Weed Identification + Control
Why this matters
Weed control is the most-visible deliverable in residential lawn care. The lawn looks bad = the customer cancels. Good weed control requires correct ID - pre-emergent vs post-emergent, broadleaf vs grassy, annual vs perennial all map to different chemistry. Wrong herbicide at the wrong time costs money + kills the desired turf.
The four weed-control regimes
- Pre-emergent broadleaf + grassy - applied to soil BEFORE seed germinates; prevents emergence
- Post-emergent selective broadleaf - kills broadleaves growing in turf; turf unharmed
- Post-emergent selective grassy - kills certain weed grasses growing in turf grass; species-specific
- Non-selective post-emergent - kills everything (glyphosate, glufosinate); for landscape beds, hardscape, total renovation
Pre-emergent timing (the highest-leverage application of the year)
Soil temperature drives germination - not calendar date.
- Spring pre-emergent: apply when 4-inch soil temp reaches 50 - 55°F for 3 consecutive days. Targets crabgrass, foxtail, goosegrass, summer annuals.
- Northern climates (USDA 5 - 6): typically late March - mid April
- Mid-Atlantic (USDA 7): typically mid March - early April
- Southern (USDA 8 - 9): typically late February - mid March
- Fall pre-emergent: apply when soil temp drops below 70°F. Targets annual bluegrass (Poa annua), henbit, chickweed.
- Common products: prodiamine (Barricade), dithiopyr (Dimension - has light post-emergent on young crabgrass), pendimethalin (Pendulum), isoxaben (Gallery - broadleaf).
- Watering-in: 0.5 inches within 7 days activates most pre-emergents.
Post-emergent broadleaf weed ID
Dandelion (Taraxacum officinale) - yellow flower, rosette, deep taproot. Easy ID. Highly responsive to 2,4-D / dicamba / MCPP three-way mix.
White clover (Trifolium repens) - three-leaflet, white flower head, low spread. Note: clover was historically intentional in lawn seed; some customers want it. Confirm scope before treating.
Black medic (Medicago lupulina) - looks like clover but with yellow flowers + black seed pods. Treated like clover.
Plantain (Plantago major + lanceolata) - broad oval (major) or narrow lance (lanceolata) leaves, seed spike. Responsive to three-way + triclopyr.
Ground ivy / Creeping Charlie (Glechoma hederacea) - scalloped round leaves, mint-family square stem, purple flower. Aggressive; triclopyr is the workhorse.
Wild violet (Viola spp.) - heart-shaped leaves, purple flower, waxy cuticle. Hardest broadleaf to control. Multiple applications with triclopyr + surfactant.
Spurge (Euphorbia spp.) - low spreading mat, milky sap when broken. Best controlled pre-emergent; post-emergent with 2,4-D + dicamba.
Oxalis / woodsorrel (Oxalis stricta) - clover-look-alike with yellow flowers + heart-shaped leaflets. Difficult. Repeated three-way + triclopyr applications.
Grassy weed ID
Crabgrass (Digitaria sanguinalis + ischaemum) - light green, flat blades, finger-like seed head. Summer annual. Pre-emergent is the answer; post-emergent (quinclorac, fenoxaprop) for escapes - costlier + window is narrow.
Goosegrass (Eleusine indica) - flat white center, ID at silver-base. Late-summer germination; needs late pre-emergent timing.
Nutsedge (Cyperus esculentus / rotundus) - triangular stem, glossy yellow-green color, faster growth than surrounding turf so it pokes up above the cut. Sedgehammer (halosulfuron), sulfentrazone, or Dismiss.
Annual bluegrass / Poa annua - light green clumps, prolific white seed heads in spring. Fall pre-emergent is the prevention; in-turf control is hard (specialty products: prodiamine + sulfonylurea).
Bermudagrass in cool-season turf - looks like tan-yellow stolons in fescue. No good selective. Spot-treat with glyphosate + reseed, OR live with it.
Selective post-emergent chemistry
- 2,4-D + MCPP + dicamba (three-way) - standard broadleaf workhorse. Cheap, broad, effective. Brand: Trimec.
- Triclopyr - adds wild-violet, creeping-Charlie, oxalis control. Combine with three-way for "four-way."
- Sulfentrazone - broadleaf + sedge + spurge. Brand: Dismiss.
- Quinclorac - crabgrass + foxtail + some broadleaves. Brand: Drive XLR8.
- Halosulfuron-methyl - yellow + purple nutsedge. Brand: Sedgehammer.
- Fenoxaprop-p-ethyl - crabgrass + foxtail in cool-season turf. Brand: Acclaim.
NEVER apply broadleaf herbicide when temperatures exceed 85°F. The volatility of dicamba + 2,4-D causes vapor drift onto neighboring landscape plants (tomatoes, redbuds, hostas all extremely sensitive). Customer + neighbor lawsuits over off-target broadleaf damage are real. Read the label temperature cap (typically 85°F) + respect it. Early morning + cool conditions are the safe windows.
Application notes
- Always include surfactant for post-emergent broadleaf (waxy-cuticle weeds need it for uptake)
- Spot-spray when possible; broadcast only when weed density exceeds 30%
- Document application: product, rate, weather, target species
- Re-treat per label intervals (typically 14 - 28 days for stubborn species)
- Maintain Pesticide Applicator License (state-required for commercial application)
Cultural prevention
- Thick turf is the best herbicide. Mow at upper end of species range; overseed thin spots in fall; fertilize properly.
- Soil pH correction (lime / sulfur per soil test) lets desired turf outcompete weeds.
- Mulched landscape beds with pre-emergent block annual broadleaf re-invasion.
References
- WSSA (Weed Science Society of America) databases
- State extension weed ID guides
- Herbicide label is the law - read + follow exactly
- Manuall internal: Common Lawn Weeds Identification Reference, Organic Lawn Care Program Reference