Step-by-Step Guide to Killing Devil's Grass in Cool-Season Lawns

Colloquially dubbed "devil's grass" by frustrated homeowners and turf professionals, Cynodon dactylon (Bermudagrass) is a premium warm-season turf in the South but an aggressive, nearly indestructible weed when it invades Northern cool-season lawns. Its dual reproductive system—above-ground stolons and below-ground rhizomes—allows it to choke out Tall Fescue and Kentucky Bluegrass while resisting standard broadleaf herbicides. Eradicating this invasive grass requires a precise, multi-phase maintenance protocol rather than a single spray-and-pray application.

Identifying the Invader: Is It Really Devil's Grass?

Before purchasing herbicides, confirm the weed's identity. Misidentifying Quackgrass or Crabgrass as devil's grass will lead to chemical failure. According to the USDA Plants Database, Cynodon dactylon exhibits distinct morphological traits:

• Stolons and Rhizomes: It spreads via tough, wiry above-ground runners (stolons) and deep, scaly underground stems (rhizomes) that can penetrate 6 to 12 inches into compacted soil.
• Ligule: The defining identifier is a short, membranous ligule at the leaf collar featuring a distinct fringe of white hairs.
• Seedheads: In late summer, it produces 3 to 7 slender, finger-like spikes at the top of the stem, resembling a bird's foot.

The Eradication Protocol: Step-by-Step Maintenance

Removing devil's grass from a desirable cool-season lawn requires selective chemistry or targeted non-selective spot treatments. Timing is the most critical variable in this procedure.

Step 1: Time the Application for Maximum Translocation

Do not spray in early summer. The optimal treatment window is late summer to early fall (August 15 through September 30). During this period, the grass is actively pulling carbohydrates—and systemic herbicides—down into its deep rhizomes to prepare for winter dormancy. Spraying in spring only burns the top growth, guaranteeing a massive resurgence by July.

Step 2: Select and Apply the Correct Herbicide

Standard 2,4-D and Dicamba weed killers will not harm devil's grass because it is a monocot (grass), not a broadleaf weed. You must use graminicides (grass-specific herbicides) or non-selective options.

Herbicide Comparison Matrix for Cool-Season Lawns

Note: Always add a non-ionic surfactant (NIS) at a rate of 1 teaspoon per gallon of mixed Fluazifop to break the waxy cuticle of the devil's grass blades. Data sourced from NC State University TurfFiles.

Step 3: The 21-Day Follow-Up

Fluazifop and Fenoxaprop are slow-acting. The devil's grass will turn reddish-purple and stop growing within 7 days, but complete necrosis takes 14 to 21 days. Do not mow the treated area for 48 hours before and 48 hours after application to maximize leaf surface absorption. Apply a second follow-up dose exactly 21 days after the first application to kill any surviving rhizome nodes.

The Organic Alternative: Soil Solarization

If the infestation is localized to a garden bed edge or a small, heavily compacted patch of lawn where chemical use is restricted, solarization is a highly effective, non-chemical eradication procedure.

1. Scalp the Area: Mow the devil's grass down to 0.5 inches and remove all clippings.
2. Hydrate the Soil: Water the area deeply to a depth of 12 inches. Moist soil conducts heat significantly better than dry soil, accelerating the cooking of the rhizomes.
3. Apply Clear Polyethylene: Cover the patch with 2-mil to 4-mil clear plastic sheeting (not black plastic). Clear plastic traps solar radiation and creates a greenhouse effect, raising soil temperatures to 120°F–140°F in the top 6 inches.
4. Seal the Edges: Bury the perimeter of the plastic 6 inches deep in a trench to prevent heat escape and block emerging stolons.
5. Wait 4 to 6 Weeks: Leave the plastic in place during the peak heat of July and August. This will cook the rhizomes and sterilize the topsoil of weed seeds.

Post-Treatment Lawn Recovery Procedures

Once the devil's grass has been eradicated, the resulting bare patches must be rehabilitated immediately to prevent secondary weed invasions like Crabgrass or Spurge.

• Core Aeration: Run a dual-spike core aerator over the bare patches to relieve the compaction caused by the dense Bermudagrass rhizome mat.
• Overseeding: Apply a premium Turf-Type Tall Fescue (TTTF) blend at a rate of 8 to 10 lbs per 1,000 square feet. TTTF's deep root system and aggressive bunch-type growth will outcompete future invaders.
• Starter Fertilizer: Apply a high-phosphorus starter fertilizer (e.g., 18-24-12) at a rate of 1 lb of nitrogen per 1,000 sq ft to accelerate seedling root development.

Preventative Maintenance to Stop Regrowth

Devil's grass seeds can remain viable in the soil profile for up to two years. A strict preventative maintenance schedule is required to protect your newly recovered lawn.

Pre-Emergent Herbicide Barrier

Apply Prodiamine 65 WDG in early spring when soil temperatures consistently reach 55°F at a 2-inch depth. The exact application rate is 0.18 oz per 1,000 square feet, watered in with 0.5 inches of irrigation. This creates a chemical barrier in the top inch of soil that halts cell division in germinating weed seeds. A 5-ounce bottle costs approximately $45 and treats up to 27,000 square feet, making it highly cost-effective.

Cultural Mowing Practices

Devil's grass thrives in full, unfiltered sunlight. By raising your mower deck to cut your Tall Fescue or Bluegrass at 3.5 to 4.0 inches, you create a dense turf canopy that shades the soil surface. This deprives germinating devil's grass seeds of the photosynthetic light they need to establish, reducing reliance on chemical controls. Always follow the EPA's Safe Pest Control guidelines when integrating chemical and cultural lawn care methods to protect local watersheds.

Frequently Asked Questions

Can I just pull devil's grass out by hand?

No. Hand-pulling is counterproductive. Cynodon dactylon rhizomes snap easily when pulled. Every 1-inch fragment of rhizome left in the soil contains a viable node that will sprout a new plant within 10 days. Hand-pulling actually multiplies the infestation by planting the fragments deeper into the soil profile.

Will applying more fertilizer kill the devil's grass?

Over-fertilizing will not kill it. In fact, applying high-nitrogen fertilizers (like 30-0-0) in the heat of summer will only stimulate the aggressive stolon growth of the devil's grass, allowing it to overtake your cool-season turf faster. Restrict heavy nitrogen applications to the fall months when cool-season grasses are actively growing, and the Bermudagrass is preparing for dormancy.