The label is the law. Rates and restrictions below are taken from the product labels listed under Sources. Your product's own label governs; read it before you mix or apply. Mixing calculator · Disclaimer
- Active ingredient
- S-metolachlor (CAS 87392-12-9)
- Chemical class
- Chloroacetamide; WSSA Group 15, thought to block very long chain fatty acid synthesis. Group 15 products typically hit weeds before they emerge without stopping seed germination.[2][1]
- Mode of action
- WSSA Group 15: Very long chain fatty acid (VLCFA) synthesis inhibitors (chloroacetamides)[2][1]
- Signal word
- Pennant Magnum Herbicide: Caution[1]
Where Pennant Magnum can't go
The label rules out greenhouses and other enclosed structures, golf greens, tees and aprons, and turf under stress from insects, nematodes or disease. It is not for homeowner use, and turf use is barred in New York State.[1]
Sale and use are prohibited in Nassau and Suffolk Counties, New York. Don't mix or load it within 50 feet of streams, rivers or lakes. Near wells and sinkholes the same 50-foot limit applies unless the work is done on a self-contained impervious pad.[1]
Seeding around a Pennant Magnum application
Don't seed or overseed desirable turfgrass for 4 months before or after treatment. Newly seeded grass should go through a winter and build a well-developed rhizome system before it is treated.[1]
Where it is used
Timing is everything: apply before weeds emerge. If no rain falls within 7 days of treating turf, irrigate with ½ inch of water to activate it.[1]
To cut runoff, avoid applying when rain or irrigation is expected within 48 hours, since that drives runoff. A level, well-kept grass buffer between treated areas and ponds or streams also helps.[1]
Fertilizing with nitrogen at or soon after application reduces the delayed spring green-up and temporary yellowing the label says can follow treatment on turf.[1]
When it is sprayed over the top of ornamental plants, follow with enough overhead irrigation to wash it off the foliage and lower the chance of injury.[1]
Products and registrations
| Product | Registrant | EPA Reg. No. | Active % | Formulation | Signal word |
|---|---|---|---|---|---|
| Pennant Magnum Herbicide[1] | Syngenta Crop Protection, LLC | 100-950 | 83.7% | emulsifiable concentrate (7.62 lb active per gallon) | Caution |
Label rates
| Product | Use | Pests | Rate |
|---|---|---|---|
| Pennant Magnum Herbicide | Warm-season turf: bermudagrass, centipedegrass, St. Augustinegrass, bahiagrass and zoysiagrass, including St. Augustinegrass sod production | Crabgrass, sprangletops, doveweed and annual bluegrass; yellow nutsedge and annual sedge at the top rate | 14–28 ml product per 1,000 sq ft[1] That is 1.3 to 2.6 pints per acre. Use the full 2.6 pt/A (28 ml/1,000 sq ft) for yellow nutsedge. Apply no more than once every 6 weeks, and no more than 2.6 pt/A a year outside sod farms. |
| Pennant Magnum Herbicide | Container and field-grown ornamentals, nursery stock and landscape plantings | Annual grasses, certain broadleaf weeds and yellow nutsedge | 14–28 ml product per 1,000 sq ft[1] 14 to 21 ml on coarse and medium soils, 21 to 28 ml on fine soils, in at least 10 gallons of carrier per acre. Total yearly use is capped at 46 ml per 1,000 sq ft (4.2 pt/A). |
Safety and environmental hazards
People
Signal word CAUTION. The label says it causes moderate eye irritation, is harmful if swallowed or absorbed through skin, and may cause skin sensitization in some people. Handlers wear long sleeves, long pants, chemical-resistant gloves, shoes and socks.[1]
Pets
Outside farm and nursery settings, nobody should enter the treated area until the spray has dried. The label also bars grazing animals on treated turf or feeding them the clippings.[1]
Bees and pollinators
There is no direct bee toxicity statement. The label warns it is toxic to plants and may harm forage and habitat for pollinators next to the treated site, so limiting drift protects them.[1]
Fish and water
Never apply it directly to water. S-metolachlor is known to leach into groundwater in permeable soils, and the label rates it high for runoff into surface water for weeks or months after application.[1]
Common questions
- Does S-metolachlor control nutsedge?
- Pennant Magnum lists yellow nutsedge and annual sedge as controlled in warm-season turf at 2.6 pints per acre, about 28 ml per 1,000 sq ft. It works before emergence or after existing plants are removed.[1]
- Can I use Pennant Magnum on a fescue or bluegrass lawn?
- No. Its turf directions cover only warm-season grasses: bermudagrass, centipedegrass, St. Augustinegrass, bahiagrass and zoysiagrass.[1]
- How soon can I reseed after S-metolachlor?
- The label says to wait 4 months after treatment before seeding or overseeding desirable turfgrass.[1]
Sources
- U.S. EPA Pesticide Product Label System, “Pennant Magnum Herbicide label, EPA Reg. No. 100-950 (EPA-stamped, Jan. 24, 2023).” www3.epa.gov/pesticides/chem_search/ppls/000100-00950-20230124.pdf (accessed 2026-10-06)
- Weed Science Society of America, “Summary of Herbicide Mechanism of Action According to the Weed Science Society of America.” wssa.net/wp-content/uploads/WSSA-Mechanism-of-Action.pdf (accessed 2026-10-06)