Oxalis, commonly known as Wood Sorrel, is a genus of plants that often challenges gardeners and homeowners. While some species are cultivated as ornamentals, the varieties that appear uninvited in lawns and garden beds are widely considered aggressive weeds. These species can quickly overwhelm an area, leading many to question their invasive status. Managing this plant is difficult due to its complex, dual methods of propagation, which necessitates a dedicated approach to removal and control.
Recognizing Common Oxalis Varieties
Accurate identification is the first step in addressing an Oxalis infestation, as appearance varies by species. The most common varieties share a distinctive feature: trifoliate leaves composed of three heart-shaped leaflets. These leaflets often fold along their central rib, especially at night or during intense heat, a behavior known as nyctinasty.
Oxalis stricta, or yellow woodsorrel, is typically upright with green leaves and small, five-petalled yellow flowers. Conversely, Oxalis corniculata, or creeping woodsorrel, tends to grow low to the ground and often displays green to purplish-red leaves. Oxalis pes-caprae, known as Bermuda Buttercup, produces large, lemon-yellow flowers on tall, thin stems.
Oxalis is often confused with true clover (Trifolium), but their leaf shapes differ clearly. Clover leaves are round or oval, whereas Oxalis leaves are distinctly heart-shaped. Tasting a small piece of a stem reveals a mild, sour flavor due to oxalic acid, which gave the plant its common name, Wood Sorrel.
The Aggressive Spread of Wood Sorrel
Oxalis is classified as an aggressive weed because of the effective biological mechanisms it employs to spread. One frustrating mechanism is the development of underground storage structures, including rhizomes and tiny bulbils. Species like Bermuda Buttercup (O. pes-caprae) develop numerous small bulbs that readily detach when the soil is disturbed.
These structures allow the plant to survive foliage removal, ensuring rapid regrowth. Even a small piece of a missed bulbil or rhizome can generate a new plant. This resilience is compounded by the plant’s unique, ballistic seed dispersal method.
The seed capsules, which look like small cylindrical pods, contain minute seeds. When mature, these capsules burst open explosively upon disturbance, flinging seeds up to 13 feet away. This rapid dispersal allows Oxalis to quickly colonize new areas, establishing a persistent seed bank in the soil that can continue to germinate for years.
Strategies for Effective Removal and Control
Controlling Oxalis requires a multi-pronged strategy addressing both the tenacious underground structures and aggressive seed dispersal.
Manual and Cultural Control
For small infestations, manual removal must be meticulously executed. When hand-pulling, it is imperative to remove the entire root system, including all rhizomes or bulbils, which is best done when the soil is moist. Disturbing the soil as little as possible is important, as excessive cultivation can trigger the germination of dormant seeds.
Cultural control focuses on prevention. Applying a thick layer of mulch, ideally two to three inches deep, is effective because Oxalis seeds require light for germination. Maintaining a healthy, dense lawn or garden bed also helps, as thick turf and closely planted ornamentals crowd out the low-growing Oxalis.
Chemical Control
For larger, widespread infestations, chemical control may be necessary, often requiring post-emergent herbicides designed for broadleaf weeds. Herbicides containing active ingredients such as triclopyr or a combination of 2,4-D and dicamba are recommended for selective control in turfgrass. These systemic herbicides are absorbed by the leaves and transported down to the root system.
Timing the application is crucial, with the best results occurring before the Oxalis begins to flower and produce seeds. Because of the persistent bulbils and the long-lived seed bank, a single application is rarely sufficient. Repeat treatments, often spaced two to four weeks apart, are required to control new plants emerging from missed bulbils or germinating seeds. Using a pre-emergent herbicide in the spring can also help manage the seed bank by preventing new seeds from sprouting.