Does Burning Weeds Kill Them? Propane Doses for 14 Weeds by Stage

Does burning weeds kill them for good? It depends on what you burn, and the University of Nebraska flame-weeding manual (Knezevic and others, 2014) measured it: the propane dose that gave 80 and 90 percent control for 14 weeds at 39 growth stages. We converted every row into pounds of propane per 1,000 sq ft and minutes of a 100,000 BTU homeowner torch, which none of the top results do. The dose for 90 percent control runs from 4 gallons per acre on a 4-leaf ragweed to 30 on a flowering green foxtail.

You also do not need to set anything alight. Flaming works by heat, not ash: University of Maryland Extension puts lethal leaf temperatures at 55 to 94°C (131 to 201°F) and says 65 to 130 milliseconds of exposure kills many annuals. If your weeds are mostly in a lawn, a torch scorches the turf too, and the selective options in our lawn weed killer guide or our list of common lawn weeds by type are the better start.

How heat kills a weed in under a second

Above about 50°C (122°F) the proteins in leaf cell membranes coagulate, and at 100°C (212°F) the sap boils and bursts the cell walls, per UMD and Stevan Knezevic at UNL. The plant then dries out. Iowa State Extension says flamed weeds turn a dull green right away and die over the next several hours to days; ATTRA (Diver, 2002) puts it at 1 to 3 days.

The test in the field is your thumb. ATTRA says foliage that keeps a thumbprint when pressed between thumb and finger has been flamed enough. UC IPM gives a time: 2 to 5 seconds of exposure is usually enough, and if the plant still looks healthy the next day, flame it again a little longer. Charring it to the soil only burns propane and raises the fire risk.

Which weeds burning kills, by type and stage

Read your weed’s row before you buy propane. Annual broadleaf seedlings are the one group that a single pass reliably kills, roots and all, because the growing point sits above ground in the flame. Grasses keep theirs below the soil surface, and perennials keep food in roots the flame never reaches, per the UNL manual (sections 5.2.1 to 5.2.3).

Weed type and stage	Control measured	Passes needed	What happens after	Source
Annual broadleaf seedling, 1 to 3 in, 1 to 6 leaves (pigweed, lambsquarters, ragweed)	90% at about 10 gal propane/acre	1	Leaves dry within a few days; no regrowth	UNL 2014, s. 5.2 and 5.2.1
Annual broadleaf at 14 leaves or taller	90% at about 15 gal/acre; velvetleaf at 39 in needed 21	1, at a slower pass	Dies if dosed; tolerance rises with size	UNL 2014, Table 2
Broadleaf at the seed-leaf (cotyledon) stage, e.g. wild sunflower, cocklebur	Can survive	Wait until 2 to 4 true leaves	Survivors regrow	UNL 2014, s. 5.2.1
White mustard, 0 to 2 leaves vs 2 to 4 leaves	95% at 42 vs 74 kg propane/ha (about 9 vs 16 gal/acre)	1	Dose nearly doubles with 2 more leaves	UMD Extension, 2022
Annual grass, 2 to 7 leaves (foxtails, barnyardgrass)	About 80% short-term suppression at 10 gal/acre; 90% took 10 to 19	2, 7 to 10 days apart	Leaves turn white, new leaves in 1 to 2 weeks	UNL 2014, s. 5.2.2
Perennial broadleaf (Canada thistle, dandelion, field bindweed)	Top growth blackened in 24 hours at 3 gal/acre (thistle)	Every 2 to 3 weeks, over more than 1 season	Regrew a few weeks later	UNL 2014, s. 5.2.3; Iowa State; Cal-IPC
Rhizome, stolon or prostrate weeds (e.g. bermudagrass, nutsedge)	Not effective	Use another method	Regrows from rhizomes	Cal-IPC, Moore
Mixed weeds on paved traffic islands	Weed cover held under 2%	5 a year on average	Seedlings keep emerging	Rask and others, 2013

Two rows surprise most people. A tiny seedling still showing only its seed leaves can be harder to kill than one with 3 true leaves, so UNL waits for 2 to 4 leaves on wild sunflower and cocklebur. And common ragweed is the easiest weed in the table: 4 gallons per acre at 4 leaves, against 18 for green foxtail at 5 leaves. Grassy weeds like foxtail and crabgrass are where a torch disappoints.

The propane dose for 14 weeds at 39 growth stages

UNL flamed each weed with non-hooded torches across the whole row and recorded the dose for 80 and 90 percent control. We added two columns: the 90 percent dose as pounds of propane per 1,000 sq ft (at NIST‘s 4.23 lb per gallon) and as minutes of a Weed Dragon VT2-23C at its 18 psi setting, which Flame Engineering lists at 2.25 lb an hour. The tractor rigs were not handheld wands, so treat the minutes as a floor.

Swipe the table sideways to see every column.

Weed	Type	Stage, height	Gal/acre for 80%	Gal/acre for 90%	Lb propane per 1,000 sq ft (90%)	Torch minutes per 1,000 sq ft (90%)
Common waterhemp (Amaranthus rudis)	Annual broadleaf	3-leaf, 2 in	6	8	0.78	21
Common waterhemp (Amaranthus rudis)	Annual broadleaf	5-leaf, 4 in	7	10	0.97	26
Common waterhemp (Amaranthus rudis)	Annual broadleaf	Flowering, 15 in	7	10	0.97	26
Redroot pigweed (Amaranthus retroflexus)	Annual broadleaf	3-leaf, 2 in	10	15	1.46	39
Redroot pigweed (Amaranthus retroflexus)	Annual broadleaf	5-leaf, 4 in	10	14	1.36	36
Redroot pigweed (Amaranthus retroflexus)	Annual broadleaf	Flowering, 18 in	12	18	1.75	47
Field bindweed (Convolvulus arvensis)	Perennial broadleaf	8-leaf, 2 in	6	8	0.78	21
Field bindweed (Convolvulus arvensis)	Perennial broadleaf	10-leaf, 4 in	5	7	0.68	18
Field bindweed (Convolvulus arvensis)	Perennial broadleaf	40-leaf, 13 in	7	12	1.17	31
Kochia (Kochia scoparia)	Annual broadleaf	6-leaf, 1 in	8	10	0.97	26
Kochia (Kochia scoparia)	Annual broadleaf	10-leaf, 3 in	6	8	0.78	21
Kochia (Kochia scoparia)	Annual broadleaf	Flowering, 18 in	10	15	1.46	39
Ivyleaf morningglory (Ipomoea hederacea)	Annual broadleaf	10-leaf, 3 in	8	12	1.17	31
Ivyleaf morningglory (Ipomoea hederacea)	Annual broadleaf	14-leaf, 4 in	10	13	1.26	34
Ivyleaf morningglory (Ipomoea hederacea)	Annual broadleaf	Flowering, 16 in	10	14	1.36	36
Velvetleaf (Abutilon theophrasti)	Annual broadleaf	5-leaf, 2 in	6	9	0.87	23
Velvetleaf (Abutilon theophrasti)	Annual broadleaf	7-leaf, 7 in	8	12	1.17	31
Velvetleaf (Abutilon theophrasti)	Annual broadleaf	16-leaf, 39 in	12	21	2.04	54
Venice mallow (Hibiscus trionum)	Annual broadleaf	3-leaf, 2 in	7	11	1.07	28
Venice mallow (Hibiscus trionum)	Annual broadleaf	5-leaf, 4 in	7	11	1.07	28
Venice mallow (Hibiscus trionum)	Annual broadleaf	Flowering, 20 in	8	15	1.46	39
Common ragweed (Ambrosia artemisiifolia)	Annual broadleaf	4-leaf, 1 in	3	4	0.39	10
Common ragweed (Ambrosia artemisiifolia)	Annual broadleaf	14-leaf, 8 in	5	6	0.58	16
Common ragweed (Ambrosia artemisiifolia)	Annual broadleaf	26-leaf, 31 in	10	14	1.36	36
Common lambsquarters (Chenopodium album)	Annual broadleaf	5-leaf, 2 in	8	10	0.97	26
Common lambsquarters (Chenopodium album)	Annual broadleaf	11-leaf, 5 in	12	15	1.46	39
Tansy mustard (Descurainia pinnata)	Annual broadleaf	9-leaf, 5 in	8	10	0.97	26
Tansy mustard (Descurainia pinnata)	Annual broadleaf	Flowering, 13 in	10	12	1.17	31
Henbit (Lamium amplexicaule)	Annual broadleaf	9-leaf, 6 in	13	15	1.46	39
Henbit (Lamium amplexicaule)	Annual broadleaf	Flowering, 12 in	8	10	0.97	26
Green foxtail (Setaria viridis)	Annual grass	5-leaf, 2 in	12	18	1.75	47
Green foxtail (Setaria viridis)	Annual grass	7-leaf, 5 in	12	19	1.85	49
Green foxtail (Setaria viridis)	Annual grass	Flowering, 26 in	22	30	2.91	78
Yellow foxtail (Setaria pumila)	Annual grass	2-leaf, 2 in	9	15	1.46	39
Yellow foxtail (Setaria pumila)	Annual grass	4-leaf, 4 in	11	18	1.75	47
Yellow foxtail (Setaria pumila)	Annual grass	Flowering, 19 in	17	28	2.72	73
Barnyardgrass (Echinochloa crus-galli)	Annual grass	4-leaf, 2 in	7	10	0.97	26
Barnyardgrass (Echinochloa crus-galli)	Annual grass	7-leaf, 7 in	9	16	1.55	41
Barnyardgrass (Echinochloa crus-galli)	Annual grass	Flowering, 33 in	16	27	2.62	70

Size raises the dose: 13 of the 14 weeds needed more propane at their largest stage than at their smallest. Velvetleaf goes from 9 gallons per acre at 2 inches to 21 at 39 inches, and the three grasses need 27 to 30 gallons once they flower. Henbit is the exception (15 at 9 leaves, 10 at flowering), and the manual does not say why. The chart puts the earliest and latest stage side by side.

1. Common ragweed4
2. Common waterhemp8
3. Kochia10
4. Velvetleaf9
5. Redroot pigweed15
6. Barnyardgrass10
7. Yellow foxtail15
8. Green foxtail18

Source: Knezevic and others, UNL and PERC, 2014, Table 2 (Ulloa and others 2010a, b), broadcast flaming Last 3 bars are grasses. 10 gal/acre = 0.97 lb propane per 1,000 sq ft.

Show the numbers

	Earliest stage tested (1 to 2 in)	Flowering or largest stage (15 to 39 in)
Common ragweed	4	14
Common waterhemp	8	10
Kochia	10	15
Velvetleaf	9	21
Redroot pigweed	15	18
Barnyardgrass	10	27
Yellow foxtail	15	28
Green foxtail	18	30

Why grasses come back after you burn them

A grass seedling keeps its growing point below the soil surface, wrapped in a leaf sheath, so the flame cooks the blades and misses the bud. ATTRA says grasses form that protective sheath by the time they are about 1 inch tall. At 10 gallons per acre, UNL got about 80 percent control of grasses, and only as short-term suppression: the leaves turned white, and within 1 to 2 weeks new leaves grew.

UNL’s fix is a second flaming 7 to 10 days after the first, or burying the scorched grass with soil within a few hours. Lawn grass has the same buried growing point, so a torch run across turf browns it rather than clearing it. To remove a grass weed from a lawn, the selective sprays in our guide to the crabgrass killer that will not kill grass work where flame cannot.

Does burning weeds kill the roots?

Only on young seedlings. A weed 1 to 2 inches tall has a fragile root that dies with the top, per a Cal-IPC training deck quoting McGill University. A perennial is different. In UNL’s trial, 3 gallons of propane per acre turned Canada thistle leaves completely black within 24 hours, yet the plants regrew a few weeks later because the heat cannot reach deep roots.

The only way flame kills a perennial is starvation by repetition. Iowa State says to flame regrowth about every 2 weeks; Cal-IPC says new shoots appear within about a week and treatments 2 to 3 weeks apart eventually exhaust the root. UNL says it can take several years. A dandelion taproot runs 6 to 18 inches deep, which is why our guide to killing dandelions without killing grass digs or sprays instead, and why our thistle control guide relies on herbicides that move to the root.

Several popular pages say otherwise. The table sets each claim against what the trial measured.

Claim in the search results	Where it appears	What the research measured
“YES, flame weeding will kill the roots too!”	Flame Engineering, Flame Weeding 101	True for annual seedlings. Canada thistle and dandelion regrew a few weeks after a full top kill (UNL 2014, s. 5.2.3).
Weeds 1 to 4 inches: “nearly 100% effective”	Flame Engineering, Flame Weeding 101	UNL designed its doses for 90 percent control; at 10 gal/acre grasses reached only about 80 percent, as short-term suppression.
“Just a split second of heat should kill unwanted weeds and grasses completely”	Flame Engineering, Flame Weeding 101	Broadleaves, often. Grass leaves turn white and regrow within 1 to 2 weeks (UNL, s. 5.2.2).
Burn the top fully so the heat damages shallow roots	Instructables, 2020	UC IPM: do not burn the weed to the ground; 2 to 5 seconds is enough. UNL: flaming cannot penetrate deep into soil.
Torching gives temporary relief and does not kill roots	Facebook gardening group in the results	Half right: annual broadleaf seedlings at 10 gal/acre dry out with no regrowth (UNL, s. 5.2.1).

How long burning weeds lasts, compared with spraying

Flame leaves nothing behind, so the next flush of seedlings is untouched; UMD lists the lack of residual control first among its drawbacks. On Danish traffic islands, Rask and others (2013) counted the treatments needed to keep weed cover under 2 percent over a season: flaming averaged 5 a year, glyphosate 2.5, hot water 3, and steam and hot air with flame 5.5 each.

1. Glyphosate2.50
2. Hot water3
3. Flaming5
4. Hot air with flame5.50
5. Steam5.50

Source: Rask, Larsen, Andreasen and Kristoffersen, Weed Research 53(4), 2013, Denmark, 2005 and 2006 Commercial machines on paved traffic islands, 3 trial sites.

Show the numbers

	Average treatments per year
Glyphosate	2.50
Hot water	3
Flaming	5
Hot air with flame	5.50
Steam	5.50

On efficacy per pass, a 2024 Indian field trial in Heliyon found a hand-pulled flame weeder at 40 psi gave 91.1 percent weed control efficiency, against 98.06 for hand weeding and 78.26 for mechanical weeding, while using 13.24 labor-hours per hectare to hand weeding’s 255.75. The two heat-and-acid alternatives work the same contact-only way; their numbers are on our own pages.

Method	What one treatment kills	Treatments a year to keep cover under 2% (Rask 2013)	Our page
Propane flame	Annual broadleaf seedlings; grass tops for 1 to 2 weeks	5	This page
Boiling water	Seedlings at about 3 fl oz per sq ft; perennials regrow	3 (hot water)	Does boiling water kill weeds?
Vinegar (20% acetic acid)	Young broadleaves it fully wets; grasses and roots regrow	Not tested	Vinegar weed killer
Glyphosate	Moves to roots, kills perennials and grass, non-selective	2.5	Does Roundup kill grass?

How much propane burning weeds takes

A gallon of propane holds 91,452 BTU (EIA) and weighs 4.23 lb, so UNL’s 10 gallons per acre works out to 0.97 lb per 1,000 sq ft. The Weed Dragon burns 2.25 lb an hour at 18 psi and 4.6 lb at full flame, per Flame Engineering, so the broadleaf dose is about 26 minutes of torch time per 1,000 sq ft. A 20-lb grill cylinder covers about 20,600 sq ft at that rate. Enter your area and weed below.

Area to flame (sq ft)MethodBroadcast flaming, non-hooded torch (UNL doses)Weeds and stage (UNL dose, gal propane per acre)Broadleaf seedlings 1 to 3 in, 90% control: 10 gal/acreBroadleaf at 14 leaves, 90% control: 15 gal/acreCommon ragweed at 4 leaves, 90% control: 4 gal/acreAnnual grass, 2 to 7 leaves, 80% short-term: 9 gal/acreGreen foxtail at 5 leaves, 90% control: 18 gal/acreFlowering foxtail or barnyardgrass, 90%: 30 gal/acrePasses1 pass2 passes (grasses, 7 to 10 days apart)5 passes (season average on traffic islands, Rask 2013)

Propane (lb)

0.97

Propane (gallons)

0.23

Weed Dragon minutes at 18 psi

26

20-lb cylinders

0.05

Default: 1,000 sq ft of broadleaf seedlings, 1 pass = 0.97 lb (0.23 gal), about 26 minutes of a Weed Dragon at 18 psi (2.25 lb/hr), 0.05 of a 20-lb cylinder. Gal/acre x 4.23 lb / 43,560 sq ft. UNL measured these doses with tractor-mounted torches; a handheld wand loses more heat, so treat results as a minimum.

Area flamed	Weeds and dose (UNL)	Propane, lb	Weed Dragon minutes at 18 psi	Share of a 20-lb cylinder
200 sq ft	Broadleaf seedlings, 10 gal/acre	0.19	5	1%
200 sq ft	Green foxtail at 5 leaves, 18 gal/acre	0.35	9	2%
500 sq ft	Broadleaf seedlings, 10 gal/acre	0.49	13	2%
500 sq ft	Green foxtail at 5 leaves, 18 gal/acre	0.87	23	4%
1,500 sq ft	Broadleaf seedlings, 10 gal/acre	1.46	39	7%
1,500 sq ft	Green foxtail at 5 leaves, 18 gal/acre	2.62	70	13%

Moisture changes the math. UNL says dew or rain on the leaves soaks up heat that would otherwise burst the cells, and recommends waiting at least 1 hour after rain or heavy dew. UMD adds that in one comparison, flaming at noon and 4 p.m. controlled weeds better than at 8 a.m. or 8 p.m. Wet ground, though, is safer, which is the trade-off in the next section.

When you should not burn weeds: fire rules and risk

A flame weeder is an open-flame torch, and fire restrictions name torches. In unincorporated Boulder County, Colorado, the Stage 1 restrictions of March 30, 2026 ban operating any torch with an open flame except in a cleared area at least 10 feet in diameter with a chemical pressurized fire extinguisher, on pain of a $500 fine. Check your county or fire district before you light up during a dry spell.

California’s Public Resources Code 4427 applies whenever burning permits are required: equipment from which a spark, fire or flame may originate, including cutting torches, cannot run on or near grass, brush or forest land without clearing 10 feet around it and keeping a 46-inch round point shovel and a backpack pump extinguisher at hand. A weed torch is not named, so ask your CAL FIRE unit.

Mulch is the usual accident. In a University of Nevada burn of 8 mulches, shredded cedar spread fire at 47.9 feet a minute; our page on cedar mulch fire risk compares it with other mulches. UVM Extension says to avoid buildings, plastic, mulch and dry plants, skip dry, windy weather and keep a bucket of water within reach.

Never burn poison ivy, poison oak or poison sumac. The CDC National Institute for Occupational Safety and Health warns that burning them releases urushiol in the smoke, which can irritate the lungs, and that an amount smaller than a grain of salt causes a rash in 80 to 90 percent of adults.

The flowchart puts the fire checks and the weed checks in the order to ask them.

1. 1Could any of it be poison ivy, poison oak or poison sumac?

   YesDo not burn it. CDC NIOSH warns the smoke carries urushiol and can irritate the lungs. Dig or spray it instead.

   NoGo to the next question

2. 2Is a fire restriction or burn ban in effect, or is the weather dry and windy?

   YesWait. Stage 1 orders such as Boulder County's (2026) allow torches only in a cleared 10-foot area with an extinguisher; UVM says skip dry, windy days.

   NoGo to the next question

3. 3Is there mulch, dry grass, a conifer, fence or siding within a few feet?

   YesUse another method here. Shredded cedar mulch spread fire at 47.9 ft a minute in a University of Nevada burn.

   NoGo to the next question

4. 4Is it a grass, or a weed that spreads by rhizomes or runners?

   YesExpect regrowth in 1 to 2 weeks. Flame twice, 7 to 10 days apart (UNL), or use a grass-selective spray or digging.

   NoGo to the next question

5. 5Is it a perennial with a taproot or creeping roots (dandelion, thistle, bindweed)?

   YesFlame only if you will repeat every 2 to 3 weeks for a season or more (Iowa State, Cal-IPC); digging or a systemic herbicide is faster.

   NoGo to the next question

6. 6Is the broadleaf weed taller than about 4 inches or past 6 leaves?

   YesSlow the pass: UNL's doses rose to about 15 gal/acre at 14 leaves and 21 for 39-inch velvetleaf.

   NoBurn it: one slow pass at about 10 gal propane/acre (about 1 lb per 1,000 sq ft) kills 90 percent of annual broadleaf seedlings. Stop when a pressed leaf keeps a thumbprint.

Source: UNL 2014; CDC NIOSH; Boulder County 2026; UVM; Iowa State; Cal-IPC; University of Nevada SP-11-04

This page is general information, not safety or legal advice. Fire restrictions change with the weather and differ by county; confirm with your local fire authority before using an open flame outdoors.