Dollar weed (Hydrocotyle umbellata), also known as pennywort, is a common and persistent broadleaf weed that can rapidly take over warm-season turfgrasses, including St. Augustine grass. Its ability to spread quickly via both seeds and an underground network of rhizomes makes it a challenging lawn pest to manage. The presence of this weed often signals a deeper issue with the lawn’s environment, particularly concerning moisture levels. Effective control requires a layered strategy combining immediate removal, targeted chemical treatment, and long-term changes to cultural practices.
How to Identify Dollar Weed and Why It Thrives
Dollar weed is easily recognizable by its bright green, shiny, coin-shaped leaves, which are responsible for its common name. The leaves are typically one to two inches in diameter and have slightly scalloped edges, resembling miniature lily pads. A distinguishing feature is the attachment point of the stem, or petiole, which connects directly to the center of the leaf blade, giving it an umbrella-like appearance. This central stem attachment differentiates it from similar-looking weeds like dichondra.
The weed is a warm-season perennial that thrives in moist, humid conditions. Dollar weed flourishes in areas with excessive water, poor drainage, or compacted soil where water tends to pool. The extensive network of underground stems allows the plant to share water and nutrients, enabling it to survive even in slightly drier spots once established.
Dollar weed easily outcompetes St. Augustine grass when the turf is weakened by improper maintenance. Low mowing heights and overwatering are two common practices that stress the grass while simultaneously creating an ideal, wet environment for the weed to germinate and spread. Correctly identifying the weed and understanding its preference for saturated soil conditions is the first step toward successful management.
Immediate Non-Chemical Suppression Techniques
For small or newly established patches, non-chemical suppression offers a targeted approach without introducing herbicides. Manual removal is a viable option for spot control, but it requires careful technique due to the weed’s ability to regrow from remaining underground parts. When hand-pulling, remove the entire plant, including the extensive network of roots and rhizomes beneath the surface. Pulling when the soil is moist makes complete root system removal easier and more effective.
A larger-scale suppression technique involves manipulating soil moisture, as dollar weed is highly moisture-dependent. Reducing the frequency of irrigation quickly stresses the existing population. Temporarily withholding water allows the soil surface to dry out, which the dollar weed cannot tolerate as well as established St. Augustine turf. This immediate reduction in water helps to deprive the weed of the conditions it needs to flourish and spread further.
Other non-chemical spot treatments include applying boiling water directly to the weed, which instantly kills the foliage and part of the root system. Horticultural vinegar solution can also be used as a contact killer for individual plants, though care must be taken to avoid contact with desirable grass.
Selecting and Applying Herbicides Safe for St Augustine Grass
When dollar weed infestations are widespread, targeted herbicide application is often necessary to achieve control. St. Augustine grass is sensitive to many common broadleaf weed killers, so selecting the correct active ingredient is important to avoid damaging the lawn. The primary recommended active ingredient for dollar weed control in St. Augustine lawns is Atrazine.
Atrazine-based products are effective post-emergent herbicides, killing the weed after it has begun growing. They also provide some pre-emergent control by inhibiting new seeds from germinating. These herbicides are selective, designed to target the dollar weed without causing significant harm to the St. Augustine turf when applied according to label instructions. Another selective option is a combination of active ingredients found in products like Celsius WG, which controls a wide range of weeds safely on St. Augustine.
Application timing is a deciding factor, with best results achieved when the dollar weed is young and actively growing, typically in the spring. Post-emergent herbicides should be applied as a spot treatment or a broadcast application, depending on infestation severity. Follow manufacturer directions regarding temperature limits, as applying Atrazine when temperatures are above 85°F can injure St. Augustine grass. Always ensure the product label specifically lists St. Augustine grass to prevent inadvertent damage to the lawn.
Long-Term Cultural Practices for Prevention
The most effective long-term solution is creating a healthy, dense St. Augustine lawn environment that discourages weed establishment. Adjusting mowing height is a significant factor in turf health and weed suppression. St. Augustine grass should be maintained at a height of 3 to 4 inches. This height encourages a deeper root system and shades the soil surface, which naturally inhibits the germination and growth of dollar weed seedlings.
Proper irrigation practices are equally influential, as dollar weed thrives on constant moisture. Lawns should be watered deeply but infrequently, applying about one inch of water per week. Allow the top layer of soil to dry out completely between watering sessions. This approach forces grass roots to grow deeper, making the turf more drought-tolerant and creating surface conditions unfavorable for the shallow-rooted dollar weed.
Addressing underlying issues with soil health and drainage is necessary for sustained prevention. Compacted soil can be relieved through aeration, which improves water infiltration and reduces standing water. An appropriate fertilization schedule supports dense turf growth, helping the St. Augustine grass outcompete the dollar weed. A soil test can help determine if high phosphorus levels, which can also favor weeds, need adjustment, promoting a balanced nutrient profile for a robust lawn.