look-alikes:

All three of these look-alikes are turfgrass weeds that can be reduced through improving the quality and thickness of your lawn.  It may be of interest to note that white clover was included in grass seed mixes until the development of broadleaf herbicides in the 1950s.  At that point, it became a goal of the herbicide companies to influence homeowners to have a "perfect" lawn, free from all broadleaf weeds, including white clover.

It is no surprise that white clover was historically considered part of a healthy lawn. Like other members of the legume family, it takes nitrogen from the air and "fixes" it into its root system. This nitrogen becomes an available fertilizer for the grass. This symbiotic relationship reduces or can even eliminate the need for supplemental nitrogen fertilizer. White clover will fill in empty spots in the turf, reducing other weeds and the need for chemical herbicides. White clover greens up early in the spring and, being drought-tolerant, stays green during the hot, dry, dog days of summer. Consider including white clover in your lawn just because it is an important nectar source for many pollinators. White clover is also a very valuable forage crop. Rabbits and other wildlife will browse it in the garden. If you have members of your household who are allergic to bee stings, you can always reduce the number of flowers by mowing beneath their height, which can reach 6 inches.

The following Penn State Extension articles provide additional detailed information about these three weeds:

Yellow Woodsorrel

Black Medic

Lawn and Turfgrass Weeds: White Clover