Oxalis, commonly known as wood sorrel or false shamrock, is a pervasive broadleaf weed that can quickly colonize a lawn. Finding a treatment that selectively eradicates this aggressive plant while leaving the surrounding grass unaffected requires specialized selective herbicides. This article focuses on the chemical and cultural strategies that provide effective control against Oxalis while maintaining the health of your lawn.
Understanding Oxalis: Identification and Growth Habits
Oxalis is often mistaken for clover due to its trifoliate, or three-leaf, structure, but its heart-shaped leaflets are a distinct identifying feature. These leaves may vary from light green to a deep purplish-red, and they characteristically fold up at night or during intense heat. The plant produces small, five-petaled flowers, most commonly yellow, which are followed by seed capsules that aggressively aid its spread.
Oxalis is difficult to control due to its reproductive complexity. Certain species, like creeping wood sorrel (Oxalis corniculata), develop a tenacious taproot and creeping stems that root at the nodes, allowing spread across the soil surface. Other varieties produce small underground bulbils or rhizomes that can remain dormant and regrow even if the above-ground foliage is removed. Furthermore, mature seed pods can explode upon contact, launching dozens of tiny seeds up to 10 feet away, ensuring rapid re-infestation.
Selective Chemical Control Methods
The most effective tools for killing Oxalis without damaging grass are selective post-emergent herbicides, which are formulated to target broadleaf weeds but not narrow-leaf grasses. These products are usually combinations of synthetic auxins, a class of plant hormones that cause uncontrolled, distorted growth in susceptible broadleaf plants, leading to their demise.
“Three-way” broadleaf herbicides containing active ingredients such as 2,4-D, MCPP (Mecoprop), and Dicamba are commonly used. This combination offers a broad spectrum of control and is generally safe for established cool-season grasses and many warm-season varieties. Dicamba and 2,4-D are particularly effective at being absorbed by the foliage and translocating throughout the plant’s system, reaching the difficult-to-kill roots and rhizomes.
For established or harder-to-control Oxalis infestations, herbicides containing Triclopyr often provide superior results. Triclopyr is highly effective against many tough, woody broadleaf weeds and can penetrate the waxy leaf surfaces of mature Oxalis plants. Homeowners must confirm the product’s safety for their specific turf type, as Triclopyr is typically safe for cool-season turf like fescue or bluegrass but can be harmful to certain warm-season grasses like bermudagrass or St. Augustine grass. Newer professional formulations may also include ingredients like Clopyralid and Diflufenican, which are effective against creeping Oxalis with fewer required applications.
Non-Chemical and Cultural Control Methods
While chemicals offer a rapid solution, cultural practices play a significant role in preventing Oxalis from becoming established in the first place. Maintaining a dense, healthy lawn is the primary defense, as thick turf naturally outcompetes weeds for sunlight, water, and nutrients. This density is achieved through proper fertilization, aeration, and overseeding.
Manual removal is possible for small, isolated patches, but it requires extreme care to ensure the weed does not regrow. It is necessary to remove the entire root structure, including any brittle taproots, rhizomes, or small bulbils, as leaving even a small fragment can lead to immediate re-sprouting. Mowing the lawn at a higher setting (usually between 3 to 4 inches) encourages deeper grass roots and allows the turf blades to shade the soil. Shading the soil makes it less hospitable for Oxalis seeds to germinate and thrive.
Timing and Application Strategy for Lawn Safety
The success of a selective herbicide application relies heavily on proper timing to maximize absorption by the Oxalis and minimize stress on the turf. Post-emergent herbicides should be applied when the Oxalis is actively growing, typically from late spring through early summer, but before the plant begins to produce mature seeds. Treating young, vigorously growing weeds ensures they absorb the chemical more readily, resulting in a more complete kill that reaches the entire root system.
The application should be made when temperatures are moderate, ideally below 90°F. Applying herbicides during high heat can cause temporary or permanent damage to the turfgrass. It is helpful to water the lawn lightly the day before treatment to reduce drought stress and promote active weed growth, enhancing the uptake of the chemical.
After application, avoid mowing the lawn for at least two days to allow the herbicide sufficient time to be absorbed and translocated throughout the weed. Spot-treating individual weeds or small clusters is the safest method to protect the surrounding grass, though broadcast application may be necessary for widespread infestations. Always follow the specific product label for mixing ratios and application rates.