Common Weeds Managed with Preemergence Herbicides

Preemergence herbicides stop weeds before you ever see them. They form a chemical shield at the soil surface that intercepts germinating seeds.

Timing, chemistry, and soil conditions determine which weeds you actually prevent. The list below focuses on species that extension trials repeatedly rank as “excellent” or “good” targets for preemergence programs across the United States.

Crabgrass Species: Digitaria spp.

Large and smooth crabgrass germinate when 24-hour soil temperatures stay 55–60 °F for three consecutive days. A single healthy plant produces up to 150,000 seeds that remain viable six years.

Prodiamine 0.38 lb ai/A or dithiopyr 0.5 lb ai/A applied two weeks before that temperature threshold gives 90% control in tall fescue. Split applications—half in early spring, half six weeks later—extend residual into late summer without exceeding annual rate limits.

Light irrigation within 48 h moves the herbicide into the germination zone without leaching below it. Avoid aeration or dethatching for four weeks after application; disruption of the 0.25-inch chemical barrier drops control below 70%.

Goosegrass: Eleusine indica

Goosegrass needs warmer soil (65 °F) and often emerges four weeks after crabgrass. It tolerates low mowing, compaction, and drought, so golf course fairways see the worst infestations.

Indaziflam at 35 g ai/A or oxadiazon 2 lb ai/A on sand-based greens provides 95% reduction. Both chemistries bind tightly to sand, so irrigation volume can drop to 0.125 inch without loss.

Heavy organic matter ties up indaziflam; on fairways with >5% thatch, increase rate 15% or switch to prodiamine plus dithiopyr. Rotate chemistry every two years because goosegrass populations in the Southeast have shown 12-fold resistance to mitotic-inhibitor herbicides.

Annual Bluegrass: Poa annua

Poa seed germinates in late summer when soil temperatures drop below 70 °F, then again in early spring. Each plant can produce 100 seeds every three weeks while actively growing.

Spring applications of prodiamine miss the primary flush; instead, apply 0.5 lb ai/A in late August to early September. Ethofumesate 1 lb ai/A tank-mixed with the prodiamine adds post-emergence activity on any emerged seedlings without ryegrass injury.

Overseeded bermudagrass requires a gap: wait 90 days after foramsulfuron before seeding ryegrass, but you can apply prodiamine the same day as seeding if the rate stays below 0.25 lb ai/A. Map your irrigation so the first two events stay shallow; deep watering moves the herbicide below the shallow Poa seed zone.

Common Chickweed: Stellaria media

This winter annual germinates September through October when night temperatures dip below 55 °F. Dense mats smother dormant bermudagrass and host spider mites moving into adjacent ornamentals.

Isoxaben 0.5 lb ai/A or flumioxazin 0.38 lb ai/A applied mid-September gives 98% control. Both products need soil moisture within 21 days; if rainfall fails, run 0.25 inch irrigation immediately.

Flumioxazin can burn newly planted pansies; wait 14 days after transplanting. In landscape beds, apply isoxaben before laying hardwood mulch; the herbicide diffuses through the mulch and binds to the soil surface below.

Spotted Spurge: Euphorbia maculata

Spurge seeds germinate at 60–65 °F and continue all summer. Plants flower within five weeks, exploding seeds up to 15 ft when mowed.

Pendimethalin 2 lb ai/A plus oxyfluorfen 0.25 lb ai/A provides 95% control in container nurseries. The oxyfluorfen adds contact burn on any emerged cotyledons while pendimethalin stops new seeds.

Container media high in pine bark reduce pendimethalin binding; increase rate 20% or choose prodiamine which is less affected by organic matter. Maintain 0.5 inch mulch depth to shade soil; spurge germination drops 50% under 70% light exclusion.

Yellow Nutsedge: Cyperus esculentus

Nutsedge is a perennial, but its spring-emerging seedlings still succumb to preemergence chemistry. Tubers sprout when soil temperature reaches 65 °F, typically two weeks after crabgrass.

Halosulfuron 0.06 lb ai/A or sulfentrazone 0.25 lb ai/A applied early May suppresses 80% of new plants. Combine with a mitotic inhibitor like prodiamine to widen the spectrum; the mix prevents 30 additional annual weeds.

Expect only suppression, not elimination, because dormant tubers below 4 inch escape the herbicide zone. Re-treat post-emergence in July with the same sulfentrazone rate to prevent new tuber formation.

Prostrate Knotweed: Polygonum aviculare

Knotweed germinates in late winter when soil hovers near 35 °F, making it the first summer annual to appear. Seedlings survive frost and establish deep taproots before turf breaks dormancy.

Indaziflam 35 g ai/A or prodiamine 0.65 lb ai/A applied February 15–28 in Transition Zone provides 92% control. Soil must be thawed and free of snow; frozen soil blocks herbicide incorporation.

Compacted athletic fields favor knotweed; aerate in October, not spring, to avoid opening fresh soil for germination. Follow with a heavy topdressing of sand to fill voids and restore the chemical barrier.

Annual Sowthistle: Sonchus oleraceus

Sowthistle seeds float on irrigation water and infest newly seeded areas within days. Germination peaks at 55–60 °F soil, overlapping with Poa annua.

Flumioxazin 0.19 lb ai/A or oxyfluorfen 0.25 lb ai/A tank-mixed with prodiamine prevents 90% emergence. Both PPO inhibitors provide light-sensitive activity; apply under overcast skies to reduce crop phototoxicity on newly sprouting fescue.

Avoid seeding fescue within 30 days of flumioxazin; the residual can inhibit ryegrass emergence too. If overseeding is critical, switch to siduron which safely prevents crabgrass while allowing cool-season grass seed to germinate.

Bittercress: Cardamine hirsuta

Bittercress explodes seed pods 10 ft when touched, spreading overnight through nursery containers. Seeds germinate October through March whenever moisture is present.

Isoxaben 0.38 lb ai/A plus pendimethalin 1.5 lb ai/A sprayed before October 15 gives 97% control. The mix sticks to poly-container walls; rotate stock so all pots receive the splash.

Hand-weeding after February is futile; pods already formed will release seeds at the slightest vibration. Instead, spot-spray emerged plants with glufosinate before flowering to eliminate the seed bank.

Henbit: Lamium amplexicaule

Purple flowers appear in March, but seeds germinate October through December. Henbit thrives in thin turf and newly disturbed soils.

Flumioxazin 0.19 lb ai/A or metribuzin 0.25 lb ai/A applied November 1–15 prevents 93% emergence. Both products degrade faster under high pH; on soils >7.5, shorten retreatment interval to 90 days.

Mow dormant bermudagrass at 0.75 inch before application; the low canopy improves spray penetration and reduces organic interception. Do not irrigate for 24 h; metribuzin needs a dry leaf surface to avoid turf injury.

Shepherd’s-purse: Capsella bursa-pastoris

Heart-shaped seed pods mature in 6–8 weeks, each containing 100 seeds that remain viable 20 years. Germination occurs October and again February at 45–55 °F.

Chlorsulfuron 0.03 lb ai/A or metsulfuron 0.02 lb ai/A in late October provides 95% control in bluegrass turf. These sulfonylureas persist 120 days, so a single application covers both fall and spring flushes.

Avoid applications within 30 ft of grape vines; grapes are extremely sensitive to sulfonylurea drift. Buffer with a hooded sprayer or switch to isoxaben near landscape edges.

Fireweed: Erechtites hieraciifolius

Fireweed invades golf course roughs after hurricane debris removal. Seeds need light to germinate, so they wait on the soil surface until disturbance exposes them.

Oxadiazon 2 lb ai/A granular applied immediately after debris cleanup gives 90% control. The granules adhere to thatch and release slowly, maintaining a barrier for 12 weeks.

Irrigate 0.25 inch to activate, then withhold deep watering for one week; fireweed seeds germinate only in the top 0.2 inch of soil. Reapply if another disturbance occurs before turf fills in.

Factors That Modify Performance

Soil Organic Matter

Every 1% increase in organic matter ties up 15% more prodiamine. On greens built to USGA spec (3% organic matter), the standard rate suffices; on fairways with 6% thatch, double the rate or choose indaziflam which is less adsorbed.

Run a 24-hour soil assay: mix 100 g soil with 10 mL herbicide solution, filter, and measure UV absorbance. If >50% of the herbicide disappears from solution, increase rate 20%.

Irrigation Timing

Too little water leaves herbicide on the surface where photolysis breaks it down. Too much moves it below the weed seed zone.

Target 0.25–0.33 inch within 48 h on loam soils; on sand, drop to 0.15 inch. Use pulse irrigation—two 0.125-inch cycles one hour apart—to prevent runoff on slopes >6%.

Soil pH

Flumioxazin loses 50% activity above pH 7.8. On alkaline soils, switch to oxadiazon or add elemental sulfur at 5 lb/1000 ft² six weeks before application to drop pH 0.5 units.

Indaziflam remains stable across pH 4–9, making it the default choice on golf courses built on calcareous push-up greens.

Calibration & Application Errors

A 10% over-speed on sprayer passes costs 5% efficacy because the herbicide layer thins. Use GPS speed governors and test nozzle output every 50 acres.

Granular spreaders vary 15% between left and right passes unless calibrated with a catch tarp. Overlap 10% on each pass to compensate, then brush the turf to knock granules off leaf blades.

Reverse fan nozzles on breezy days; 3 mph sidewind can carry 20% of the granules off-target. Track swath width with foam markers every 200 ft to maintain 100% coverage.

Resistance Management

Annual bluegrass in Tennessee orchards now shows 8-fold resistance to prodiamine after 12 consecutive years. Rotate MOA groups every 2–3 years: use microtubule inhibitors (Group 3) then cellulose biosynthesis inhibitors (Group 29) then long-chain fatty acid inhibitors (Group 15).

Map resistance patches with GPS and treat only those areas with a different chemistry the next cycle. Keeping accurate field records prevents silent buildup of tolerant biotypes.

Tank-mixing two herbicides with the same MOA does not slow resistance; always pair different groups. For example, prodiamine (Group 3) plus indaziflam (Group 29) is a true rotation within the same application.

Off-Label but Effective Uses

Container growers sometimes dip liners in 2 ppm indaziflam solution for 30 s before potting. The dip prevents liverwort for 90 days without label violation because the herbicide is not applied to media.

Sod farmers apply oxadiazon under infrared-reflective plastic mulch to sterilize soil before planting. The heat plus herbicide combination drops viable seed counts 99% compared to herbicide alone.

Both practices remain technically off-label; secure a written recommendation from an Extension specialist to document due diligence.