Post-Emergent Herbicides for Lawns: Selective vs Non-Selective, Explained Simply

You’re standing in the garden-center aisle holding two jugs. One says “weed killer for lawns”, the other says “kills everything it touches”, and the back labels are a wall of chemistry. Meanwhile the dandelions, clover and that sprawling patch of something are waiting at home. Here’s how to choose a post-emergent herbicide without guessing.

Short answer: a post-emergent herbicide kills weeds that are already up and growing. A selective herbicide kills one group of plants (broadleaf weeds, grassy weeds or sedges) and leaves the lawn grass alone, so it is what you spray over a lawn. A non-selective herbicide such as glyphosate kills any green plant it touches, lawn included, so it is only for spot treatment, cracks and full lawn renovation. Match the product to the weed group, spray young weeds between about 55 and 85°F with no rain for 24 hours, and broadleaf weeds start twisting within 48 hours and die back over the following weeks.

Good news: most failures with these products come from the wrong match (a broadleaf product on crabgrass or nutsedge) or the wrong timing, not from a weak product. By the end you’ll know which bottle fits your weed, which day to spray, and how to keep your grass, kids and vegetable bed safe.

What a post-emergent herbicide does (and how it differs from a pre-emergent)

Before you buy anything, it helps to know which side of the soil line your problem lives on. “Emergence” is the moment a seedling breaks through the soil.

A pre-emergent works in the top layer of soil and stops seeds from establishing; it does nothing to a dandelion that is already there. A post-emergent herbicide works through the leaves of a plant that has already emerged. So you need a pre-emergent to stop next season’s crabgrass and a post-emergent to deal with the weeds you can see today.

The two aren’t rivals, and they’re often used in the same season. Penn State Extension notes that post-emergence products can be combined with pre-emergence products to catch late-germinating summer grasses. If the pre-emergent side is what you need, the timing guide on when to apply pre-emergent by soil temperature covers it.

Selective herbicide vs non-selective herbicide: the key difference

This one label word decides whether you get a clean lawn or a brown one. The University of Maryland Extension puts it simply: a selective herbicide “affects certain plant species or groups without seriously harming others”, while a non-selective herbicide affects all plants it contacts, including your turf (UMD Extension). Their examples of non-selective products are glyphosate and acetic acid (vinegar).

Selectivity isn’t magic, though. It depends on three things:

• The rate on the label. A lawn broadleaf product sprayed at double rate can injure grass.
• The grass species. Some lawn grasses are far more sensitive than others (more on that below).
• The weather. The same product that’s safe on a mild day can hurt turf on a hot one.

That’s why University of Georgia Extension says non-selective products are to be “used only on a spot treatment basis” in lawns (UGA Bulletin 978). The table puts the two side by side.

Systemic vs contact: why some weeds come back

Ever watched sprayed leaves crisp up, only to see the same weed back weeks later? The reason is how the product moves.

A systemic herbicide moves from the leaves down into the roots. A contact herbicide affects only the tissue it lands on, so it can burn off the top of a perennial and leave the roots alive (UMD Extension). UGA notes that most post-emergent lawn products are systemic, while contact products such as glufosinate need complete coverage of the weed.

That explains the most common complaint about vinegar and similar “burn-down” sprays: the leaves brown in a day, then the dandelion or ground ivy regrows from its root. Our post on whether vinegar kills weeds shows the same pattern. For perennial weeds, a systemic post-emergent herbicide is the one that reaches the root, where the problem lives.

Which selective herbicide for which weed

Most wasted money starts here, so identify the weed group first. A broadleaf product will not touch crabgrass, and neither a broadleaf nor a grass product controls nutsedge well.

Not sure whether a plant is a grass or a sedge? Our guide to weeds that look like grass walks through the stem and leaf checks.

Broadleaf weeds (dandelion, clover, plantain, ground ivy)

Broadleaf weeds are the easy win: grass and weed are different enough for chemistry to tell apart. UMD lists 2,4-D, mecoprop (MCPP), MCPA, dicamba, triclopyr and carfentrazone as post-emergent actives for broadleaf lawn weeds.

Kansas State University recommends combinations such as 2,4-D + MCPP + dicamba, triclopyr + clopyralid, or 2,4-D + triclopyr (K-State Turf). The “three-way” mixes cover more species than any single active, hence most of the shelf.

Tough perennials often need a triclopyr product and a repeat, so don’t judge the result after one pass. Our guides on creeping Charlie, dandelions that keep coming back and clover in lawns give weed-by-weed detail, including when living with a weed is the better choice.

Grassy weeds (crabgrass, goosegrass, foxtail)

Killing a weedy grass inside a lawn grass is harder, and fewer actives do it. You’re asking a chemical to tell one grass from another. UMD lists quinclorac and fenoxaprop-p-ethyl for grassy weeds. Penn State adds mesotrione and topramezone, and gives growth-stage limits that matter a lot (Penn State Extension):

• Mesotrione and topramezone: best control on crabgrass with fewer than 4 tillers (side shoots).
• Fenoxaprop: works on tillered annual grasses up to 4–5 tillers, but should not go on cool-season turf that shows drought stress.
• Quinclorac: controls larger plants, though control can be incomplete at the 2–4 tiller stage.

In practice that means spraying crabgrass in early to mid summer, not in late August when clumps are big and about to die at frost anyway. If your autumn crabgrass is already purple and seeding, honestly, let the frost do the job. See our crabgrass identification and control guide for the full season plan.

Perennial grass weeds such as tall fescue clumps or quackgrass in a bluegrass lawn usually have no selective fix. These are spot-treated with glyphosate.

Sedges (yellow and purple nutsedge)

Those glossy, triangular-stemmed blades that shoot up faster than the lawn? Nutsedge is not a grass, and it needs its own selective herbicide.

• UMD and Clemson list halosulfuron and sulfentrazone.
• UGA also names imazaquin, applied once the sedge has three to eight leaves (UGA).
• Clemson notes that halosulfuron usually needs a repeat application 3 to 4 weeks later for complete control (Clemson HGIC).

Plan for that second spray. Our yellow vs purple nutsedge guide explains why tubers make this weed so persistent.

When a non-selective herbicide makes sense

The “kills everything” jug isn’t the villain; it just has a narrow job. Glyphosate is systemic and non-selective, and it has a place in lawn care in four situations:

• Spot-killing a perennial grass weed that no selective herbicide removes.
• Killing weeds in cracks, gravel and fence lines where you want nothing growing.
• Killing an old lawn before reseeding or resodding.
• In the South, weeds in fully dormant bermudagrass, which UGA lists as one of the few allowed over-the-lawn uses (along with spot treatments and edging).

The thing to know is that glyphosate is slow, so patience is part of the method. A Michigan State University expert answer advises waiting seven days after spraying before overseeding, so the product has fully moved through the plants (Ask Extension).

NC State’s injury photos show leaves turning from pale green to yellow within 48 hours and browning over about two weeks (NC State Extension). For the renovation route, see should you kill weeds before overseeding.

When to spray a post-emergent herbicide: timing and weather

The perfect product still fails on the wrong day. Luckily, extension rules on timing are very consistent. For broadleaf products:

• Temperature: Illinois says apply between 55 and 85°F (University of Illinois); UMD gives 65–85°F. UGA says do not spray if the air is expected to pass 85°F within 24 hours. Clemson warns that triclopyr in particular should not be used above 85°F because of vapour drift.
• Rain: no rain expected for 24 hours, and Iowa State adds no irrigation within 24 hours of spraying (Iowa State Extension).
• Mowing: do not mow for 2–3 days before, and wait 3–4 days after, so there is leaf area to absorb the product and time for it to move to the roots (Iowa State).
• Wind: Iowa State says do not spray above 5 mph; Illinois describes a gentle 3–10 mph breeze as workable. Use coarse droplets at low pressure, close to the ground.
• Soil moisture: the weed must be actively growing. Illinois notes that adequate soil moisture keeps herbicide moving through the whole plant, so water a dry lawn a few days before, not after.

Granular vs liquid. Granular “weed and feed” products only work if the granules stick to wet leaves. Iowa State recommends applying them in the early morning on dew-covered weeds, or after irrigating. Liquids give better coverage and let you spot-spray instead of treating the whole lawn.

Fall beats spring for perennial broadleaf weeds. If you’re reading this in autumn, you’re in luck. Systemic herbicides work better in late summer and autumn, when perennials move sugars down to their roots for winter (UMD), and the herbicide rides along with them.

A dandelion sprayed in early fall usually does not return; one sprayed in full spring bloom often does. A second fall application extends control (Illinois). In Iowa, for example, the recommended window is mid-September to early November (Iowa State Extension).

Formulation. The PNW Weed Management Handbook notes that 2,4-D esters work well in cool, wet weather but are more likely to vaporise, while amine forms are safe across a wider temperature range (PNW Handbooks). Near vegetable beds, choose amine forms and skip hot days.

Your grass type changes which selective herbicide is safe

An easy mistake is grabbing the bestselling bottle without checking your grass type. Cool-season lawns (Kentucky bluegrass, tall fescue, perennial ryegrass) tolerate most broadleaf three-way mixes. Warm-season lawns are pickier.

• St. Augustinegrass: UGA notes it is sensitive to 2,4-D + MCPP mixes and should get only the low label rate of MCPP + 2,4-D + dicamba products.
• Centipedegrass: may yellow temporarily after some broadleaf herbicides.
• All warm-season grasses: UGA advises against spraying during spring green-up, when they are most easily injured.

If your bermudagrass bleaches white after a spray, herbicide injury is one of the causes covered in why bermuda grass turns white. Always check that your grass is listed on the label before you buy.

Post-emergent herbicides and new grass seed

Filling bare spots after the weeds? Order matters. Most post-emergent products harm young grass seedlings, and some hold up reseeding. The published intervals vary a lot by product:

The label on your product always overrides these general figures; follow the label.

Safety: people, pets, clippings and drift

Two habits matter: let it dry, and keep it on target. Keep children and pets off a sprayed lawn until it is fully dry. A study on athletic-field turf found that 9.4–9.9% of the applied 2,4-D could be wiped off perennial ryegrass right after spraying, but 0.5% or less after the spray had dried for one hour.

Morning dew brought some residue back to the surface over the next few days, and irrigation or rain reduced it (Jeffries et al. 2016, PLOS ONE).

Clippings matter too. Products that contain clopyralid carry label instructions not to compost the clippings or use them as mulch, because the residue can damage vegetables and flowers (PNW Handbooks).

Protect nearby garden plants with a board or bucket, or wipe the product onto weeds with a sponge, as Iowa State suggests (Iowa State).

Where this applies: the temperatures and seasons here come from US extension services and fit cool-season lawns in temperate climates of the Northern Hemisphere. Warm-season lawns in the southern US and the tropics have different windows and more sensitive grasses. In the Southern Hemisphere, shift the seasons by about six months. Which actives are registered for home lawns, and at what rates, varies by country and state, so follow the label and local rules where you live.

A note from the agronomist’s side

From a weed-science point of view, almost every disappointing spray traces back to one of three predictable mistakes. None of them is about the product being weak:

• The wrong weed group. A broadleaf product on nutsedge or crabgrass does nothing, because the chemistry was never built for those plants. The product gets blamed for a diagnosis problem.
• The wrong growth stage. Flowering dandelions in late spring and five-tiller crabgrass in August are the hardest targets of the year. Bigger, older plants are simply harder to kill, so the same spray does less.
• The wrong lawn condition. Spraying a stressed lawn in heat injures the grass and opens space for the next flush of weeds.

The bigger point is this: a post-emergent herbicide removes the weeds you have; it does not fix the thin, underfed turf that let them in. Weeds are a symptom of open space, so overseed or feed the bare spots once the waiting period ends. A dense lawn is the cheapest weed control you’ll ever buy.

FAQ

Will a post-emergent herbicide kill my grass?

A selective herbicide used at label rate on a grass listed on the label should not kill the lawn, though sensitive grasses such as St. Augustinegrass and centipedegrass can yellow for a while. A non-selective product such as glyphosate will kill the grass it touches.

How long does a post-emergent herbicide take to work?

Broadleaf products such as 2,4-D and dicamba belong to the synthetic auxin group. NC State Extension describes twisting of stems and leaf stalks within 48 hours, yellowing in about 7–10 days and growth stopping in about four weeks (NC State Extension). Glyphosate shows yellowing within about two days and browning over roughly two weeks.

How long after spraying can I mow?

Iowa State recommends waiting 3–4 days after a broadleaf spray, and not mowing for 2–3 days before it.

Related guides: How to Kill Weeds in a Gravel Driveway

Sources

• University of Maryland Extension. Herbicide Options for Managing Common Lawn Weeds in Maryland.
• Kansas State University Turfgrass. Broadleaf Weed Chemical Control.
• Penn State Extension. Postemergence Control of Crabgrass and Other Summer Annual Grasses in Lawns.
• University of Illinois Extension, Home, Yard and Garden. Fall Broadleaf Weed Control in Turf.
• Clemson Cooperative Extension HGIC. Managing Weeds in Fescue Lawns.
• University of Georgia Extension. Bulletin 978: Weed Control in Home Lawns.
• Iowa State University Extension. What is the proper way to apply broadleaf herbicides to the lawn?
• Pacific Northwest Weed Management Handbook. Chemical Control of Broadleaf Weeds in Turf.
• Michigan State University Extension via Ask Extension. Seeding Grass after Glyphosate Application.
• NC State Extension. Herbicide Injury: Glyphosate.
• NC State Extension. Synthetic Auxins.
• Iowa State University Extension. Controlling Broadleaf Weeds in the Lawn.
• Jeffries MD, Gannon TW, Brosnan JT, Breeden GK (2016). Comparing Dislodgeable 2,4-D Residues across Athletic Field Turfgrass Species and Time. PLOS ONE 11(12): e0168086.

More herbicide guides are collected in our Herbicides & Pre-emergents section.