Short answer: crabgrass is a light green summer annual grass that grows as a low, spreading clump, with stems radiating from the centre like a crab’s legs and leaf blades about 1/2 to 3/4 inch (1.3 to 2 cm) wide. From July onward it sends up seedheads of 3 to 5 thin “fingers” spread in a V. It dies with the first frost, but each plant leaves thousands of seeds behind. To get rid of it, stop the seeds germinating with a pre-emergent applied before soil at 2 inches (5 cm) holds about 55°F (13°C) for several days in a row, kill any escapes while they are young with a post-emergent such as mesotrione, fenoxaprop or quinclorac, and keep the lawn thick and mowed high so next year’s seeds have nowhere to start.
What crabgrass looks like
Crabgrass is the common name for grasses in the genus Digitaria (family Poaceae), also called finger grass. Two species cause almost all lawn problems: large (hairy) crabgrass, Digitaria sanguinalis, and smooth crabgrass, Digitaria ischaemum. Both look the same from standing height:
- Growth habit: stems grow outward from a single centre in a rosette, lying low to the ground, which is why the plant slips under the mower blade.
- Colour and texture: paler, yellower green than most lawn grasses, with wider, softer blades that stand out as coarse patches.
- Leaves: according to Penn State Extension, blades are 1/2 to 3/4 inch wide and 2 to 10 inches (5 to 25 cm) long, with a short membranous ligule (1 to 1.5 mm) and no auricles where the blade meets the stem.
- Seedheads: 3 to 5 slender spikes radiating from the stem tip, produced from July through September.
- Stems: flattened and often purplish at the base. Older large crabgrass roots where the lower stem joints touch soil, so one plant can cover a patch the size of a dinner plate or more.
Large crabgrass vs smooth crabgrass
You only need to tell them apart if you want to know which one you are fighting; the control is the same. Smooth crabgrass is the more common lawn species in the northeastern and mid-Atlantic US, according to Rutgers Cooperative Extension.
| Feature | Large (hairy) crabgrass | Smooth crabgrass |
|---|---|---|
| Hairs | Long, stiff hairs on both leaf surfaces and on the sheath | Nearly hairless; a few long hairs only near the base of the blade |
| Size | Bigger plant; clumps can reach about 2 1/2 ft (80 cm) if unmown | Smaller, lower plant |
| Base colour | Green to purplish | Often distinctly purple at the base |
| Ligule tip | Somewhat jagged | Smoother edge |
Crabgrass seedlings and look-alikes
Crabgrass seedlings are pale green, short and broad-bladed, and those of large crabgrass are fuzzy with coarse hairs. The new leaf comes out rolled, not folded. The most useful clue is timing: crabgrass is an annual, so there are no mature crabgrass plants in April or May. A coarse clump that is already full-sized in spring is a perennial grass, and those need different treatment (most crabgrass herbicides will not touch them). The University of Wisconsin makes the same point: crabgrass does not usually show up as a visible plant until late June or July in the upper Midwest.
| Plant | Life cycle | Key identification clue | How it differs from crabgrass |
|---|---|---|---|
| Crabgrass | Summer annual | Pale green rosette, rolled new leaf, membranous ligule, no auricles; seedhead of 3 to 5 fingers | Reference plant |
| Goosegrass (Eleusine indica) | Summer annual | Whitish-silver, flattened stem base; folded new leaf; seeds in a “zipper” or herringbone pattern on 2 to 5 thick spikes | Germinates 3 to 4 weeks after crabgrass; common in compacted, trampled spots |
| Quackgrass (Elymus repens) | Perennial | Claw-like auricles that wrap the stem; long, sharp-tipped white rhizomes | Green in early spring; spreads underground |
| Tall fescue (Festuca arundinacea, as a weed in fine lawns) | Perennial | Wide, stiff, dull blade with prominent veins; tight bunches that shred when mowed | Full-sized clumps already in April and May |
| Dallisgrass (Paspalum dilatatum) | Perennial | Stiffer leaves, tall papery ligule, short knotty rhizomes; 3 to 6 drooping spikes set at different points along the stem | Returns from the same crown each year |
| Nimblewill (Muhlenbergia schreberi) | Perennial | Short, fine, bluish-green blades in wiry patches; stems root at nodes | Grey-brown and dormant much of spring and fall; prefers moist shade |
The dallisgrass details come from UC IPM, which notes that crabgrass leaves are soft while dallisgrass leaves are stiffer. If you are not sure which grasses you have, our guide to different grass types in a lawn walks through the other common intruders.
Life cycle: why timing is everything
Crabgrass lives for one season. Seeds germinate in spring, the plants grow fast through the heat of summer, flower from July, drop seed in late summer and die at the first hard frost. It uses the C4 photosynthetic pathway, which is why it keeps growing in hot, dry weather when cool-season lawn grasses slow down. It reproduces only by seed, and Rutgers notes that each plant can produce thousands of seeds that stay viable in the soil for several years. That is why pulling every plant one year can still leave you with more the next: last year’s plants had already shed their seed.
So the aim is simple: stop seeds from establishing, and kill any plants before they seed.
Soil temperature, forsythia and growing degree days
Crabgrass follows soil temperature, not the calendar. Three ways to time it:
- Soil thermometer: Penn State says seeds germinate once soil reaches about 55 to 58°F (13 to 14°C) at daybreak for 4 to 5 days. Most germination happens once the top 2 inches warm to 60 to 70°F (16 to 21°C). Put the pre-emergent down before this, while soil is in the 50 to 55°F (10 to 13°C) range.
- Forsythia: Rutgers uses forsythia bloom as its signal that crabgrass germination is about to start. Aim to have the product down while forsythia is in full bloom, before the petals drop.
- Growing degree days (GDD): Michigan State University’s GDDTracker runs a free crabgrass timer by ZIP code. It counts GDD from a base of 32°F (0°C) and recommends applying between 250 and 500 GDD, which lines up with 0 to 2 inch soil reaching about 50 to 55°F.
In practice that means roughly mid-March to mid-April across much of the US transition zone, April to early May further north, and late February to March in the South. Penn State’s own windows run from 15 March to 15 April in southeastern Pennsylvania to 20 April to 10 May in the northern tier.
Pre-emergent herbicides: actives and split applications
A pre-emergent forms a chemical barrier in the top layer of soil that kills crabgrass as the seed sprouts. It does nothing to plants you can already see (with one exception, below), and it must be watered in by rain or irrigation as the label directs. The three main actives in homeowner products are:
| Active ingredient | Strength | Watch out for |
|---|---|---|
| Prodiamine | Long soil residual; the usual choice for one early application or a split program | No effect on emerged crabgrass. Blocks grass seed too. |
| Dithiopyr | Also kills very young crabgrass up to the 1 to 2 tiller stage, according to Purdue University, so it forgives a late application | Blocks grass seed; the early post-emergent effect ends once plants tiller |
| Pendimethalin | Widely sold and inexpensive | Shorter residual than prodiamine; blocks grass seed |
| Mesotrione (at seeding) | One of the few options allowed at seeding for Kentucky bluegrass, perennial ryegrass and tall fescue | Not for new fine fescue; short pre-emergent life |
Pre-emergents stay active for about 6 to 8 weeks or longer, but crabgrass keeps germinating into midsummer. A split application covers that gap: apply half the label’s yearly rate at the normal spring timing and the second half 6 to 8 weeks later, as Rutgers recommends. In the South, the University of Arkansas Extension suggests a first application in late February and a second in early April.
The big trade-off is seeding. Most pre-emergents stop lawn grass seed as well as crabgrass, so you cannot seed bare patches in spring and use prodiamine or pendimethalin on the same spot. Check the label’s reseeding interval, and plan overseeding for early autumn instead. Our page on killing weeds before overseeding covers the sequence.
Post-emergent options by growth stage
If crabgrass is already up, choose the product by how big the plants are. Count tillers, the side shoots coming from the base of the plant. Rutgers reports that crabgrass starts to tiller from mid to late June in New Jersey, and every extra tiller makes it harder to kill.
| Active ingredient | Best growth stage | Lawn grasses it suits | Notes |
|---|---|---|---|
| Dithiopyr | Seedlings to 1 to 2 tillers | Most cool- and warm-season lawns (check label) | Also provides residual pre-emergent control |
| Mesotrione | Fewer than 4 tillers | Kentucky bluegrass, perennial ryegrass, tall fescue; label also lists some warm-season grasses such as centipede and St. Augustine | Usually needs a repeat spray; causes temporary white bleaching of weeds (and of bermudagrass, which it injures). Add a non-ionic surfactant. |
| Topramezone | Fewer than 4 tillers | Cool-season lawns | Injures bermudagrass (it is used to suppress it). Add methylated seed oil. |
| Fenoxaprop | Up to 4 to 5 tillers (rate rises with size) | Cool-season lawns and zoysia | Can injure some Kentucky bluegrass in high heat; do not use on drought-stressed turf; weaker when mixed with 2,4-D or MCPP |
| Quinclorac | Young seedlings and large plants with 5+ tillers; less reliable at 2 to 4 tillers | Kentucky bluegrass, ryegrass, tall fescue, bermuda and zoysia | Not recommended on St. Augustine or centipede. Add methylated seed oil. |
The stage and safety details come mainly from Penn State’s post-emergence crabgrass guide, which also gives waiting periods after seeding (for example, quinclorac can go on tall fescue seedlings 7 days after emergence but Kentucky bluegrass seedlings only after 28 days). The Arkansas guide adds that quinclorac products mixed with 2,4-D can burn warm-season lawns in summer heat. Always read the label for your grass type before you spray. If you see white or bleached patches after using mesotrione, see why bermuda grass turns white.
A few practical rules: spray actively growing plants when the lawn is not drought-stressed, do not mow for a couple of days either side of the application, and expect large plants to take one to three weeks to die. From late August, killing a big crabgrass plant that is about to die from frost anyway rarely pays off. Pulling it or cutting off seedheads is often the better use of your time.
Organic methods and what the evidence says
- Hand pulling: works well for a few plants, especially young ones before they root at the nodes. Pull after rain, get the whole crown, and bag plants that already carry seedheads. In beds, a 2 to 3 inch layer of mulch stops most seedlings; see how to stop weeds growing through mulch.
- Corn gluten meal: a natural pre-emergent that stops roots forming on germinating seeds. Iowa State University, where its herbicidal effect was discovered, recommends 20 lb per 1,000 sq ft (about 1 kg per 10 m²) three to five weeks before crabgrass germinates. It contains about 10 percent nitrogen, so it also feeds the lawn. The catch is that it only works if the soil dries out after the seeds sprout. Heavy rain or irrigation weakens it, it does nothing to emerged plants, and control improves only over several years of use. Illinois Extension sums up the research: university trials often show limited control in the first year, results are better in dry springs than in rainy ones, and it will stop new lawn seed germinating too.
- Vinegar and boiling water: these burn the leaves and any lawn grass they touch, and crabgrass often regrows from the crown. See does vinegar kill weeds for what horticultural vinegar really does.
Cultural control: the long-term fix
Crabgrass seed needs light and warm, bare soil to germinate. A thick lawn with a tall canopy shades the soil and keeps it cooler, and this does more than any single product. Rutgers found that mowing at the highest recommended height gives significantly less crabgrass than close mowing.
- Mow high: keep cool-season lawns at the top of their range, often 3 to 4 inches (7.5 to 10 cm) for tall fescue and Kentucky bluegrass. Scalped edges along drives and paths are where crabgrass starts.
- Water deeply and infrequently: light daily sprinkling keeps the soil surface moist, which crabgrass seedlings like. Water less often but more deeply.
- Feed at the right time: for cool-season lawns, put most of the year’s nitrogen on in early autumn, when the grass benefits and the crabgrass is dying. Heavy summer feeding helps the weed more than the lawn.
- Fill gaps in autumn: overseed thin areas in late summer to early autumn so the bare soil is covered before spring. Our soil preparation guide covers the steps.
- Fix compaction: goosegrass and crabgrass both take over compacted, heavily trodden spots where lawn grass thins.
How crabgrass looks through the year
In spring it appears as tiny pale green seedlings in bare spots and along edges, often weeks after the lawn greens up. In summer it forms spreading pale clumps, then finger-like seedheads from July. In autumn the plants turn purplish or reddish-brown and die at the first frost. In winter crabgrass is not dormant but dead: what you see are flat, straw-coloured mats or bare patches. Rake them out and treat or overseed those spots before next spring.
Month-by-month crabgrass calendar
This calendar fits a cool-season lawn in the US transition zone or a similar climate. Move each step 3 to 4 weeks earlier in warm southern regions and 2 to 3 weeks later in the far north or at high altitude. Where winters are frost-free, crabgrass can survive year-round.
| Month | What crabgrass is doing | What to do |
|---|---|---|
| January to February | Seeds dormant in soil | Buy product; in the South, first pre-emergent split in late February |
| March | Soil warming | Watch soil temperature and forsythia; apply pre-emergent (first split) in the transition zone |
| April | Germination begins as soil passes 55°F | Northern lawns apply pre-emergent; do not rake or dethatch treated areas afterwards |
| May | Seedlings emerging | Second split 6 to 8 weeks after the first; spot-treat seedlings with dithiopyr or mesotrione; pull by hand |
| June | Tillering starts mid to late June | Post-emergent while plants have fewer than 4 tillers; raise mowing height |
| July | Rapid growth in heat; seedheads appear | Fenoxaprop or quinclorac on larger plants; water deeply, not daily |
| August | Heavy seed drop | Pull or bag plants with seedheads; plan autumn overseeding |
| September | Growth slowing, plants purpling | Overseed bare patches; main nitrogen feeding for cool-season lawns |
| October to November | Killed by first frost | Rake out dead mats; keep mowing new seedlings |
| December | Nothing living above ground | Note where crabgrass was worst for next spring’s treatment |
Is crabgrass ever useful?
Large crabgrass is grown as summer grazing for cattle and goats in parts of the southern US, and its quick cover holds bare soil against erosion. In a lawn, though, it leaves bare patches every autumn, so it is worth keeping out.
FAQ
Will crabgrass come back next year?
The plants die each autumn, but the seed bank in the soil does not. Expect crabgrass for at least two or three seasons after a bad year unless you use a pre-emergent and thicken the lawn.
Does nutsedge count as crabgrass?
No. The shiny, bright green “grass” that grows faster than the lawn in midsummer is often nutsedge, a sedge with triangular stems that crabgrass products do not control. See yellow nutsedge vs purple nutsedge.
Related guides: Poa Annua in Lawns: How to Identify and Control Annual Bluegrass · Weeds That Look Like Grass: 9 Grassy Lawn Weeds and How to Tell Them Apart · How to Get Rid of Foxtail Barley (Wild Barley Grass) in Lawns and Pastures · When to Apply Pre-Emergent: Soil Temperature Timing
Sources
- Rutgers NJAES – FS1309: Crabgrass and Goosegrass Identification and Control in Cool-Season Turfgrass
- Penn State Extension – Lawn and Turfgrass Weeds: Smooth Crabgrass and Large Crabgrass
- Penn State Extension – Postemergence Control of Crabgrass and Other Summer Annual Grasses in Lawns
- Purdue University Turfgrass Science – Smooth Crabgrass
- Michigan State University – About GDDTracker (Crabgrass Preemergence Timer)
- University of Wisconsin Extension – Perennial Grass Weeds in Lawns
- UC IPM – Dallisgrass identification
- University of Arkansas Cooperative Extension – Crabgrass
- Iowa State University Extension – How to Use Corn Gluten Meal as Herbicide
- Illinois Extension – Corn Gluten Meal: Can it naturally prevent weeds and feed your lawn?