Forbs, seedlings
Moderate
Moderate-high ($1000–$5000/gross)
Low/no risk
Low risk
Low/no risk
Low (ground-dwelling animals, nesting birds)
Overview
Steaming and hot water are physical techniques of weed management where high temperature water is applied to damage plants through membrane rupture, protein denaturation, and enzyme deactivation resulting in leakage and loss of organelle function. High temperatures cause damage quickly while lower temperatures need more time. Plant parts exposed to temperatures as low as 113°F for extended periods of time can be killed.
Steam plus boiling water at about 221°F is applied using specialized equipment powered by diesel, gasoline, or electricity. Water is supplied either by a large tank attached to the machine or continuously through a hose. Another method uses much lower temperature water (~135°F) but the water is mixed with a biodegradable foam that insulates the application surface allowing for longer heat retention. Damage to susceptible plants can be observed within minutes to 3 hours.
Both water and steam are applied through a wand-type applicator. The steam applicator uses a hooded spray head or shower head to dispense the mix of a saturated steam and boiling water. The foam machine’s wand end is designed to distribute the foam quickly.
While these machines do not require extensive training to operate, they are labor intensive due to the time required to move hoses during the application and move the machine itself from site to site. Because treatment only kills plant tissue that is contacted, retreatment is usually necessary within 3 weeks in all but the coolest part of the year. At the time of this writing, costs of the machines (not including trailers and additional options) range from about $13,000 for the smallest steamer to over $40,000 for the largest foam system.
In general, grasses tend to be more tolerant of steaming and hot water treatment than are broadleaf species. This is likely due to the ability of grasses to produce new growth from vegetative reproductive structures such as stolons and rhizomes or protected meristems rather than any special tolerance of the plant to high heat. Consequently, this technique is most effective on young annuals, especially dicots. More mature annuals will be suppressed or controlled but the labor and time needed will be increased. These techniques should not be considered effective for control but rather for local suppression of plants, since many species of plants, especially those larger than the 2-leaf stage, will regrow. Herbaceous perennials and perennial grasses can be suppressed but will regrow in as little as two weeks.
For both grasses and broadleaf plants, the thicker the leaves are the less injury that can be expected with steaming. Additionally, broadleaf species with wider leaves are injured more than those with narrower leaves.
How to Use
Specialized equipment has been developed for steaming or hot water applications to weeds. Use of equipment developed for other uses, for example clothes steamers or boiling water in a pot, is not recommended.
On hard surfaces and along fence lines, first cut down as much vegetation as possible using a string trimmer or other vegetation removal method.
Water temperature on the gauge for the saturated steam system should be 225 to 250oF. Adjust the pressure down to increase the temperature. The foam steam system has a lighted button that indicates when the appropriate temperature is reached.
For long hose distances (greater than 100 ft from an access point), it is helpful to have an assistant move the hose.
For saturated steam, the covered applicator head is placed over the weeds for a short time, generally less than 2 seconds and moved in a similar motion as if using an upright vacuum cleaner to cover an area. For larger weeds, the head can be used to knock the plant over and allowed to dwell on the plant for 5 to 10 seconds. Treated foliage will change color when the cells begin to leak. When using a “shower head” attachment, hold the opening about 8 to 12 inches above the plant and move it as you see the leaf color change to bright green.
For the hot water-foam system, the wand will expel foam and the operator should move quickly to make sure it is applied uniformly on the desired area. Foam application is much more rapid than saturated steam but it is also less precise. It is important to follow the directions to ensure that the correct concentration of foaming additive is in the tank. Using the system to only apply hot water is not effective.
Either system can be used on herbaceous vegetation right up to the base of woody shrubs and trees without injuring them, though any leaves contacted may be injured slightly. Exposed roots should not be affected if they are mature and woody.
Efficacy depends on the amount of steam applied, the water temperature, the weed species, and the growth stage. Injury is rapid and often observed immediately after treatment. Maximum injury is usually observed at three days, but after this point most plants start to recover (at least partially).
Non-Chemical Weed Control: Steaming, Cheryl Wilen from Cal-IPC on Vimeo.
Special Tips
If a plant has large leaves, make sure to contact directly under the top leaves, which may be shielded.
For plants with tillers or lateral branching, the plant can continue to grow or recover even if all parts of the plant are thoroughly treated.
Most studies recommend that weeds should be controlled between the cotyledon and the two–true leaves stage. Otherwise, the effectiveness of the thermal treatment is reduced and an increase in fuel consumption is required. Steam has been shown to be more effective on certain erect-growing broad-leaved weed species than on prostrate-growing weeds.
There is no evidence that weed seeds in the soil are killed by saturated steam or hot water treatment if the equipment is operated as recommended. If seeds on the soil surface are exposed to extended periods (30–60 seconds) of saturated steam, germination may be reduced. However, in practice, saturated steam is not applied to treated areas for that length of time.
Optimal Conditions for Use
Steaming and hot water can be used in nearly all types of sites as long as they can be applied safely. Equipment is heavy, so hoses must be long enough to reach the treatment area from a staging area where the equipment resides. As noted above, the system is most effective on young plants. It is ideal for treating newly emerged flushes of weeds.
Depending on the choice of equipment and availability of water to refill the tanks, one can expect to treat about 0.5 to 1 acre/day. Therefore, it is best suited for small areas and in special circumstances such as cracks and crevices around schools and other sensitive areas.
Steaming is fairly indiscriminate as a treatment tool but can be focused on small areas and is therefore moderately selective. The operator also has control over where to apply the steam or foam.
Treating at 3- or 4-week intervals appears to be the optimal time for weed suppression during periods of active growth. Waiting longer will make treatment more difficult by allowing the plant to recover more and a larger number of other germinating weeds to grow before treatment.
One study reported that plants were more sensitive (almost two times as sensitive) when treated in the late afternoon than in the morning.
Caveats
- Water and heat from the treatment may stimulate seed germination.
- During periods of active weed growth, expect to treat at 3-week intervals if suppression is to be maintained.
- It is often better to have two people work the area—one to hold the wand and the other to move the hoses.
- This is a very slow technique—it is probably faster than hand-weeding but slower than mowing or trimming. Expect to treat about 0.5 to 1 acre/day using one of the larger machines, factoring in the time needed to refill the water tank.
- A water source must be readily available to refill tanks or otherwise supply the machine, depending on the model.
Potential Hazards to Humans, Environment, and Cultural Resources
Human safety. Low risk. In general, these systems do not pose a hazard to operators, though operators should wear gloves and protective eyewear to protect from the hot hose fittings or in the event of a hose break.
Cultural resources. Low risk. This technique does not disturb the soil surface and poses no impact.
Habitat. Low risk. However there is uncertainty about the environmental impact of the foaming additive, which is currently based on plant-based oils and may contain sugar, especially around water resources. As a precaution, it is best to stay away from waterways when using a foam system.
Sensitive species. Low-moderate risk. Noise may disrupt nesting birds and direct contact with foam or steam could harm ground-dwelling animals.
Erosion. Low risk.
Consider Combining with the Following Non-Chemical Methods
Mowing large areas or string trimming along fences, walls, and tree wells will reduce the time to apply the treatments. Efficacy will likely be increased for the foam system when upright growing plants are mowed as efficacy is improved the longer the foam can stay on the plant.
Don’t Use This Technique When/For
Perennial plants, especially woody plants, will not be controlled with this technique. Plants with rhizomes, bulbs, and tubers will resprout if those organs have formed before the plant is treated. Plants with protected meristems tend to recover better than those with unprotected meristems.
These systems are about as loud as a gas-powered leaf blower so they may not be preferred in some areas where noise would impact animals.
There is uncertainty about the use of the foaming additive (currently plant-based oils, may contain sugar) around water.
References
De Cauwer B., S. Bogaert, R. Claerhout, D. Bulcke, and D. Reheul. 2015. Efficacy and reduced fuel use for hot water weed control on pavements. Weed Res. 55:195–205.
De Cauwer, B., A. De Keyser, N. Biesemans, S. Claerhout, and D. Reheul. 2016. Impact of wetting agents, time of day and periodic energy dosing strategy on the efficacy of hot water for weed control. Weed Research 56, 323–334.
Kolberg, R.L. and L.J. Wiles. 2002. Effect of Steam Application on Cropland Weeds. Weed Tech. 16:43–49.
Leon, R.G. and D.T. Ferreira. 2008. Interspecific Differences in Weed Susceptibility to Steam Injury. Weed Tech. 22:719–723.
Martelloni, L., C. Frasconi, M. Sportelli, M. Fontanelli, M. Raffaelli, and A. Peruzzi. 2019. The Use of Different Hot Foam Doses for Weed Control. Agronomy 9:490. https://doi.org/10.3390/agronomy9090490
Peerzada A.M. and B.S. Chauhan 2018. Thermal Weed Control: History, Mechanisms, and ImpactsIn: Non-Chemical Weed Control Chapter 2 Pp. 9-31. Academic Press. https://doi.org/10.1016/B978-0-12-809881-3.00002-4
Shrestha, A., M. Moretti, and N. Mourad. 2012. Evaluation of Thermal Implements and Organic Herbicides for Weed Control in a Nonbearing Almond (Prunus dulcis) Orchard. Weed Tech. 26:110–116.
Authors and Credit
Lead Author: Cheryl Wilen, University of California Integrated Pest Management Specialist, UC ANR
Suggested Citation:
Wilen, C. Steaming. Weed Control User Tool (WeedCUT) – Methods for Managing Weeds in Wildlands. weedcut.ipm.ucanr.edu. California Invasive Plant Council (Cal-IPC) and University of California Statewide Integrated Pest Management Program (UC IPM). Accessed [date].
Efficacies
| Rating | |||
|---|---|---|---|
| Plant | Flowering Period | ||
| Winter | Excellent | ||
| Spring | Excellent | ||
| Summer | Excellent | ||
| Fall | Excellent | ||
| Multiple Seasons | Excellent | ||
| None | Poor | ||
| Plant | Germination | ||
| Winter | Excellent | ||
| Winter / Spring | Excellent | ||
| Spring / Summer | Excellent | ||
| Opportunistic | Fair | ||
| Plant | Palatability | ||
| Yes | No Information | ||
| No | No Information | ||
| Partial | No Information | ||
| Plant | Plant Growth Form | ||
| Grass | Good | ||
| Forb | Excellent | ||
| Shrub | Poor | ||
| Tree | Poor | ||
| Vine | Poor | ||
| Plant | Plant Type | ||
| Annual | Good | ||
| Biennial | Fair | ||
| Perennial | Poor | ||
| Plant | Propagule Production | ||
| Low (<1000/square meter) | Excellent | ||
| Moderate (1000–10,000/square meter) | Fair | ||
| High (>10,000/square meter) | Fair | ||
| Plant | Rate of Spread | ||
| High (doubling in <10 year) | Excellent | ||
| Moderate (50–75% increase in 10 years) | Excellent | ||
| Slow Rate (25% increase in 10 years) | Excellent | ||
| Plant | Resprouting / Regenerative Capacity | ||
| Low | Good | ||
| Moderate | Poor | ||
| High | Poor | ||
| None | Excellent | ||
| Plant | Seed Life | ||
| Short (≤3 years) | Excellent | ||
| Moderate (4–10 years) | Excellent | ||
| Long (>10 years) | Fair | ||
| Plant | Type of Reproduction | ||
| Seed | Excellent | ||
| Vegetative | Fair | ||
| Seed & Vegetative | Fair | ||
| Plant | Type of Vegetative Reproduction | ||
| Rhizome / Stolon / Stem | Fair | ||
| Bulb / Corm / Tuber | Fair | ||
| Root sprout / Sucker / Crown sprout | Poor | ||
| Site | Existing Desirable Plant Cover | ||
| <10% | Excellent | ||
| 10–25% | Excellent | ||
| 26–50% | Excellent | ||
| 51–75% | Excellent | ||
| >75% | Fair | ||
| Site | Ground Condition | ||
| Muddy | Excellent | ||
| Smooth | Excellent | ||
| Cobbly | Excellent | ||
| Rocky | Excellent | ||
| Site | Habitat | ||
| Marsh / Wetland | Excellent | ||
| Riparian | Excellent | ||
| Grassland | Excellent | ||
| Shrubland | Excellent | ||
| Woodland / Forest | Excellent | ||
| Site | Level of Tolerable Disturbance | ||
| Low | Fair | ||
| Medium | Good | ||
| High | Excellent | ||
| Site | Slope | ||
| Flat | Excellent | ||
| Moderate (10–40%) | Excellent | ||
| Steep (>40%) | Fair | ||
| Site | Target Area | ||
| <40 square feet | Excellent | ||
| 0.001–0.01 acre | Excellent | ||
| 0.02–0.1 acre | Good | ||
| 0.2–1 acre | Good | ||
| 2–10 acres | Poor | ||
| 11–50 acres | Poor | ||
| 51–100 acres | Poor | ||
| >100 acres | Poor | ||
| Site | Targeted Invasive Plant Cover | ||
| <1% | Excellent | ||
| 1–10% | Excellent | ||
| 11–25% | Excellent | ||
| 26–50% | Excellent | ||
| 51–75% | Good | ||
| >75% | Fair | ||
| Site | Vehicle Accessibility | ||
| Roadside | Excellent | ||
| <100 feet from road | Excellent | ||
| 100–1000 feet from road | Fair | ||
| >1000 feet from road | Poor | ||
