Does Boiling Water Kill Weeds? What the Hot-Water Trials Show

The part no top result gives is the dose. Swedish and Belgian hot-water trials report energy per square meter, which we converted to kitchen units: about 3 fl oz of just-boiled water per sq ft cut seedling weight by 90 percent, and a 6-leaf weed took about 9 fl oz. A 1.7-liter kettle therefore covers about 19 sq ft of young seedlings or about 6 sq ft of older ones. Roots are a different job: a broad-leaved dock needed 1.6 liters at 194°F driven into the soil per plant (Latsch and Sauter 2014). Use the calculator below for your own area.

What boiling water does to a weed, and how fast

Heat kills plant cells by denaturing proteins and rupturing membranes. A USDA-ARS paper (Boyette and others, 2018) puts the start of protein damage at about 113°F (45°C) and lethal leaf and stem temperatures at 131 to 203°F (55 to 95°C), with faster heating doing more damage than slow heating. Water off the boil is far above that line, so any leaf it wets dies. What it does not wet, including the growing point under a rosette or a root an inch down, may survive.

The timeline is short. Iowa State Extension says treated plants wilt almost at once but do not brown until 1 to 2 days later, and plants with established roots resprout after several days. UC IPM WeedCUT puts maximum injury at about 3 days, after which most plants start to recover at least partly. So judge a kettle treatment at 2 to 3 weeks, not the next morning: brown leaves on day 2 are expected even for a weed that will be green again by week 3.

Temperature	What happens at it	Source
113°F (45°C)	Protein damage in plant tissue can begin; plant parts held this hot for long periods can die	USDA-ARS 2018; UC WeedCUT
120°F (49°C)	Third-degree burn to adult skin possible in 5 minutes; CPSC’s suggested water heater setting	CPSC 5098
130°F (54°C)	Adult third-degree burn in 30 seconds	CPSC 5098
131 to 203°F (55 to 95°C)	Lethal range for leaf and stem tissue	USDA-ARS 2018
135°F (about 57°C)	Water temperature used by hot-foam machines, which rely on foam to hold the heat in	UC WeedCUT
140°F (60°C)	Adult third-degree burn in 6 seconds	CPSC 5098
150°F (66°C)	Adult third-degree burn in 2 seconds	CPSC 5098
172 and 190°F (78 and 88°C)	Most of 7 pavement weeds were 2 to 6 times less sensitive than at 208°F	De Cauwer 2015
194°F (90°C)	Enough to kill broad-leaved dock roots when 1.6 liters were driven into the root zone	Latsch and Sauter 2014
208°F (98°C)	Recommended operating temperature for hot-water weeders on pavement	De Cauwer 2015
212°F (100°C)	Boiling at sea level; cooler at altitude and cooler still once poured	Physical constant

Read the two columns together. The gap between a lethal leaf temperature and a lethal skin temperature is small: water hot enough to kill a weed in a second is hot enough to give you a third-degree burn in 2 seconds. That is the one real hazard of the method, and it gets its own section below.

How much boiling water it takes per square foot

Hot-water research measures dose as heat energy per square meter. Hansson and Ascard (2002) found that a 90 percent cut in weight of white mustard at the 2-leaf stage took 340 kJ per square meter, one third of the energy needed at the 6-leaf stage. De Cauwer and others (2016) found four treatments in 12 weeks at 589 kJ per square meter highly efficient on dandelion, broadleaf plantain and two pavement grasses. Those are machine doses: even coverage, little runoff, water at about 208°F.

To turn them into kitchen units we divided by the heat 1 liter of just-boiled water carries above 61°F tap water, about 352 kJ (4.186 kJ per kg per degree C times 84 degrees). Treat the results as a floor: a kettle pours unevenly and loses heat to the air and the pavement, so real use will run higher.

Goal in the trial	Published dose	Our conversion, per sq ft	One 1.7 L kettle covers	Source
White mustard, 2-leaf stage, 90% weight cut	340 kJ/m²	0.090 L (3.0 fl oz)	19 sq ft	Hansson and Ascard 2002
Dandelion, plantain, 2 grasses, each of 4 passes in 12 weeks	589 kJ/m²	0.156 L (5.3 fl oz)	11 sq ft	De Cauwer 2016
White mustard, 6-leaf stage, 90% weight cut	About 1,020 kJ/m² (3 times the 2-leaf dose)	0.269 L (9.1 fl oz)	6 sq ft	Hansson and Ascard 2002
Hard-surface weed cover cut 90%, lasting 15 days instead of 7	50% more energy	1.5 times the 7-day dose	Two thirds the area	Hansson and Ascard 2002
Broad-leaved dock, 80% plants dead at 12 weeks, rotary nozzle	1.6 L per plant at 194°F	Per plant, not per sq ft	About 1 plant	Latsch and Sauter 2014
Broad-leaved dock, spiked injection heads A and B	3.7 and 2.6 L per plant	Per plant	Under 1 plant	Latsch and Sauter 2014
Soil under plastic film heated to 163°F at 2.4 in deep	120 L/m² at about 194°F	11.1 L (2.9 gal)	0.15 sq ft	Ushiki and others 2007

1. Seedlings, 2-leaf0.09
2. Each of 4 passes0.16
3. Weeds at 6 leaves0.27
4. Heat top 1 in of soil0.88
5. Heat top 2 in of soil1.75

Source: Green Industry Report arithmetic from Hansson and Ascard 2002, De Cauwer 2016 and Kodesova 2013 heat capacities Leaf doses are machine-trial floors; soil bars assume moist loam warmed from 68 F to 131 F with zero heat loss.

Show the numbers

	Liters per sq ft
Seedlings, 2-leaf	0.09
Each of 4 passes	0.16
Weeds at 6 leaves	0.27
Heat top 1 in of soil	0.88
Heat top 2 in of soil	1.75

The jump down the table is the whole story. Seedlings need about 3 fl oz per sq ft, a splash. A single dock plant needed roughly a full kettle forced into the root crown by a pressure washer nozzle, and the Japanese soil trial used 120 liters per square meter under plastic film to push 163°F down 2.4 inches. A 1.7 liter kettle cannot reach that zone by pouring from the surface.

Area of cracks, joints or gravel to treat (sq ft)WaterStraight off the boil, about 208 FWeed stageSeedlings, up to 2 true leaves (340 kJ/m2)Repeat program, each of 4 passes (589 kJ/m2)Older weeds, about 6 leaves (1,020 kJ/m2)

Boiling water (liters)

4.5

Boiling water (US gallons)

1.2

Full 1.7 liter kettles

2.6

Electricity to heat it, theoretical minimum (kWh)

0.45

That electricity at 18.16 cents per kWh (cents)

8.2

Floor doses converted from machine trials (Hansson 2002; De Cauwer 2016); a kettle pours less evenly, so expect to use more. Does not reach roots.

Why the roots survive a kettle

Soil soaks up heat. Using the van Wijk and de Vries heat capacities tabulated by Kodešová and others (2013) (soil minerals 1.9 MJ per cubic meter per degree C, water 4.18), we worked out what it takes to bring the top of a moist loam to 131°F, the bottom of the lethal range. Assumptions: 50 percent solids, starting at 68°F, and every joule staying put, which never happens.

Soil layer under 1 sq ft	Heat to warm it 1°C	Heat to reach 131°F from 68°F	Boiling water needed, zero losses	Room for added water in that layer
Top 1 in, moist (25% water by volume)	4.7 kJ	165 kJ	0.88 L (30 fl oz)	0.59 L
Top 1 in, dry (10% water)	3.2 kJ	113 kJ	0.60 L (20 fl oz)	0.94 L
Top 2 in, moist (25% water)	9.4 kJ	330 kJ	1.75 L (59 fl oz)	1.18 L

Two things fall out. Heating just the top inch of moist soil takes about 0.88 liters per sq ft, about 10 times the seedling dose, before any loss. And that inch has room for only about 0.59 liters of extra water, so most of a heavy pour drains deeper or runs off, carrying its heat with it. Wet soil makes it worse: the dry case needs about a third less, which matches the dock trial, where drier soil needed less hot water.

The trials agree. Iowa State says boiling water will not kill roots or other plant parts in the soil. Latsch and Sauter note the regrowth tissue of dock sits in a hypocotyl about 4 inches long; a 10-second dip in 190°F water killed every root in the lab, but in the field it took pressure heads injecting water 3.5 inches deep, or a high-pressure rotary nozzle, to get there. For creeping grasses, our bermuda grass control guide covers why rhizomes 6 inches down regrow after any contact treatment.

Which weeds boiling water kills, and which come back

Leaf shape decides a lot. De Cauwer and others (2015) found sensitivity highest in species with large, flat-lying leaves and lowest in grasses with small upright leaves, and UC WeedCUT adds that grasses regrow from stolons, rhizomes and protected growing points. Treat by type:

Weed type	Likely result of one pour	What the evidence says	Source
Broadleaf seedlings, cotyledon to 2 true leaves	Often killed	2-leaf white mustard needed one third of the 6-leaf energy; UC calls this the best window	Hansson 2002; UC
Annual broadleaf at 6 leaves or taller	Top killed, some regrow	3 times the energy; UC says plants past 2 leaves often regrow, cut tall growth first	Hansson 2002; UC
Young grasses with upright leaves (perennial ryegrass, red fescue)	Burned, often regrow	Least sensitive group; red fescue more sensitive than perennial ryegrass	De Cauwer 2015; 2016
Dandelion (Taraxacum officinale)	Leaves die, taproot regrows	More sensitive than grasses; good control took 4 passes in 12 weeks	De Cauwer 2016
Broad-leaved dock (Rumex obtusifolius)	Leaves die, root crown regrows	1.6 to 3.7 L per plant at 194°F, driven into the soil, for 80% kill	Latsch 2014
Herbaceous perennials and perennial grasses	Suppressed only	Regrow in as little as 2 weeks	UC WeedCUT
Weed seeds already in the soil	Not killed	No evidence hot water kills buried seed; water and heat may trigger germination	UC WeedCUT

1. 1Is the weed in a lawn or a bed with plants you want to keep within splash range?

   YesDo not pour. Dig it, or use a selective product on its label terms; boiling water kills turf and roots near the surface.

   NoGo to the next question

2. 2Is it a seedling with 2 true leaves or fewer, in a crack, joint or gravel?

   YesOne pour of about 3 fl oz per sq ft, straight off the boil, late afternoon. Check at 2 to 3 weeks.

   NoGo to the next question

3. 3Is it a taprooted or creeping perennial, such as dandelion, dock or bermuda grass?

   YesExpect regrowth. Plan 3 to 6 pours about 3 weeks apart, or dig out the crown; a kettle does not reach the roots.

   NoOlder annual: cut it low, pour about 9 fl oz per sq ft, and repeat in 3 weeks if it greens up.

Source: Hansson and Ascard 2002; De Cauwer 2015; UC IPM WeedCUT; Iowa State Extension

If the weed is in turf, boiling water is the wrong tool: it kills the grass blades it touches just as fast. Dandelions in a lawn are covered on our page on killing dandelions without killing grass, and selective products on our lawn weed killer guide. Not sure what you have? Start with common lawn weeds.

How often you have to repeat it

On traffic islands, Rask and others (2013) counted how many treatments kept weed cover under 2 percent. Hot water needed 3 a year on average, glyphosate 2.5, flaming 5 and steam 5.5. That is a well-equipped crew with a dedicated machine. On gravel embankments with an established weed flora, Hansson’s SLU dissertation found 6 hot-water treatments per season were needed for good control.

1. Glyphosate2.50
2. Hot water3
3. Flame5
4. Hot air and flame5.50
5. Steam5.50

Source: Rask and others, Weed Research 53 (2013), 2005 to 2006 seasons Averages across three sites; machine-applied treatments.

Show the numbers

	Treatments per year
Glyphosate	2.50
Hot water	3
Flame	5
Hot air and flame	5.50
Steam	5.50

Spacing matters as much as count. Hansson and Ascard found the required interval averaged 25 days, and that after 3 to 4 weeks almost no reduction remained because perennials had regrown. De Cauwer and others (2015) found 3-week intervals up to twice as effective and energy efficient as 6-week intervals. UC WeedCUT gives the same 3-week figure during active growth. For a resprouting crown, Iowa State suggests a second pour 7 to 10 days after the first.

How to pour boiling water on weeds

1. Pick the spot: cracks, paver joints, gravel and bare edging, with no lawn or bed plant you want within splash range.
2. Treat young. Pour when weeds are between the seed-leaf and 2 true-leaf stage; the 2-leaf dose is a third of the 6-leaf dose.
3. Cut tall weeds down first, as UC WeedCUT advises for hard surfaces, so the water reaches the crown instead of running off leaves.
4. Go in the late afternoon, on dry leaves. Plants were about twice as sensitive then as in the morning in De Cauwer 2015, and wet plants raised the required dose in Hansson 2002.
5. Pour straight off the boil. At 172 to 190°F most species were 2 to 6 times less sensitive than at 208°F, so do not let the kettle sit.
6. Use a kettle, held low, at about 3 fl oz per sq ft for seedlings and 9 fl oz for older weeds; wet every leaf and the center of the rosette.
7. Wear closed shoes and long pants, keep children and pets indoors, and never carry an open pot.
8. Check at 3 days for injury and at 2 to 3 weeks for regrowth. Pour again at 3 weeks, or 7 to 10 days for a resprouting crown, until the root gives out or you dig it.

Do not add salt. It turns a residue-free method into one that leaves sodium and chloride in the joint; our salt dose and soil math shows what each spoonful leaves behind. Vinegar has its own evidence on our vinegar weed killer page, and bleach on our bleach label review.

Burn risk: the number that matters

The U.S. Consumer Product Safety Commission says most adults suffer third-degree burns from 150°F water in 2 seconds and from 140°F water in 6 seconds. Water just off the boil is about 60 to 70 degrees hotter than either. CPSC also notes that most tap-water scald injuries and deaths are among older adults and children under 5, the two groups most likely to be nearby or to do the pouring on a patio.

Splash is the usual route to a burn: water hitting hard pavement bounces onto feet and shins. Pour from 2 to 3 inches up rather than from waist height, and refill indoors on a stable counter. Electricity is not a reason to skip it: heating 1 liter from 57°F takes about 0.1 kWh, under 2 cents at the August 2026 U.S. residential average of 18.16 cents per kWh.

This page is general information, not medical or safety advice. If a scald blisters or covers more than a small area, get medical care.

Where boiling water makes sense

The extension sources agree on the niche. Iowa State points to places where other plants are not nearby, such as patio and sidewalk cracks. USDA-ARS lists spot treatments around poles and fences, cracks in concrete and asphalt, and gravel. Our pavers versus concrete patio comparison and mulch versus rock page cover the surfaces where it fits.

One magazine result warns against pouring within 2 feet of shallow-rooted shrubs, while UC WeedCUT says commercial steam and foam can be used right up to the base of woody shrubs, with mature woody roots unaffected. Both can be right: the machines put down a thin film, while a pot emptied onto one spot sends liters into the root zone. UC also says household methods such as boiling water in a pot are not recommended. Above about 100 sq ft, a rented or contracted steam or foam unit is the practical step up; UC lists machines from about $13,000 to over $40,000 that treat 0.5 to 1 acre a day.