Dallisgrass (Paspalum dilatatum) is a highly invasive and problematic perennial weed in turfgrass and agricultural settings. This warm-season perennial grass is notorious for its aggressive growth and resistance to common management techniques, making it one of the most difficult weeds to control in lawns. Its ability to thrive in a wide range of conditions, including highly irrigated sites and compacted soils, contributes significantly to its reputation as a troublesome pest.
Identifying Dallisgrass in the Landscape
Dallisgrass is easily recognized by its distinctive growth habit, forming coarse, circular clumps that stand out from desirable turfgrass. The leaf blades are noticeably broader than many common lawn grasses, often measuring between a quarter and a half-inch wide. They possess a coarse, grayish-green texture with a distinct mid-vein and tend to grow upright and taller than the surrounding lawn.
The most telling feature is the production of unique seed heads, which emerge on tall, slender stems from late spring through fall. Each seed head, known as a raceme, consists of three to five or more finger-like spikes branching loosely from the central stalk. These spikes contain numerous oval-shaped seeds that often have distinctive black or purplish spots.
Beneath the surface, the plant anchors itself with a deeply rooted, fibrous crown, supplemented by a network of short, thick rhizomes, or underground stems. These rhizomes are often brown and have distinctive rings, allowing the plant to spread slowly outward. This dense root structure contributes to the plant’s ability to withstand environmental stress.
Biological Factors Contributing to Persistence
The primary reason Dallisgrass is so challenging to eradicate stems from its perennial life cycle. The established plant does not die off during the winter months; instead, it goes dormant in cold weather while the above-ground foliage dies back. The underground crown and rhizomes remain alive, allowing the plant to regrow vigorously from the same spot year after year, often becoming larger and more entrenched.
Its robust root system and short, thick rhizomes are highly resilient, acting as underground storage organs that resist drought and physical disturbance. This extensive root structure makes manual removal difficult, as even small fragments of the rhizome left in the soil can regenerate into a new plant. The fibrous crown is also tolerant of close mowing.
Dallisgrass relies on prolific seed production as a mechanism for spreading to new areas. A single plant produces a large number of seeds, which are easily dispersed by wind, water runoff, and lawn mowers. These seeds can remain viable in the soil for extended periods, leading to continuous cycles of reinfestation.
Effective Control and Eradication Methods
Controlling Dallisgrass requires a combination of methods tailored to its perennial nature and robust root system.
Mechanical Removal
For small or newly emerged clumps, mechanical removal is a viable, non-chemical option, but it must be executed with care. The entire plant, including the deep fibrous roots and all rhizomes, must be dug out completely using a sharp spade or specialized tool. This process is most effective when the soil is moist, which helps ensure the entire root mass is extracted without leaving behind regenerative fragments.
Chemical Control
Chemical control is necessary for widespread or established infestations, but the selection of herbicides must be precise. Pre-emergent herbicides prevent seeds from germinating and should be applied in early spring when soil temperatures reach 60 to 65 degrees Fahrenheit to prevent new seedlings from establishing. Since Dallisgrass is a perennial, pre-emergents are ineffective against existing plants.
Existing clumps require the use of post-emergent selective herbicides, which target the weed without harming the surrounding turfgrass. Active ingredients such as Quinclorac, Mesotrione, or professional products like Monument 75WG or Fusilade II are commonly recommended. Multiple applications, typically four to six weeks apart, are usually necessary for complete eradication due to the plant’s ability to recover.
Application timing is important, as the plant is most sensitive during its active growth period, typically in the spring and early summer or again in the late summer and early fall. Non-selective herbicides, like glyphosate, can be used for spot treatments on isolated clumps, but this will kill the surrounding desirable turfgrass. If a non-selective product is used, a barrier should be placed around the clump to protect the lawn from accidental spray drift.
Cultural Practices
Cultural practices are a long-term preventative measure to suppress the establishment of Dallisgrass seedlings. Maintaining a dense, healthy lawn is the best defense, as thick turf competes aggressively with the weed for sunlight, water, and nutrients. Proper lawn maintenance includes mowing at the recommended height for the specific turf species and ensuring correct irrigation. This prevents overly wet or compacted soil conditions, which Dallisgrass prefers.