Nutsedge, often mistaken for a common grass, is actually a weed in the sedge family, making it notoriously difficult to eradicate from a St. Augustine lawn. Unlike true grasses, nutsedge has a glossy, waxy texture and is easily identified by its rapid growth, often appearing taller than the surrounding turf just a day or two after mowing. Because St. Augustine grass is easily damaged by many broadleaf herbicides, controlling this perennial and aggressive weed requires specific, selective chemical solutions.
Understanding Nutsedge: The Challenge of the Tuber
The persistence of nutsedge lies underground in its survival mechanism: the tuber, often called a “nutlet.” These small, starchy storage organs form on the ends of rhizomes, which are underground stems that allow the plant to spread horizontally. A single plant can produce several hundred new tubers in a single growing season, which remain dormant and viable in the soil for multiple years.
When the visible shoot is pulled or severed, the tubers are left intact and immediately sprout new shoots. This mechanism means that simply hand-pulling the weed often encourages the underground reserve to produce even more plants. Therefore, any control approach must target and exhaust the energy stored within these tubers to prevent rapid re-sprouting and recurrence. Tubers are typically found within the top six to eight inches of the soil.
Non-Chemical Strategies for Immediate Removal
For small, isolated patches, physical removal is a viable option, but it must be performed correctly to eliminate the underground tubers. Instead of pulling the weed, which leaves the nutlet behind, use a small garden trowel to dig up the entire plant cluster. The excavation needs to be deep and wide enough to encompass the full root system, generally at least 8 to 10 inches deep.
Solarization offers a non-chemical method for heavily infested, localized areas where the turf can be sacrificed temporarily. This process involves thoroughly watering the affected soil and then covering the patch with a sheet of clear plastic, sealing the edges to trap heat. Leaving the plastic in place for four to six weeks during the hottest summer months raises the soil temperature high enough to kill the dormant tubers.
Reducing soil moisture is a cultural practice that immediately suppresses nutsedge growth. Nutsedge thrives in wet, poorly drained, or over-irrigated areas, so its presence often signals an underlying drainage issue. Adjusting irrigation to water deeply but less frequently, allowing the top layer of soil to dry out, makes the environment less hospitable. Correcting low spots where water collects will also naturally reduce the weed’s ability to flourish.
Selecting Herbicides Safe for St. Augustine Grass
Because St. Augustine turf is sensitive to many common broadleaf weed killers, specialized sedge-targeting herbicides are necessary for chemical control. The most effective active ingredients safe for St. Augustine grass include Halosulfuron-methyl, Sulfentrazone, and Imazosulfuron. These compounds are selective, post-emergent herbicides that are absorbed by the nutsedge foliage and translocated down into the tubers to stop growth.
Halosulfuron-methyl (e.g., SedgeHammer or Certainty) is highly selective and the safest option for St. Augustine, showing excellent turf tolerance. Imazosulfuron (e.g., Celero) is also safe and provides excellent nutsedge control. Sulfentrazone is effective and fast-acting but requires caution, as it can cause temporary leaf discoloration on turf stressed by heat or drought. Avoid applying these herbicides when temperatures consistently exceed 85 to 90 degrees Fahrenheit to minimize the risk of injury.
Timing the application is crucial, as the herbicide must be applied when the nutsedge is actively growing, typically in late spring or early summer, before it produces new tubers. The weed is most susceptible when it has three to five leaves. Many systemic products work best when mixed with a non-ionic surfactant, which helps the herbicide stick to and penetrate the waxy leaves. Due to the nutsedge’s persistence, a second application is nearly always required six to ten weeks after the initial treatment.
Preventing Recurrence Through Turf Management
The most effective long-term strategy involves cultural practices that favor St. Augustine grass, allowing it to naturally outcompete the weed. Mowing height is a key control factor; St. Augustine should be maintained at a tall height, ideally between 2.5 and 4 inches. This height creates a dense canopy that shades the soil, blocking the sunlight necessary for nutsedge seeds and tubers to germinate. Mowing the lawn too short encourages nutsedge growth and stresses the turf.
Proper fertilization promotes a dense, healthy lawn that naturally suppresses weed encroachment. St. Augustine grass typically requires between two and four pounds of nitrogen per 1,000 square feet annually, applied in multiple applications. Using a slow-release, complete fertilizer ensures consistent nutrient availability, which supports strong, lateral growth without causing excessive top growth that can stress the turf.
Correcting any underlying issues with soil drainage or compaction will remove the preferred growing conditions for nutsedge. Aerating the lawn in compacted areas improves water infiltration and air circulation, reducing the standing moisture that nutsedge prefers. By managing water, mowing height, and fertility to maximize St. Augustine health, the lawn becomes a natural defense against recurrence.