Information: Flame vs. Steam

2010 March update - please also see the article Thermal weed management for crop production (621KB pdf file) written for eOrganic (USA). Please note that the article states that steam weeders are not widely available. The article was written for a USA audience and PhysicalWeeding does not yet have a presence in North America, however, if you are interested in steam weeders they can be purchased direct from us.

Introduction

The decision whether to choose a steam or flame weeder is reasonably straight forward and depends on the following issues. While the decision may be clear cut in many situations, for others it may well be more nuanced. Please do not hesitate to contact Merf to discuss your particular situation in more detail.

Fire hazard

Flame weeders are an inherent fire hazard while our steam weeders are not. If it is not possible to use open flames due to the risk of starting a fire or if a weeder will be used in an area where there are fire bans (which typically prohibit all forms of open flames outside of buildings) then steam is the only option.

Potential damage to irrigation, plastic mulches and other materials

Not only are flame weeders a significant fire hazard, the high flame temperatures (1,700°C / 3,000°F) will also burn and melt a wide range of materials such as plastic mulches and irrigation pipes (especially thin walled drip / trickle systems) and ignite combustible materials such as wood chip and paper mulches. In comparison the temperature of steam is much lower at around 150°C / 300°F and can generally be safely used on the above materials for durations much longer than required to control weeds. Where damage to plastic or other combustible or low melting temperature materials is an issue, steam is the clear choice.

Fuel type

Flame weeders run on natural or petroleum gases, e.g., propane, butane and LPG while steam weeders are powered by diesel, vegetable oils or similar forms of liquid fuels. It is theoretically possible to build liquid fuelled flame weeders but we have not designed these yet as there are a number of practical issues to overcome. Its also quite feasible to power a steam weeder with a gas type rather than liquid fuel, however, its less practical as the fuel tanks are much more expensive as they are pressurised, and gas fuels are less readily available than diesel as all modern tractors use it so it is readily available on practically every farm. It also only requires the burner nozzle to be changed at a cost of 30-40 euros to convert a steamweeder to run on biofuel, vegetable oil, even on the likes of used cooking oil. If accessing sufficient gas based fuels is a problem, then steamweeders are the clear option, while if gas fuels are significantly cheaper, then flame weeders are likely to be the better option. If there is a desire to power a thermal weeder with a renewable fuel such biofuels, raw or processed vegetable oil, especially ‘waste’ products such as used cooking oil, then steam weeders are the best option.

Weight

Steam weeders use at about 120 litres of water per hour for every 100 kilowatt of heat produced (equivalent to burning approx. 10 l diesel). PhysicalWeeding recommends at least 200 kw heat output per meter working-width of both flame and steam weeders. This means around 500 l of water per hour (half a tonne) would be required for a machine with a working width of two meters. This illustrates that considerable amounts of water may be required for steam weeding. Flame weeding does not use water at all so the machines are much lighter, even when empty, so if weight is an issue, flame weeding will be the preferred choice.

Cost vs. effectiveness

Steam weeders are inherently more complex machines than flame weeders, so they generally cost more. However, scientific research has shown that steam is a more effective and quicker means of getting heat into weeds. Therefore, there is a trade-off between the purchase price and effectiveness. If capital cost is the major issue compared with running costs then a flame weeder will be the preferred choice, while if running costs are the greater concern, steam weeders will be a better option. The former scenario would by typical of a smaller scale producer who only use a thermal weeder for a few handfuls of hours a year, while the latter situation would be more typical of larger scale producer using a weeder for hundreds of hours a year. However, the point of balance between flame and steam is not fixed but depends on a wide range of variables, so it is not possible to give a general statement of where flame or steam would be the preferred option.